SCIENCE SCIENCE FOR FOR SOUTH SOUTH AFRICA AFRICA
ISSN 1729-830X ISSN 1729-830X
Int e rnat iona l Yea r o f
A st ronomy 2009
VOLUME 5 • NUMBER 1 • 2009 VOLUME 3 • NUMBER 2 • 2007 R29.95 R20
The hi st or y o f t he Unive r se The myst er y o f da rk e ne rgy The lif e a nd deat h o f st a r s I s t he r e a nyb o dy out t he r e?
A C AACDAEDMEYM YO FO FS C I EI ENNCCEE OOFF SS O U TT HH AAFFRRI C I CA A SC OU
South African PhD Project
Expanding the frontiers of knowledge www.nrf.ac.za
National Research Foundation
Cover stories Astronomy for Africa Kevin Govender The International Year of Astronomy 2009 is important to Africa, celebrating the continent’s long history of astronomy and our world class research facilities. 6
The history of the Universe: cosmology up to 1995 George Ellis Cosmology is the grandest study of all, asking the really big questions, such as how the Universe began, which we have only begun to unravel in the last 90 years. Dark energy: cosmology since 1995 Bruce Basset and Renée Hlozek The Universe is not only expanding, but it is accelerating as it does so, under the influence of a dominant energy form that no-one has yet seen.
Contents VOLUME 5 • NUMBER 1 • 2009
Is there somebody out there? John Menzies and Rudi Kuhn Are we alone in the Universe or is there life elsewhere? Since the early 1990s we have had the technology to search beyond our own Solar System for extra-solar planets. Black holes in our Galaxy and beyond Marissa Kotze and Phil Charles Black holes are no longer the stuff of science fiction stories or even just theoretical constructs, but objects that are present in the Universe in some numbers, even in the centre of our own Galaxy. The Milky Way and other galaxies Petri Vaisenen We sit inside the Milky Way, just one of many galaxies in the Universe; some spiral and some elliptical, but all fascinating.
The life and death of stars Enrico Oliver and Patricia Whitelock The Sun is our nearest star and without it the Earth would be a very different place, but there are countless numbers of other stars, which have been forming and dying over the ages. Big eyes on the stars David Buckley Humankind has been looking at the stars for centuries, but in the past few decades the technology to do so has become more and more complex, providing us with more and more information. When a crocodile eats the Sun: indigenous astronomy Thebe Medupe and Themba Matamela Africa has a long history of astronomy, shown by rock engravings by the San people and the rich heritage of the Timbuktu manuscripts.
MeerKAT coming on track during IYA2009 South Africa has put in a bid to host the Square Kilometer Array, an array of telescopes that will help to answer the big questions about the Universe and the MeerKAT, the Karoo Array Telescope may form its core.
The physics behind astrophysics Lisa Cruse, Kevin Govender and Nicola Loaring Physics is not just a dry, academic subject, but the backbone of the exciting world of astronomy and astrophysics.
When the light goes out: Pluto and Charon Amanda and Eric Gulbis Astronomers have developed techniques that allow us to characterise distant objects in the Universe that cannot be observed directly.
Astronomy at the Cape Brian Warner As with many other things, the Cape has a long history of astronomy, initially developed through the necessity to find ways to successfully navigate past it.
Books Starwise: A beginner’s guide to the universe. • 365 Awesome facts and records about Nature.
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Quest 5(1) 2009 1
SCIENCE SCIENCE FOR FOR SOUTH SOUTH AFRICA AFRICA
ISSN 1729-830X ISSN 1729-830X
Int e rnat iona l Yea r o f
VOLUME 5 • NUMBER 1 • 2009 VOLUME 3 • NUMBER 2 • 2007 R29.95 R20
A st ronomy 2009
The hi st or y o f t he Unive r se The myst e r y o f da rk e ne rgy The lif e a nd deat h o f st a r s I s t he r e a nyb o dy out t he r e?
A C AACDAEDMEYM YO FO FS C I EI ENNCCEE OOFF SS O U TT HH AAFFRRI C I CA A SC OU
Sutherland star party. Images: NASA; Tony Hallas. Sutherland Star Party 2008–9. CJ Ödman
SCIENCE FOR SOUTH AFRICA
Editor Dr. Bridget Farham Editorial Board Roseanne Diab (University of KwaZulu-Natal) (Chair) Michael Cherry (South African Journal of Science) Phil Charles (SAAO) Anusuya Chinsamy-Turan (University of Cape Town) George Ellis (University of Cape Town) Jonathan Jansen (Stanford University) Correspondence and The Editor enquiries PO Box 663, Noordhoek 7979 Tel.: (021) 789 2331 Fax: (021) 789 2233 e-mail: email@example.com (For more information visit www.questsciencemagazine.co.za) Advertising enquiries Barbara Spence Avenue Advertising PO Box 71308 Bryanston 2021 Tel.: (011) 463 7940 Fax: (011) 463 7939 Cell: 082 881 3454 e-mail: firstname.lastname@example.org Subscription enquiries Andrea Meyer (012) 843 6484/81 and back issues Tel.: e-mail: email@example.com Copyright © 2009 Academy of Science of South Africa
To infinity & beyond
hen Buzz Lightyear (in Toy Story) discovered that he couldn’t actually fly into space he was totally devastated. And looking at this issue of Quest I can understand his sorrow. What can I say about our special edition for the International Year of Astronomy 2009? Wow! South Africa is ideally situated to observe the skies. We have a relatively pollution free atmosphere and lots of wide open spaces that are not polluted by light at night – death to sky watching. The Cape has also been on major sea routes for centuries – requiring astral observations to allow navigation. As a result, we have an enviable history of astronomical research and have, over the years, developed truly excellent research facilites. These research facilities in turn, have allowed astronomy to grow as a major scientific discipline within South Africa and our research is world class. In fact, anyone with a gift for mathematics and physics should think seriously about a career in astronomy in South Africa. Astronomy and astrophysics are about big issues – life, the Universe and everything (to borrow from Douglas Adams). When you read about the history of the Universe, the life and death of stars and the research into extrasolar planets, you realise how small and insignificant we – and our Earth – are. NASA and other collaborative organisations have telescopes and probes that go deep into the Universe. These instruments are relaying information back to us all the time. Go to www.nasa.gov for constant updates on what is happening in the field and for truly awe-inspiring photographs and videos of what is in our Solar System and beyond. On our doorstep we have SALT, an optical telescope, and are builing MeerKAT, a radio telescope array, that, in the hands of our body of astronomers and astrophysicists, are constantly contributing to the body of knowledge about the Universe. And we hope to win the bid to host the Square Kilometre Array – a radio telescope so powerful that it will be able to look back in time to the Big Bang. Take yourself to infinity and beyond in the International Year of Astronomy, looking into the glory that is our Universe. This issue of QUEST is dedicated to Anthony Fairall (1943–2008) whose love of astronomy inspired generations of students and colleagues.
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Astronomy for Africa Kevin Govender explains why the International Year of Astronomy is so important to us all.
his special issue of QUEST is devoted to the International Year of Astronomy (IYA), 2009, and celebrates astronomy’s long history, Africa’s indigenous knowledge of the subject and South Africa’s remarkably wide range of world-class research facilities. But I would like to begin by addressing the question as to what astronomy (and specifically the IYA) means to Africa and the developing world. I write this on behalf of numerous individuals and organisations across the African continent that have come together through the common appreciation of astronomy and its benefits to the people of Africa. I have found that our sentiments, although focused on our experiences in Africa, apply readily to developing regions across the world. I live close to the southern tip of Africa and am employed by the South African Astronomical Observatory, home to the Southern African Large Telescope (SALT), the largest single aperture optical telescope in the Southern Hemisphere. In fact, Africa is also the home of the High Energy Stereoscopic System (HESS), the largest Gamma Ray detector in the world, as well as the Karoo Array Telescope, a large array of radio telescopes currently under construction. Along with these remarkable facilities, a number of countries in Africa could soon be home to the largest radio telescope array ever built, the Square Kilometre Array, if our bid to host it is successful. The message here: Africa is a world player in the astronomy field and the IYA means a lot to us, particularly in terms of education and the development of research communities.
Education The astronomy celebrations of 2009 can be used as a rallying point for the development of a strong learning culture. Astronomy is one of the most accessible of all sciences, with the biggest laboratory (a dark night sky) being most available to people in rural areas. Astronomy is used to spark curiosity
What is astronomy? So what is astronomy? What does it mean to people in developing regions? First it must be acknowledged that astronomy belongs to us all. Virtually every culture in the world had already established a relationship with the stars, Moon and Sun hundreds if not thousands of years ago. In fact, for as long as people have walked the Earth we have looked up at the night sky and wondered about the objects we saw. In Africa, people have used the stars for centuries, be it for navigation, for agriculture, or even for story telling.
Today, astronomy as a field of study has developed into something that attempts to answer some of the biggest questions imaginable. It is a field that challenges the limits of human understanding and yet never ceases to expand on it. Astronomy, as I have seen for myself on the faces of children, is also a spark that triggers the curiosity and wonder that is so often suppressed in a world of distractions – a curiosity that is so effective for the development of a person and thus the development of a people. Coming back specifically to the IYA, there are four areas where the IYA would play a major role in Africa and developing countries: education; development of research; public understanding of science; and development of partnerships.
Top: Contained within the most massive and active star-forming region in the Small Magellanic Cloud, star cluster NGC 346 delivers energetic radiation that excites nearby gas, causing it to glow. The result is one of the most dynamic and intricately detailed images of a nearby starforming region that has ever been taken by the Hubble Space Telescope. Image: NASA, ESA and The Hubble Heritage Team (STScI/AURA). Acknowledgment: A. Nota (STScI/ESA)
Above: Atop a modified Boeing 747 carrier aircraft, space shuttle Endeavour began the journey from Edwards Air Force Base on the first leg of its ferry flight back to the Kennedy Space Center just after sunrise on Dec. 10, 2008. Image: NASA/Tom Tschida
Quest 5(1) 2009 3
to contribute to the global knowledge and understanding of the Universe. In our quest as human beings to discover things that often fall outside of our wildest imaginations, Africa wants to play a role! The IYA is an opportunity to stimulate and support these researchers, and to spark the next generation of great minds.
Top: A violent and chaotic-looking mass of gas and dust is seen in this Hubble Space Telescope image of a nearby supernova remnant. Denoted N 63A, the object is the remains of a massive star that exploded, spewing its gaseous layers out into an already turbulent region. This supernova remnant is a member of N 63, a star-forming region in the Large Magellanic Cloud (LMC). Image: NASA, ESA, HEIC, and The Hubble Heritage Team (STScI/AURA) Acknowledgment: Y.-H. Chu and R. M. Williams (UIUC)
Above: Spitzer Space Telescope set its infrared eyes upon the dusty remains of shredded asteroids around several dead stars. This artist’s concept illustrates one such dead star, or ‘white dwarf’, surrounded by the bits and pieces of a disintegrating asteroid. These observations help astronomers better understand what rocky planets are made of around other stars. Image: NASA/JPL-Caltech
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and interest, not only in Science and Mathematics, but education in general. Very young people can be inspired by the beauty and scale of the universe, directly influencing the Millenium Development Goal (MDG) of Universal Primary Education. Development of research The need for the development of a strong research community is clear from policies and structures already in place within the African Union and the New Partnership for Africa’s Development (NEPAD). All across Africa I have found students and researchers who capture this spirit. people who are driven by the innate human curiosity within us all, people who want
Public understanding of science Scientific knowledge among the public is often not very strong in many developing countries. In many cases traditional knowledge or superstition dominates, leading to misconceptions. Astronomy can play a key role in addressing this often sensitive relationship between traditional and scientific knowledge systems. Astronomy, as mentioned earlier, is something that virtually every culture already has a relationship with. Indigenous astronomy can often be used as an easy way to start a gradual process of introducing a modern understanding of the universe. By bringing to the public the things we know about the universe and more importantly, how we know them (technologies employed, scientific method, etc.), we can spread knowledge of science and technology. However, the conversation goes both ways. Indigenous astronomical knowledge in Africa, such as the constellation isiLimela (aka Pleiades) which indicated the planting season, serves as proof of the advanced thinking and observations of our ancestors. In fact, archeological finds in Mali and other parts of Africa strongly suggest that Africans were interested in astronomy hundreds if not thousands of years ago. Developing partnerships Partnerships are essential for development and their importance is clear from their choice as a MDG as well as the spirit of the African Union and regional cooperation bodies such as the Southern
African Development Community (SADC), the East African Community (EAC), and the Economic Commission for West African States (ECOWAS). The IYA will not end at the end of 2009. One of the biggest legacies of the IYA will be the establishment of strong partnerships for astronomy both within Africa and globally. This applies to all developing countries and is reflected in the latest ‘IYA Global Cornerstone Project’ which deals with developing astronomy globally. Those are the four focus areas that the IYA addresses in terms of development. The choice of the four was not without motive, of course – and it brings me to the subject of our status and aspirations in Africa. Astronomy in Africa In Africa, where education is probably the only sustainable solution to challenges facing the continent, a group of astronomy-, space science- and education-related individuals and organisations have come together to harness the opportunity for the benefit of Africa as a whole. The IYA will be used as a launching pad for a network of African individuals and organisations who intend to work together into the future using astronomy to enhance education in Africa. In pursuit of this we have already drafted a plan that is based strongly on the theme of ‘Astronomy for Education’. In its current version the vision of this plan reads: ‘The continent of Africa, with an evergrowing astronomy research community, united in the fields of astronomy education and promotion, working together and sharing resources, such that the people of Africa are educated, especially in the fields of science, engineering and technology.’ The choice of four development areas are in line with the four core missions of this plan, namely:
1. Enhance the teaching and interest in astronomy in schools. 2. Enhance the teaching and interest in astronomy in universities. 3. Increase the awareness and knowledge of astronomy among the public. 4. Support and encourage an African network on astronomy. Fundamental to this plan are the issues of sustainability and development with specific objectives for what is to be in place after 2009. This plan, which has been written with the spirit of NEPAD, can easily be extended to the rest of the developing world as it specifically focuses on countries that may have little or no astronomical communities. It has been developed thus far and continues to develop with input being sought from over 100 people, the overwhelming majority of whom are based in over 20 African countries. Such a pro-active and dynamic effort by the continent has certainly given Africa significant status in terms of the global IYA activities. This status is evident from International Astronomical Union (IAU) plans to hold the next international ‘Communicating Astronomy with the Public’ conference in Africa, as well as a new IYA global cornerstone project currently being driven by the Africans entitled ‘Developing Astronomy Globally’. In a nutshell, the IYA means a lot to Africa and thus to the developing world. Having said this one may still ask the question ‘Do you really believe that looking at the stars can make a difference to a person living a hard life in Africa?’ My response is simple. I believe it because I’ve experienced it! I work with some of the poorest schools and communities in southern Africa. I’ve seen the wonder on a child’s face when they look through a telescope for the first
Above left: This image of Centaurus A shows a spectacular new view of a supermassive black hole’s power. Jets and lobes powered by the central black hole in this nearby galaxy are shown by submillimeter data (colored orange) from the Atacama Pathfinder Experiment (APEX) telescope in Chile and X-ray data (colored blue) from the Chandra X-ray Observatory. Image: X-ray: NASA/CXC/CfA/ R.Kraft et al.; Submillimeter: MPIfR/ESO/APEX/A.Weiss et al.; Optical: ESO/WFI
Above: Apollo 8, the first manned mission to the moon, entered lunar orbit on Christmas Eve, Dec. 24, 1968. That evening, the astronauts-Commander Frank Borman, Command Module Pilot Jim Lovell, and Lunar Module Pilot William Anders-held a live broadcast from lunar orbit, in which they showed pictures of the Earth and moon as seen from their spacecraft. Said Lovell, ‘The vast loneliness is awe-inspiring and it makes you realise just what you have back there on Earth.’ Image: NASA
time. I’ve watched youth change their career plans after a long conversation about the Universe – and I’ve seen the naughty ones stop their games and listen. I’ve seen the smiles on wrinkled faces as they grasp the mechanism of eclipses for the first time after long lives of fear and misconception. I’ve seen people smile peacefully when presented with a view of the Earth from space – an Earth with no borders, no nationalities or skin colour, no wars or anger – simply a little blue planet, a part of the Universe – with life, life that belongs to the Earth as much as the Earth belongs to it! Astronomy is a powerful tool. It is something that can alter one’s perception of the Universe. It can make one realise how fragile our lives are. Above all, astronomy is a subject that makes one think. It broadens the mind and allows us to appreciate different perspectives – to appreciate the diversity of people and of life – and it is certainly something that is to be celebrated. ■ Kevin Govender is a physicist and the chairperson of the South African steering committee for the International Year of Astronomy.
Quest 5(1) 2009 5
The lonely star burst galaxy, NGC 1569 was discovered by William Herschel in 1788. It is the home of three of the most massive star clusters ever found in the local Universe. Each cluster contains more than one million stars. Image: NASA, ESA, the Hubble Heritage Team (STScI/AURA), and A. Aloisi (STScI/ESA)
The history of the Universe: Cosmology up to 1995 George Ellis leads us to the origins of the Universe.
osmology is the study of the Universe on the grandest scale: what exists on the largest scales, what it is doing, and how it got there. These topics have been the subject of speculation for thousands of years, but it is only in the last 90 years that we have attained a solid scientific understanding of the nature of the cosmos. This progress has been based on a parallel growth of our understanding of physical processes on the one hand, and the development of observational instruments and techniques that have given us ever better images and data about astronomical objects on the other. The interplay of theory and observation, proceeding step by step, has given us an amazing picture of a universe expanding from a hot condensed early phase, when all that existed was radiation interacting with a gas composed of hydrogen and helium and a background of dark matter, to the galaxy-filled universe that today serves as the home for the solar system and humanity. I will trace this growth of understanding in its
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historical order, emphasising that at many stages of this progress we did not succeed in imposing our view of things on the cosmos: to our surprise, again and again we had to recognise what was there, and that what was there was unexpected and often not wanted. Things did not turn out to be the way we expected. The dynamics of the Universe is governed by the force of gravity, for that is what controls the motions of planets, stars, and galaxies. The starting point for cosmology therefore was theoretical: Albert Einsteinâ€™s development of the theory of General Relativity, the best understanding of gravity we have. It embodies startling ideas. Theory 1
Riemann might be a better description. Space could possibly be positively curved (like the surface of a sphere) or negatively curved (like the two surfaces represented in Escher drawings). Furthermore, geometry can change with time. What can cause this?
Space can be curved
Geometry is not necessarily the way Euclid and everyone since him has supposed! Rather the curved spaces that were first explored in detail by the German mathematician Bernhard
A typical image by MC Escher. Image: Cordon Art, Baarn, the Netherlands
Right: The spiral galaxy NGC 1300. In addition to a pair of spiral arms it has an elongated structure called the ‘bar’, at the end of which the arms start from. Most new blue stars are in the spiral arms, while older yellow stars are in the bar and central bulge. Image: NASA, ESA, and The Hubble Heritage Team STScI/AURA
Theory 2 Matter causes space-time curvature
This is the key feature of Einstein’s theory: the cause of space time curvature is the matter in it, as expressed by Einstein’s gravitational equations. This is what underlies the dynamics of the Universe. Einstein applied these equations to the problem of cosmology in 1917, and discovered it was difficult for the Universe to be unchanging. Theory 3 The Universe can be static if and only if there is dark energy (a ‘cosmological constant’) together with positive spatial curvature
Einstein realised that if the Universe was not held apart by a form of dark energy, then it would just collapse
under gravity. Furthermore, in order to avoid this fate, it could not be spatially infinite: it must close up on itself spatially. His static cosmology was the first mathematical model of the cosmos (Newton had not succeeded in making such a model): it was eternal and unchanging in
time. But it was drastically wrong. You can’t just develop models without comparing them with observations: and the developing astronomical observations gave a completely different picture. Observation 1 The Universe is large
It has been known for centuries that the Earth and planets are small compared with the Sun, which in turn is small compared with the Galaxy. In 1924, Edwin Hubble dramatically showed that the Universe was much larger than anyone had thought up to that time. Using Cepheid variable stars to measure distances, he determined the distances of faint clusters of stars that were only just visible, and showed that they were huge galaxies made of billions of stars, just as large as our own Milky Way Galaxy, which in turn is hugely larger than the Solar System that is the home of Planet Earth. Observation 2 The Universe is homogeneous and isotropic about us When the Sun and its nine planets are shown together on the same scale, tiny Pluto is so small that it could easily be mistaken for a background star. There is no room to show more than a section of the Sun’s 1.4 million kilometer disc. Image: Adapted from The Solar System; Amber Books.
Table of sizes and distances in the Universe Distance to moon
1.27 light seconds
= 385 000 km = 3.85 ×105 km
Distance to Sun
8.31 light minutes
= 150 000 000 km = 1.5 ×108 km
Distance to nearest star
4.27 light years
= 2 244 312 light min = 4 × 1013 km
Diameter of our Galaxy
100 000 light years
= 9.5×1017 km
Distance to Andromeda galaxy
2 520 000 light years
= 2.4×1019 km
Distance to Hydra cluster
133 000 000 light years
= 1.2×1021 km
Size of observable universe
14 000 000 000 light years
= 1.3×1023 km
The word isotropic means something that has physical properties that do not vary with direction.
Galaxies stretched away in all directions, with no region looking particularly different from any other, and Hubble could see no end to them: the Universe has no observable edge. ▲ ▲
1 light year = 525 600 light min = 31 536 000 light sec = 9.5×1012 km
How many such galaxies are there? Hubble counted galaxies in all directions, and found more and more at larger and larger distances. Also, he could not see any direction in the sky that looked different from any other direction: so the Universe is isotropic about us.
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Left: This cluster of hundreds of galaxies, Abell 2218, is 2 billion light years away. But we can also see arcs of galaxies, that look like blue and red streaks of light, in the picture. These are galaxies much further away, behind the Abell cluster, and their images are distorted and amplified because the foreground cluster acts like a gigantic lens, called a gravitational lens. Image: NASA, ESA, and Johan Richard (Caltech, USA) Acknowledgement: Davide de Martin & James Long (ESA/Hubble)
Indeed the Universe is the same everywhere: that is, it is spatially homogeneous. You need not bother asking for directions to the centre of the Universe: there is none!
in a steady state: the expansion might be unchanging in time. Whether or not this was so had to be determined by observation.
The Universe is evolving (not steady state)
The Universe is expanding
Radial velocities can be measured from the spectra of galaxies. Hubble’s momentous discovery in 1929 was that distant galaxies are moving away from us. He deduced this because he could see that their spectra are red-shifted, with spectral lines observed to be redder than when they were emitted. Also, the further away the galaxies are, the faster they are moving away from us. The Universe is not static, as everyone had supposed: the galaxies are all speeding away from each other. This was very difficult for many to accept, as the idea of an unchanging eternal Universe is deeply engrained. A rearguard action was fought by Fred Hoyle and others in the 1950s, suggesting that even though the Universe was expanding, it might be Hubble Law recession speed = H0 × distance C
D C B
Distance The graph represents Hubble’s Law. This states that the redshift in light coming from distant galaxies is proportional to their distance.
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This was shown by counting numbers of radio sources of different brightness: there were many more at large distances than a steady state allowed. The implications are immense: the Universe itself is changing with time. Given that the Universe is changing in time, how is it changing? We work that out by applying Einstein’s gravitational equations. Theory 4 The Universe had a beginning
This is not just a statement that matter came into being a long time ago, and had not existed before. It is also probable that space, time, and the laws of physics themselves came into being at that time. It is not that there was nothing before: there was no ‘before’ before! Our language cannot express this properly. Physics cannot unequivocally tell us how the Universe came into being, but one of the biggest questions that humankind has asked through the ages has been answered: there was a start to everything, when physical existence itself came into being. Theory 5
by the high temperature radiation. Atoms are made of nuclei composed of protons and neutrons, surrounded by electrons; above this temperature, the electrons would be knocked out of the atoms by radiation, and would travel feely from the nuclei. All that could exist would be matter and radiation in a very hot state and in equilibrium with each other because of the high energy collisions between the radiation and the matter. The universe would also then be opaque to radiation, because the most distance that light could travel between collisions with the free electrons would be a few centimetres. We call this early phase when the Universe was dominated by hot matter and radiation, the Hot Big Bang (HBB). Theory 6 The light elements were created in the Hot Big Bang
Above a temperature of 108 K, any nuclei present would be decomposed into protons and neutrons by the energy of collisions with radiation. As the Universe cooled through this temperature when it expanded, nuclei were able to form, because they were stable at lower temperatures. This is the process of nucleosynthesis. Using our understanding of nuclear physics, we can show that deuterium and helium are created by this process, plus a very small amount of lithium. The heavier elements out of which the Earth and living beings are made (iron, carbon, nitrogen, oxygen, phosphorus, for example) do not exist in the early Universe; they were created much later in the very hot interiors of massive stars, and were then spread through space in huge explosions at the end of the life of massive stars. Observation 5
The Universe had a hot early phase
Relic radiation from the HBB exists and has a blackbody spectrum
If you follow the gas in the Universe back towards the start, it was more condensed and so would be hotter. At early enough times (temperatures greater than 4 000 K) there would be no complex objects such as atoms or biomolecules, for if any were to exist they would immediately be destroyed
If there was this hot early phase of the universe, is there any way we can detect its existence today? Yes indeed: because it was in equilibrium with hot matter, the radiation that existed in the universe necessarily had a blackbody spectrum as predicted by quantum theory.
theory and observation agree
0.8 0.6 0.4 0.2 0.0
10 15 Waves/centimetre
Cosmic Microwave Background (CMB) spectrum plotted in waves per centimeter v. intensity. The solid curve shows the expected intensity from a single temperature blackbody spectrum, as predicted by the Hot Big Bang theory. A blackbody is a hypothetical body that absorbs all electromagnetic radiation falling on it and reflects none whatsoever. The graph shown was generated from data collected by the Far Infrared Absolute Spectrophotometer (FIRAS) instrument aboard NASA’s Cosmic Background Explorer (COBE). The FIRAS data match the curve so exactly, with error uncertainties less than the width of the blackbody curve, that it is impossible to distinguish the data from the theoretical curve. These precise CMB measurements show that 99.97% of the radiant energy of the Universe was released within the first year after the Big Bang itself. All theories that attempt to explain the origin of large-scale structure seen in the Universe today must now conform to the constraints imposed by these measurements. The results show that the radiation matches the predictions of the HBB theory to an extraordinary degree.
COBE stands for the Cosmic Background Explorer satellite. This satellite was developed by NASA’s Goddard Space Flight Center to measure the diffuse infrared and microwave radiation from the early Universe to the limits set by our astrophysical environment. It was launched November 18, 1989 and carried three instruments, a Diffuse Infrared Background Experiment (DIRBE) to search for the cosmic infrared background radiation, a Differential Microwave Radiometer (DMR) to map the cosmic radiation sensitively, and a Far Infrared Absolute Spectrophotometer (FIRAS) to compare the spectrum of the cosmic microwave background radiation with a precise blackbody.
Observation 6 Element abundances agree with theory
By observing the spectra of stars and by measuring element abundances in meteorites, we observationally determine the abundances of light elements in the early Universe. There is good agreement between theory and observation, provided the density of ordinary matter (i.e. baryons) is low. The theory of nucleosynthesis agrees with the observations of element abundances: a triumph for the application of nuclear physics in the context of the HBB, and further confirmation that there was indeed such a hot early phase in the history of the Universe.
The 53 GHz DMR sky map (top) prior to dipole subtraction (middle) after dipole subtraction, and (bottom) after subtraction of a model of the Galactic emission, based on data gathered over the entire 4-year mission.
The predicted abundance of elements heavier than hydrogen, as a function of the density of baryons in the Universe (expressed in terms of the fraction of critical density in baryons, Omega_B and the Hubble constant, h). Image: Adapted from the University of California, Berkeley Astronomy Department
Observation 7 The Relic radiation (CBR) is almost isotropic (with a dipole)
Not only can we measure the CBR temperature, we can determine how it varies over the sky, and we find that it is extraordinarily constant over the whole sky, its fractional variation being only one part in 1 000. There is a temperature variation at that level where the CBR sky is hotter in one direction and cooler in the opposite direction.
This image shows a 5-year microwave sky. The image reveals 13.7 billion-year-old temperature fluctuations (shown as colour differences) that correspond to the seeds that grew to become the galaxies. The signal from our Galaxy was subtracted using the multi-frequency data. This image shows a temperature range of ± 200 microKelvin. Image: NASA/WMAP Science team
This is interpreted as being due to the Solar System’s motion of 300 km/ sec relative top the cosmic rest frame defined by the CBR. Subtracting this motion off, we find CBR fractional temperature variations across the sky only at an incredible one part in 100 000. This represents fluctuations in the gravitational potential on the last scattering surface – the spatial surface where electrons combined with nuclei to form atoms, and the CBR decoupled from matter (because there were no free electrons left) and started to travel freely towards us. ▲ ▲
It was released by the matter as the temperature dropped below 4 000 K, and since then has travelled freely towards us, cooling as the Universe expanded. This was predicted in 1948, and the radiation was first detected directly in 1965. It was then
shown by satellite measurements that its spectrum is a perfect blackbody spectrum, with a temperature of 2.75 K today (that is, -270 °C). Detection of this Cosmic Blackbody Background Radiation (CBR) radiation confirms there was an HBB era in the early Universe.
Intensity (10-4 ergs/cm2 sr sec cm-1)
Cosmic microwave background spectrum from COBE
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The frames show the evolution of structures in a 43 Mpc box from redshift of 10 to the present epoch (from left to right). Image: National Center for Supercomputer Applications by Andrey Kravtsov (The University of Chicago) and Anatoly Klypin (New Mexico State University). Visualizations by Andrey Kravtsov
Theory 7 Structure can form in the expanding Universe
The background Universe is extraordinarily simple: it is spatially homogeneous and isotropic, hence is completely smooth. The real Universe is not like that: it is full of galaxies and clusters of galaxies that on very large scales form great walls and voids with a foam-like texture. How do these come about? The dominant force on these large scales is gravity: and if there is a set of small perturbations superimposed on the smooth background, then (as can be shown by use of Einstein’s equations) these will grow by gravitational attraction, and eventually form large-scale structures such as clusters of galaxies. Without either gravity or these small fluctuations in the early Universe, we would not be here, as there would be no galaxies to provide our home. Theory 8 Seeds for structure formation can come from exponential early expansion, and this will be seen in CBR anisotropies
For this mechanism to work, there needs to be some way of providing those tiny fluctuations to form inhomogeneous seeds to the start of the process of gravitational collapse. A truly remarkable theoretical development was the realisation that if there was a super-fast accelerating expansion of the very early universe
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(‘inflation’) before the HBB era began, then minute quantum fluctuations would occur that could be blown up to vary large scales by this inflationary expansion and form the seeds for structure formation. Furthermore, these fluctuations will lead to associated small-scale variations in the CBR sky with a particular pattern of anisotropies that should in principle be detectable. The article by Bassett and Hlozek (p.12) will pick up this story by showing how in recent times we have determined that, as well as the matter we can see around us in galaxies, there is lots of dark matter in the Universe; there is dark energy that is causing a late time accelerating expansion; and the CBR anisotropies predicted do indeed exist, and fit the theory. I will close this article with a final important theoretical discovery. Theory 9 The part of the Universe we can see is limited by horizons
Because the Universe is of a finite age, and light can only travel at the speed of light (as large as that is), the part of the Universe we can see and be influenced by is strictly limited. There is a visual horizon separating what we can see from what we cannot see, no matter what detector technology we may use; and there is a particle horizon separating us from all the matter in the Universe that can have had no causal influence on us whatever. There is a caveat here: we could possibly live in a ‘small Universe’, which is spatially closed on such a small scale (perhaps 300–600 million light years) that we have already
seen right around it, and in fact see multiple images of everything there is. While this is a possibility, and can indeed be tested, it is perhaps unlikely. Assuming this is not the case, what we can know about the Universe through astronomical observations is strictly limited. For example, unless we live in a small Universe (which is obviously finite), we can never prove whether or not the Universe is spatially finite or infinite. It is fun to speculate on which might be the case, but we can never determine observationally which is true – so this is not a scientifically ascertainable fact. Indeed despite all our successes, apart from the issues of dark matter and dark energy (dealt with in the next article), there are a number of significant things we don’t know: ■ we don’t know if the Universe is finite or infinite ■ we don’t know if it is positively or negatively curved ■ we don’t know what its future fate is ■ we don’t know how it started. There is much still to be done on cosmology! ■ Professor George Ellis is is the Distinguished Professor of Complex Systems in the Department of Mathematics and Applied Mathematics at the University of Cape Town in South Africa. He co-authored The Large Scale Structure of Space-Time with University of Cambridge physicist Stephen Hawking, published in 1973, and is considered one of the world’s leading theorists in cosmology. He is an active Quaker and in 2004 he won the Templeton Prize. From 1989 to 1992 he served as President of the International Society on General Relativity and Gravitation.
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A brief history of the Universe – our understanding of the cosmos starts with the Big Bang (far left) followed by rapid expansion and the formation of the galaxies and finally the late-time acceleration driven by dark energy which is shown schematically by the way that the tube opens outwards. Image: NASA/WMAP science team
Dark energy & cosmology since 1995 I
Bruce Bassett and Renée Hlozek look at our accelerating Universe.
106 Distance (parsecs)
2 × 106
Mapping the cosmic expansion – Edwin Hubble’s original plot of the velocities of galaxies (as they move away from us) against the distance to the same galaxies indicated that on average the further away from us the galaxy is, the faster it is moving away from us. This was the first observational evidence that the Universe was expanding.
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magine standing outside. You throw a ball in the air and instead of falling back to Earth it suddenly accelerates upwards, disappearing from view among the clouds, contrary to everything you thought you knew about gravity. On a cosmic scale this is the strange situation that cosmologists find themselves in today. Since 1995 our understanding of the Universe has been turned on its head. The riddle of the expanding Universe ‘A riddle wrapped in a mystery inside an enigma’ is a fitting description of the Universe today as it unfolds before our eyes. It is like a complicated cosmic jigsaw puzzle that has a lot of missing pieces that cosmologists and astronomers are trying to put together. We now know that not only is the Universe expanding but it is also expanding at an ever-increasing rate, much like a drag-racing car accelerating. This means that the Universe appears to be in the grips of a form of anti-gravity that is pushing on the cosmic accelerator pedal. Understanding this new antigravity force is, according to many scientists, the greatest mystery in pure
science today. While poverty, global warming and AIDS are all much more important to humanity, no problem is at the same time so grand, so wide-ranging and so demanding of new ideas about our place in the Universe as the problem of cosmic acceleration. It was less than 80 years ago that the first shock was revealed. In 1929 the noted astronomer Edwin Hubble and others found that the cosmos is actually expanding. On average, every galaxy is moving away from every other galaxy. The original graph of the velocity of the galaxies Hubble used against their distances is shown here. This discovery overthrew the ideology of the day – upheld even by Einstein – that the cosmos had to be static and infinitely old. In our earlier ball analogy a static Universe corresponds to finding a ball hovering a few feet off the ground. To achieve such a strange state of affairs, Einstein had to add a repulsive force that would balance the attractive force of gravity. This repulsive force became known as Einstein’s Cosmological Constant, and is denoted by the Greek letter (pronounced ‘lamb-duh’). When the expansion of the
Left: A supernova remnant: N49 is the beautiful left-over material from a supernova that exploded in the Large Magellanic Cloud thousands of years ago. Image: NASA and The Hubble Heritage Team (STScI/AURA). Acknowledgment: Y.-H. Chu (UIUC), S. Kulkarni (Caltech), and R. Rothschild (UCSD)
Above: Dying suns in distant galaxies – during their explosion that marks the death of these stars, these supernovae can outshine the galaxies in which they live (the supernova is the bright ‘star’ in the bottom left corner). Image: NASA
Cosmic lighthouses Like cosmic lighthouses, supernovae have been detected so far away that their light has been travelling for ten billion years to reach us! As an aside, since light travels at a finite speed, we are always looking into the past. Normally this is not important, but in cosmology the distances are so huge that the light from objects of interest has always been travelling for at least a million years. What does ten billion years mean for the cosmos? When these stars exploded the Universe was less than half its current size. The Sun and the
Earth did not yet exist and would have to wait another five billion years to form and even then, ages would have to pass before the dinosaurs would roam the Earth. Supernovae are cosmic time capsules, providing us with snapshots of the teenage Universe. An additional remarkable feature of one type of supernovae is that they are very reliable. When they explode they emit almost the same amount of light each time. By comparing distant supernovae to other nearby supernovae, we can work out how far away they are, and hence trace the expansion of the Universe back in time, like an archeologist unearthing the fossilised trail of a long-disappeared dinosaur. In 1998, a startling trend was found among
FAINTER (Farther) mB(Further back in time)
000 000 000 000 000 000 000 000 000 000 000 000 000 000, making it probably the worst prediction in the history of science! It is one of the reasons why explaining the cosmic acceleration is such an important problem – the cosmos is rubbing our noses in our ignorance of the laws governing nature on the very largest and smallest scales. Before the discovery of cosmic acceleration, our best way out of this embarrassment was to assume that the true value of was actually exactly zero, meaning that there would be no anti-gravity force at all. However, in 1998 this escape route was blocked by observations of the explosions which mark the end of life for distant suns. These supernovae occur when a star reaches the end of its life and for a couple of weeks the dying star becomes a supernova that shines with the brightness of one hundred billion suns, making it visible across the vast expanse of the observable Universe.
Universe was discovered in 1929 this cosmological constant was thrown away as unnecessary. Popular legend states that Einstein later called the introduction of the ‘greatest blunder of his life’. He missed the chance to predict the expansion of the cosmos and illustrated how important it is as a scientist to keep an open mind, even to radical ideas. The true value of Ironically, not only did Einstein miss the opportunity of predicting the expansion of the cosmos, but he also missed the chance to predict that it would be accelerating! Ninety years after Einstein introduced his cosmological constant, is the leading contender to explain the observed acceleration of the cosmos today. This time would not balance the force of gravity to produce a hovering ball, but would get the ball to accelerate upwards by overpowering the normal gravity we know. The big problem, however, is that the cosmos as we know it can only withstand a tiny amount of . Like eating too much poison, more than a sliver of would prove fatal to life in the cosmos as we know it since the Universe would have expanded so fast that galaxies, stars, planets (and us) would not have had time to form before being ripped apart. This fatal limit is much, much bigger than our best theoretical predictions for the value of , which are way off. In fact the estimates made by scientists overpredicted it by a factor of 1000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000
26 24 22 20 18 16 14
0.05 0.1 redshift
MORE REDSHIFT (More total expansion of universe since supernova explosion)
The accelerating Universe and the need for dark energy – a modern Hubble diagram showing that the data from the standard candles, Type Ia Supernovae, suggest that the Universe is not only expanding, but also accelerating expanding at an ever-increasing rate. Image: Perlmutter, et al., Supernova Cosmology Project
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All eyes on the cosmos – the multiple dishes of the MeerKAT array (www.ska.ac.za/ meerkat) will allow scientists to probe the Universe to great depths. Image: SKA
Imaging a time-capsule: Supernova SDSS131951 captured using the Southern African Large Telescope (SALT) towards the end of 2006. The supernova is the bright knot to the left. The host galaxy is a spiral whose arm structure is washed out by atmospheric effects and the great distance of the galaxy from Earth. Image: Bruce Bassett, Kurt van der Heyden and Petri Vaisanen, SAAO and UCT
flux (erg cm-2 s-1 Å-1) 10-18 2 × 10-18
0 4 000
6 000 wavelength (Å)
Fingerprinting supernovae: This SALT spectrum shows the supernova light broken up into its component colours like light through a prism, allowing us to identify this as a good candidate for measuring cosmic distances. This spectrum was taken six days after the brightness of the supernova was at a maximum. Although the light arrived on Earth in September 2006, the supernova had actually exploded and faded into oblivion more than a billion light years ago (and hence lies at a distance of over a billion light years). Adapted courtesy of: Bruce Bassett, Kurt van der Heyden, Masao Sako, Petri Vaisanen and Chen Zheng, SAAO, UCT and Stanford University
the sample of collected supernovae. The supernovae appeared dimmer than was possible if the expansion of the Universe was slowing down with time. The obvious explanation would seem to be that they are in fact further away than expected, which means that the cosmos had to have been accelerating in the intervening time. To make this a bit clearer, imagine seeing a car go past you on a completely flat road. It is travelling at 60 km/hour and you believe that it is free-wheeling with the engine off and therefore should be slowing down. After an hour you expect it to be less than 60 km away since you think it is slowing down. However, a friend tells you that instead the car is now
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90 km away. How can this be? You must surely conclude that the car was not in fact slowing down, but that it had instead accelerated at some point and the extra speed that resulted was converted into the extra distance. This is the same logic we use with the cosmos. The supernovae are further away than we expected if the universe were not speeding up. Hence they tell us that the expansion of the cosmos is speeding up. Finding the mystery cause of this acceleration – the analogue of the car’s engine – is the main and most exciting quest of 21st century cosmology. Dark energy Why did cosmologists expect the cosmos to be slowing down? Like the slowing of the ball thrown in the air, we expect the cosmic expansion to slow down under the attractive pull of gravity. We all know that gravity sucks, right? In contrast, acceleration requires a dominant new form of energy that provides this anti-gravity. This mysterious entity is affectionately known as dark energy since it is currently invisible except through its accelerating effects on the cosmic expansion. Dark energy is so weird it cannot be anything familiar. It needs to have very strong negative pressure and it needs to dominate the total energy budget of the cosmos. Indeed, cosmologists now understand that everything we see, everything we know and love, is only a small part of all the energy in the Universe, perhaps about 5% of the total. The rest is totally dark and invisible. The idea that 95% of the cosmos is invisible to us is amazing and revolutionary. But reality is even more extreme than that: our new understanding based on dark energy implies that our knowledge of the laws of physics on very large scales - including Einstein’s famous theory of relativity – is fundamentally incomplete or perhaps totally flawed. It is within this exciting arena that South African astronomy is taking bold steps to join the quest to answer these and other cosmic mysteries. The South
African Large Telescope (SALT, www. salt.ac.za) is the one of the largest telescopes in the world and has already been used to observe supernovae as part of a worldwide collaborative hunt for dark energy. SALT will make ground-breaking astronomical contributions deep into the next two decades and beyond. SALT is operated by the South African Astronomical Observatory (www. saao.ac.za), which has open nights in Observatory as part of an active International Year of Astronomy programme. An even more ambitious project is the $2 billion Square Kilometer Array (SKA, www.ska.ac.za) telescope to be built in either South Africa or Australia around 2018. As a forerunner to this revolutionary ‘world-telescope’, South Africa has committed to building MeerKAT, one of the world’s largest radio telescopes, to demonstrate our capability to host such a groundbreaking facility. Set to begin operations around 2010, MeerKAT will be situated in the Karoo and will allow astronomers to undertake detailed studies of the dark cosmos not using the optical wavelengths we use with our eyes, but rather the radio wavelengths that we use for communication. While all this may not change your day-to-day life, the quest to understand dark energy and the origin of cosmic acceleration is one of the most epic challenges we can take part in and the new facilities being built in the next two decades will probably revolutionise our understanding of the universe yet again and you can be a part of the action! May the (repulsive) force be with you. ■ Bruce Bassett is a research astronomer at the South African Astronomical Observatory and Professor in the Department of Mathematics and Applied Mathematics at the University of Cape Town. Renée Hlozek is a graduate of the Department of Mathematics and Applied Mathematics at the University of Cape Town and is now a DPhil student in the Astronomy Department at Oxford University.
Is there somebody out there? John Menzies and Rudi Kuhn discuss extrasolar planets and the search for extraterrestrial life.
rom the late 1950s a succession of space probes has allowed us to develop a good understanding of the planets of the Solar System. There is much more to be learned and searches are ongoing, but at present it seems that life as we know it is unique to planet Earth. Or is it? Perhaps there are planetary systems associated with stars other than our Sun; possibly some of them provide conditions suitable for life. But first we need to find such planets. Of course, with only the Solar System to work with, we don’t know how it was formed, though there are plausible theories, or whether the appropriate conditions existed elsewhere for planet formation.
In astronomy, Kepler’s Laws of Planetary Motion are three mathematical laws that describe the motion of the planets in the Solar System. The first law states: The orbit of every planet is an ellipse with the sun at a focus. The second law states: A line joining a planet and a sun sweeps out equal areas during equal intervals of time. The third law states: The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.
Blue shifted Doppler shift due to stellar wobble
associated with 261 stars. There are 30 stars with more than one planet, the record being held by 55 Cnc with five planets. These multi-planet systems are very different from our own. The Solar System has four inner rocky planets, of which Earth is the most massive (0.001 MJ), with two gas giants and two icy outer planets of decreasing mass from Jupiter to Uranus (0.05 MJ). In the 55 Cnc system, there are four inner planets, probably one gas giant and three icy planets with masses from 0.03 to 0.8 MJ, and one 3.8 MJ outer gas giant. Of course, the ‘wobble’ method is biased towards finding massive inner planets since they will be most effective in swinging their parent star around, so there may be a selection effect here such that less massive planets are being missed. Furthermore, it can only be used on relatively nearby stars (within about 300 light years). This is also the case with a second way of finding planets, the ‘transit’ method. Transits and microlensing Planets orbiting their parent star tend to lie in or close to a plane. If we were to view this planar system edge on, then a planet could appear to cross the surface of the star, blocking out a little of the light as it does so. Thus if we were to monitor the brightness of the star we would see that it was constant except for a short dip as the planet ‘transited’ the star. More than 50 planets have been discovered this way, again among bright, nearby stars. Images of fields containing tens of thousands of stars are obtained at regular intervals and the brightness of each star is measured and plotted against time. Sophisticated methods are used to find
0.3 Solar system
0.8 0.03 0.17 0.14
3.5 5.5 Cnc
planet Brightness 1
Time From the top: To measure the velocity of a star it is necessary to record the apparent wavelengths of spectral lines and compare them with the wavelengths measured in the laboratory. Different elements in the star’s atmosphere produce characteristic lines in the spectrum with known laboratory wavelengths. If the star is approaching us, the lines are shifted to bluer wavelengths, and if receding the lines shift to the red. Doppler shift caused by stellar woble. A comparison of our Solar System and 55 Cnc, showing the relative masses of the planets in each. S = Saturn, M = Mercury, V = Venus, E = Earth, M = Mars, J = Jupiter.
Extrasolar planets It was only in the early 1990s that advances in technology and the availability of large telescopes made it possible to conduct serious searches for extrasolar planets. Success came in 1995 with the discovery of a planet associated with 51 Peg, a Sun-like star that can be seen with the naked eye, which is about 40 light years away from the Sun, in the northern constellation, Pegasus. The observations extended over a period of 18 months and required the measurement of the velocity of the parent star with a precision of 3 meters/second (11 kilometers/hour), which was at the limit of what was possible at the time. In a binary star, the two components revolve about their common centre of mass – for two equal mass stars, this is mid-way between the two. For a star with a planet the centre of mass will be close to, but not exactly at, the centre of the star, so the star will appear to an outside observer to move back and forth – to ‘wobble’ – and hence its apparent velocity will change cyclically. Using the amplitude of the velocity change, the period, knowing the parent star’s mass and then applying Kepler’s third law of motion, it is possible to deduce the mass of the planet. For 51 Peg b it turns out to be 0.47 Jupiter masses (MJ). The planet revolves about its parent star in 4.23 days at a distance of 0.052 astronomical units (AU). The ‘wobble’ method has proved to be the most effective way of finding planets, and has led to the discovery (as of November 2008) of 303 planets
This graphic shows approximately what we would expect to see as the planet begins to cross in front of its parent star and how the light coming from that star will lessen slightly because the planet is blocking a little bit of the light. Studying the details of the light curve – how deep the dip is, how wide, how steep the drop off – reveals subtle clues about the planet. Image: NASA/JPL-Caltech/UMD/GSFC
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3 3 2.5
Ogle Robonet Canopus
planetary deviation 9.5 10 10.5 11 Danish Perth MOA
0 -20 0 20 Days since 31 July 2005 UT A graph showing how planetary deviation can be used to find out whether distant stars have planets associated with them. Image: PLANET Group
Planetary deviation – an explanation This is a plot of brightness versus time for the background star of OGLE-2005-BLG-390Lb. As the unseen foreground star aligns with the background star its gravity acts like a giant lens. As the alignment changes, the brightness of the background star varies in a characteristic manner; a smooth rise and fall. If the foreground star has a planet, the planet’s gravity distorts the gravitational lens, adding extra micro-images and causing extra brightening of the background stars overall image. The length of this extra bump (anomaly) depends mostly on the planet’s mass. The larger the planet the longer the deviation in brightness. For OGLE-2005BLG-390Lb the deviation lasted about 12 hours. (See the insert of the lightcurve plot.) In this case the Perth Observatory got most of the brightness measurements during the anomaly caused by the planet. Each colour on the plot represents a separate telescope. The hanging bands of colour shows how, as daylight approaches at one telescope, the next telescope takes over, giving roughly continuous coverage.
the star(s) in the field which undergo short dimming episodes at regular intervals. The change in brightness is expected to be less than about 2%. The detection of such changes is made possible by the use of sensitive CCD cameras and substantial computing power. Since it is very unlikely that we would see a planetary system edgeon, the method favours the discovery of planets very close to the parent star, and also planets with relatively large diameters. Nevertheless it provides extra information that cannot be derived from other techniques. The dimming of the star depends on the ratio of the area of the planet’s disk to that of the star. Since we know the diameters of common stars such as the Sun, we can find the diameter of the
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Above: An artist’s impression of the smallest extrasolar planet yet found around a normal star. Image: NASA, ESA and G. Bacon (STScI) Right: The star is represented by the multicoloured circular patch, while the planets, b, c and d, are shown as deduced from images obtained at different times. Image: National Research Council of Canada
planet. From the period of the planet we can find its mass, and hence its mean density. We can say that all of the transiting planets so far discovered are gas giants (more than 0.5 MJ) with radii close to that of Jupiter. What about stars very much further away: do they also have planets? There is a rather esoteric way of answering this question that depends on Einstein’s theory of General Relativity. When a foreground massive object passes in front of a more distant star, the apparent brightness of the distant star increases in a predictable way thanks to the bending of the starlight by the foreground object. If the foreground object has an associated planet there will be a perturbation of the predicted change in brightness that allows the mass of the planet to be determined. This method (gravitational microlensing) has allowed us to find eight planets associated with stars more than 10 000 light years away. This includes a rocky/icy planet of only 5.5 Earth masses associated with a star only about one-fifth as massive as the Sun, and consequently intrinsically cooler and fainter (an artist’s impression of this planet and its dim parent star is shown above). One of the most recent discoveries is of a system that is very much like our Solar System in that the parent star has a mass about half that of our
Sun, while there are two planets, an inner one with about half the mass of Jupiter and an outer one with about half the mass of Saturn. This is the first system to resemble our Solar System and, given the very low probability of finding it, it suggests there must be many more yet to be discovered. Imaging The methods described so far can be called ‘indirect’. We know the planets are there, but we cannot see them. Efforts have been made to image extrasolar planets, but this is fraught with difficulty. Planets are seen only by reflected light and are extremely difficult to distinguish in the glare of their parent stars. For example, if we could look at our Solar System from, say, Alpha Centauri, our nearest neighbour, Jupiter would be a small dot a billion times fainter than the Sun. It requires very sophisticated technology to be able to image a planet, especially from the ground, where the Earth’s atmosphere has a major impact on the resolution. Attempts to use the infrared part of the spectrum, where the contrast is expected to be much
mass of star relative to Sun
1 Mars Earth Venus
radius of orbit relative to Earth’s Above: The habitable zone is centred around Earth. Above right: The Kepler space craft.
Image: Adapted from an image from NASA
Image: Ball Aerospace and Technologies Corporation
Right: The target area that will be searched by the Kepler space craft in the Milky Way. Image: NASA
better, have produced six candidates. Their status is uncertain since there is no mass information in an image and it may be that they are brown dwarfs, the least massive stars possible, with an internal energy source, rather than inert planets. Nevertheless, optical images have recently been achieved, both from space and from the ground, of unambiguous planets associated with two different stars. One of them has three planets. Habitable zone We know there are at least 300 planets associated with stars other than the Sun. Some of them are rocky/icy planets not too different from Earth. There are indications that Earth-mass planets should be very common. Not all of them will orbit the parent star at a distance suitable for the maintenance of life. As we understand it, liquid water is essential, and the temperature range allowed is very limited. A planet must be neither too hot nor too cold to maintain the chemical reactions that are the basis of life – it must lie in the ‘habitable zone’ around the parent star. In the Solar System the zone is centred on Earth. On the star side of the zone, there will be a tendency to produce copious gases, leading to a runaway ‘greenhouse’ effect and very high planetary surface temperatures, while on the far side of the zone the temperature will be sufficiently low that the water will freeze out. Indicators of life Once we identify planets that might conceivably harbour life forms that are similar to those we know on Earth, we require some tests to demonstrate that they do exist. Until now this could only be done spectroscopically. The atmospheric constituents of a
planet will impress their particular signatures on the spectrum. Thus, indications of ozone would suggest the presence of oxygen associated with living organisms; water would suggest that there are oceans. Since they would be expected to react strongly together, ozone and methane imply an atmosphere that is not in equilibrium. This would suggest continuous replacement of these molecular species and hence some kind of living organism responsible for their production. Prospects Meanwhile, searches for more planets, and in particular, Earth-like ones, are continuing, both from the ground and from space. The French Space Agency’s COROT spacecraft, currently in orbit, is looking for transiting planets around nearby stars. In 2009, NASA expects to launch the Kepler mission, specifically aimed at detecting transiting Earth-like planets in the parent star’s habitable zone. This will survey 100 000 stars in the Milky Way over a period of 3–5 years. The James Webb Space Telescope is expected to be launched around 2013, and will have a 6.5-m mirror optimised to work in the infrared part of the spectrum. One of its major projects will be to examine extrasolar planets spectroscopically for signs of life. The discussion here has been very conservative in considering only life forms similar to those we find on Earth, but the possible characteristics of any other kind of life are completely unknown and how we would recognise such organisms is not clear. Nevertheless, the search for life will continue and we may have an answer to our original question in the next decade or two. ■
John Menzies is a primary mirror expert at SALT. His research focuses on the search for extrasolar planets using the gravitational microlensing technique as a member of the PLANET collaboration, and for a bit of light relief, an infrared survey of Local Group galaxies for long-period variable stars. Rudi Kuhn is a NASSP MSc student currently working on the search for extrasolar planets along with his supervisor John Menzies. He was also involved with the construction and deployment of the KELTSouth telescope located at Sutherland and owned by Vanderbilt University in the USA. If you want to know more: The Extrasolar Planet Encyclopaedia http://exoplanet.euv Planet Quest – NASA site http://planetquest.jpl.nasa.gov
Further information 1 Astronomical Unit (AU) = Earth’s distance from the Sun 1.496 x 106 Km Jupiter mass (MJ) 1.93 x 1024 Kg = 316 Earth masses Extrasolar planet names Star name + lower case letter, with b = first discovered, c = second, etc., e.g. 55 Cnc f is the fifth planet found around 55 Cnc. For a microlensing event, add ‘L’, e.g. OGLE-2005BLG-390Lb is the 5.5 Earth mass planet mentioned above
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in our galaxy and beyond
Marissa Kotze and Phil Charles discuss the facts behind black holes, which are no less exciting than the science fiction stories about these fascinating features of our Universe.
Artist’s impression of GRO J1655-40. Image: NASA/CXC/M.Weiss
n 1967 the starship Enterprise, under the command of Captain Kirk, was caught in the gravitational field of a ‘black star’, which resulted in them travelling back in time. Of course, that was pure science fiction, created by the producers of the popular television series Star Trek. Nevertheless, it played an important role in the public perception of black holes and other astrophysical compact objects, such as neutron stars, which is even stronger today. At that time, black holes were still theoretical constructs, but they have been the object of serious astronomical study for much of the last century. More recently, advances in telescope technology, both in space and on the ground, have led to observations that in the case of black holes show that the truth may actually be far stranger than fiction. A solar mass is simply the mass of our Sun and is the standard unit used to express mass in astronomy.
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Scanning the Universe Technological innovation has allowed observations of distant galaxies across the entire electromagnetic spectrum, from longer wavelengths such as radio, microwave and infrared, through the optical to the shorter wavelengths of ultraviolet, X-ray and gamma-rays. Using multiple wavelengths to look at each area of the Universe has revealed that there are large numbers of black holes in every galaxy, and more importantly that there are extremely large (millions of solar masses) black holes at the centres of apparently ‘normal’ galaxies. Our own galaxy is one! Each wavelength represents a different energy of radiation and a combination and comparison of observations over a wide range of wavelengths will show many different physical processes. One of the best examples of this is shown in observations of Centaurus A using observations at different wavelengths, the nearest ‘active’ galaxy to us. In different wavelengths
it is hard to believe you are looking at the same object! Gravity and what it means in astronomy Gravity is a universal force that describes the mutual interaction between all matter, and it affects everything, from the very large (such as galaxies in galaxy clusters) to the very small (such as the photons that make up light). The surface gravity of a body increases with increased mass or decreased size, so that it is highest for the most compact, that is the most dense objects. This means that the most massive objects in the Universe can remain invisible if they are massive enough to prevent any radiation from escaping their extreme gravitational field. Such objects are subsequently only detectable by their gravitational influence on matter or other bodies in their neighbourhood. The most useful examples of this involve binary star systems where one of the components is a compact object. In the artist’s representation of GRO
Above: An artist’s impression of the central region of our Galaxy. The supermassive black hole is surrounded by a disc of cool (red) gas, but it heats up as it approaches the black hole. Image: NASA/CXC/M.Weiss Left: Centaurus A seen at different wavelengths: a jet is visible in X-rays, radio lobes stretch far beyond the stellar disc and a dark dust band obscures the nucleus in the optical, but in the infrared we can see right through it. Image: NASA/CXC/SAO
Left below: This is a summary of the birth, ageing and death of stars. The youngest stars, the protostars, are at the left of the diagram. Stars of different mass have a different life cycle, which is shown from left to right in the diagram. Image: NASA/CXC/M.Weiss
black hole it moves faster and faster, getting hotter and hotter. This means that the radiation produced is at high energy (X-rays) in the inner disc, but at lower energy (optical, IR) in the outer disc. Stellar evolution Stellar mass black holes within our galaxy are the remnants of stars that were at least 8–10 times more massive than the Sun. They represent the end products of stellar evolution, once gravity has won its last battle
J1655-40, the binary consists of a black hole and a normal star (shown in blue). Gas leaves the normal star and enters the gravitational field of the black hole where it forms an accretion disc spinning around the black hole. It does not fall directly onto the black hole because of its angular momentum, but spirals in gradually, generating large amounts of light and heat along the way because of the effects of friction within the disc. As the matter gets closer to the
over all opposing forces. Throughout the lifetime of a star, a balance is maintained between inward and outward forces, but eventually a star runs out of fuel and gravity wins, thereby resulting in the star’s ultimate collapse. Depending on the initial mass of the progenitor (the ancestral star), the compact object that forms is either a white dwarf, neutron star or black hole. This graphic summary of our current understanding of stellar evolution shows their birth, ageing and death, with the lowest mass stars at the bottom and the highest mass stars at the top. At the other end of the scale, we now know that supermassive black holes of millions of solar masses exist in the nuclei of many galaxies, including our own! Any stars straying too close to them will be literally torn apart by tidal forces, just like those that our Moon raises on Earth’s oceans, and ‘gobbled up’ by the nucleus. However, these extraordinary objects also appear to play an important role in star formation. This artist’s depiction demonstrates what scientists think is happening near Sgr A*, in the centre of our own galaxy, where a supermassive black hole is surrounded by a disc of gas.
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Above: A schematic representation of how Einstein thought that matter curved space around it, using the Earth as an example.
The picture on top shows an artist’s impression of a stellar mass black hole, while the picture above is an impression of a supermassive black hole. Images: NASA/CXC/M.Weiss
Massive, and therefore young, hot stars (shown in blue) have formed in this disc and are still forming. Observations in the infra-red (which can peer through the obscuring dust and gas in the plane of the Milky Way) have been enhanced in the last 15 years by ‘adaptive optics’ techniques, which correct for the smearing effects of the Earth’s atmosphere. In this way, teams of German and US astronomers have watched these hot stars in their orbits around the unseen central black hole, thereby providing a direct measurement of its mass at 3 million solar masses! Despite the similarities between stellar mass black holes in low mass X-ray binaries and supermassive black holes in the centre of galaxies, the formation of the latter is not well understood. However, it is believed to be linked to the evolution of the galaxies in which these types of black holes occur. High-energy jets can be produced by both, as a result of electromagnetic forces in the disc and accretion of matter into the black hole. In the case of a supermassive black hole in the center of a galaxy, the production of a jet leads to the galaxy being defined as having an ‘active galactic nucleus’ (AGN), like Centaurus A.
Stellar orbits at the centre of the Milky Way This diagram shows the directly observed position of star S2 in its 15.2 year orbit around an unseen object at the centre of our galaxy. The orbit is highly eccentric and has a semi-major axis of five light-days, coming just 17 light-hours from the galaxy nucleus at its closest. It is possible to estimate the mass of the unseen object about which S2 is orbiting by applying Kepler’s 3rd law, which is simply a (AU)3 / P (yr)2 = Mstar + Mnucleus (i.e. the same physics that applies to the orbits of the planets around the Sun). Remember that the Earth’s distance from the Sun (which defines the Astronomical Unit, or AU) is just eight lightminutes, so you can put the numbers into this equation to show that Mnucleus is three million solar masses! (Mstar can be ignored since it is only about 10 solar masses).
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S2 orbit around SrgA* 1992.23
0.05” (2 light days)
1996.25 1996.43 1997.54 1998.36 1999.47
S2 SgrA* 2002.66 2002.58 2002.50 2002.40 2002.33
The theory of general relativity was nicely summarised in the words of John Archibald Wheeler, ‘Space tells matter how to move; matter tells space how to curve’.
Where Newton’s laws end Einstein’s theory of general relativity describes the physics in environments where gravity is strong enough to accelerate matter to nearly the speed of light, where the Newtonian laws of gravity and motion no longer hold. Enstein postulated that gravitational fields are the result of the curvature of space and that matter distorts the space around it, much like a lead weight resting on a thin rubber sheet will stretch and distort it. The resultant four-dimensional space (i.e. three-dimensional space with time as a fourth dimension) is called ‘spacetime’. The straightest path between points in ‘spacetime’ can therefore become curved if space is distorted. This creates the impression that a force is acting on a body moving along it. For slow-moving bodies in weak gravitational fields, the theory of general relativity agrees with the Newtonian laws of gravity and motion. Gravitational field strength is characterised by the square of the ratio of the escape velocity to the speed of light, so that larger field strengths require much higher escape velocities at which a body can break free from the gravitational grip of another. As fantastic as Einstein’s theory sounds, it appears to hold true, and has survived a hundred years of experimental tests. Arthur Eddington’s observations of a solar eclipse in 1919 confirmed that the theory of general relativity correctly described the amount by which light is deflected around the Sun. NASA’s Cassini-Huygens probe did the same in 2002. The probe was able to measure the deflection of electromagnetic waves as it travelled along the curvature of space. Indentations in the curvature of space are deeper for denser bodies. White dwarfs are denser than the Sun
and Neutron Stars are denser than white dwarfs, while black holes are even denser than Neutron Stars. A black hole creates an infinitely deep indentation in the curvature of space, resulting in a gravitational well from which nothing can escape, because the escape speed is equal to the speed of light. The speed of light is the maximum speed at which matter can travel. The minimum radius from which even light can escape is called the ‘event horizon’ or Schwarzschild radius, since it was Karl Schwarzschild who first described black holes in the context of the theory of general relativity in 1916. Whatever lies beyond it, is therefore invisible to external observers and hence the term ‘black hole’ is a rather fitting description, but was only coined in the 1960s by John Archibald Wheeler. The theory of general relativity predicts the existence of a ‘singularity’ within the invisible part of a black hole, which is a singular point where the gravitational field strength becomes infinite. This is a result of the theory’s inability to describe gravitational variations over a very small area. Another strange effect present in strong gravitational fields is called ‘time dilation’, which is the slowing down of time as the gravitational field increases in strength. To an external observer, time would appear to have come to a complete standstill for something at the event horizon of a black hole. Beyond the event horizon So what do we think will happen if you venture close to a black hole? The situation will look very different to those actually at the black hole (explorers) and to preople watching (external observers). As the explorers approach the event horizon, they will appear redder due to gravitational redshift and their clocks will also appear to run slower because of time dilation, until they finally come to a complete stand-still at the event horizon and fade away. External observers will never see the explorers pass the event horizon and will observe the time distortion effects of strong gravitational fields. The explorers will experience a stronger gravitational pull on their feet than their heads (spatial distortion) once they reach the event horizon. This effect will increase as they fall faster and faster towards the singularity, stretching them in the process (spaghettification), until tidal disruption pulls them apart. Smart explorers who do not cross the event horizon will effectively have travelled in time. Because of time dilation in regions of extreme gravity, more time has passed in the Universe than they have spent near the black
Sun White dwarf Neutron star
Black hole Event horizon
This diagram shows the different depths to which objects of different mass will curve space.
The fate of an explorer who crosses the event horizon.
hole, so when they return, they will be in the future. There is no doubt that black holes are fascinating objects to study and that we still have a lot of unanswered questions about them. Many of those may very well have to remain unanswered, since nothing can return from a position beyond the event horizon. However, much more can be learned from the detection of gravitational waves, which represent another phenomenon predicted by Einstein’s theory of general relativity. Gravitational waves are ripples in spacetime and are a product of very high gravitational fields that are moving at high velocities. We therefore expect them to be generated by merging or spinning black hole pairs and also by compact object pairs that spin at very high velocities. Black holes remain secretive, but they still have a lot to give up to those who are prepared to listen. ■ Professor Charles is Director of the SAAO and has worked for 37 years in High Energy Astrophysics, observing X-ray sources (particularly those involving neutron stars and black holes) from space observatories and with the largest telescopes around the world. Marissa Kotze is a UCT student, currently doing research on these X-ray sources with her supervisor Prof. Phil Charles, at the South African Astronomical Observatory.
This illustration shows the deflection of an electromagnetic wave (in green) as the probe travels along the curvature of space (in blue).
Redshift occurs if the wavelength of radiation is lengthened because red light has a longer wavelength than blue light. ‘Gravitational redshift’ occurs for radiation originating near a compact object, as those wavelengths are stretched as the photons are pulled back by strong gravity and become longer than they were when they were emitted. Further reading Gravity’s fatal attraction – Black Holes in the Universe. Mitchell Begelman and Martin Rees, Scientific American library; Black Hole information booklet by NASA, available at: imagine.gsfc. nasa.gov
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The Milky Way and Petri Vaisanen talks about galaxies, including our own.
Above: The ‘Milky Way’ stretches across the night sky from lower left. It is actually the next spiral arm of our own Galaxy, while the individual stars over the whole sky are in our arm. The two fuzzy wisps on the right are the Magellanic Clouds, our neighbouring galaxies. The picture was taken in January 2007, when the Comet McNaught was visible. Image: NASA, M. Druckmuller
Left: The Milky Way, it turns out, is no ordinary spiral galaxy. According to a massive new survey of stars at the heart of the galaxy by Wisconsin astronomers, including professor of astronomy Edward Churchwell and professor of physics Robert Benjamin, the Milky Way has a definitive bar feature – some 27 000 light years in length – that distinguishes it from pedestrian spiral galaxies, as shown in this artist’s rendering. The survey, conducted using NASA’s Spitzer Space Telescope, sampled light from an estimated 30 million stars in the plane of the galaxy in an effort to build a detailed portrait of the inner regions of the Milky Way. Image: NASA/JPL-Caltech/R. Hurt (SSC/Caltech)
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ooking up to the sky at a dark location far from city lights, you are struck not only by the myriad of stars, but also by a white cloud-like strip across the sky called the Milky Way. Even looking with binoculars you can immediately see that it actually consists of thousands and thousands of individual stars. In fact, it was Galileo 400 years ago who first saw this with his small telescope! Gazing at the night sky further from the Milky Way, you might pick up two other, smaller, hazy patches of light. These are the Magellanic Clouds, and again they consist of millions of individual stars. And with a small back-yard telescope you would see hundreds more, much fainter and smaller, hazy clumps in the sky. What are they? Over the last 80 years or so we have found out that they are galaxies: enormous systems of stars. The Milky Way We live inside just such a system of stars, a galaxy named the Milky Way, or just ‘the Galaxy’. The few thousand individual stars that you see with your naked eye in the sky are stars fairly close to the location of our Sun in our particular Galactic neighbourhood in our particular spiral arm. The strip on the sky is the ‘next’ spiral arm, which is far enough away that the individual stars are not resolved without binoculars or telescopes. In addition to the spiral arms, spiral galaxies, such as the Milky Way have a brighter central part, called the bulge. If we look towards the constellation of Sagittarius on the night sky, the Milky Way looks fatter and brighter in this region. We are actually looking toward the centre of the Galaxy where there are many more stars, and behind the next arm we see the glow of the central bulge much further away. The distance to the centre of the Galaxy from where we are is almost 30 000 light years. The Earth takes more than 200 million years for one full tour around the Milky Way. Looking carefully at the strip of the Milky Way, you will also see that it has many dark patches and wisps, often running close to the middle of the strip. You can see this in the picture above. These are dust clouds obscuring our view of the stars behind them. Most of the dust is concentrated in the spiral arms. The spiral arms also contain a lot of gas, which is most evident as glowing
other galaxies fact other galaxies, far outside our own, and that our Milky Way is just one of billions of ‘island universes’ that occur within the whole Universe. The Local Group and beyond As you now know, you can see our two closest neighbouring galaxies on the South African skies, the Large and Small Magellanic clouds. Although these two are galaxies in their own right, they are much smaller than the Milky Way, and are two of many small satellite systems around our Galaxy. From the northern hemisphere you can see another much larger star system with the naked eye, the Andromeda galaxy, or M31. Andromeda is actually somewhat larger than the Milky Way and is about three million light years away. Our Galaxy, together with Andromeda, form the two major galaxies in what is called the Local Group, and both of these giants are surrounded by a couple of dozen smaller systems each. It’s a dynamic system, with some dwarfs being ripped apart by the giants, and ▲ ▲
or reflecting ‘nebulae’, gas and/or clouds, close to bright stars and seen in pictures taken by telescopes. These locations are very important in the history and evolution of the galaxies and life because it is in these gas and dust clouds that new stars are born, along with the planets around them. There are a couple of hundred billion stars in our Galaxy. Many of them are binary stars, and many of them occur in clusters. There are young star clusters, such as the well-known Pleiades. Young star clusters are groups that are born together, but which have not yet had time to disperse. Then there are globular clusters, distributed around the Galaxy in its halo. These are also stars born at the same time, but way back in the ancient history of the Milky Way. In fact, it was the spherical distribution of the globular clusters that, in the early 1920s, led astronomers to realise the size and shape of the galaxy we live in. Around the same time it was finally proven that many other ‘nebulae’ in the sky were in
The Andromeda galaxy, or M31, is the closest giant galaxy to us and is very much like our own Milky Way. Its distance is about 3 million light years. You can also spot a dwarf elliptical galaxy, an Andromeda satellite, on the right hand side of the picture. Image: NASA, ESA and T.M. Brown (STScI)
‘Milkomeda’ Andromeda and our own Milky Way galaxy are gas-rich spirals, apparently closing in on each other with a relative speed of about 150 km/s. What will happen? Will our Solar System be affected? Astronomers Cox and Loeb calculated what might happen in the collision that is expected to occur in about three billion years (Monthly Notices of the Royal Astronomical Society, 2008, 386, 461). As with the Bird galaxy which you will see later in the article, and others in the image (right), there will be streams of gas and stars flying around that are tens of thousands of light-years in length. The now orderly spiral arms will be disrupted, and starbursts will light up the Local Group. Individual stars will not collide with each other, though. They are so far apart that all the billions of stars will simply fly past each other. So the Sun and the Earth will be safe from that point of view. What may happen, however, is that a stellar intruder from Andromeda may pass sufficiently close by to trigger an avalanche of comets toward the inner Solar System where we are, from the Oort cloud far outside and around the Solar System. The researchers have calculated that there is a very real possibility that our Sun will end up in one of the tidal arms far outside the galaxies. Our night skies would then be very dark, containing few stars bright enough to see with a naked eye. On the other hand, there is a smaller possibility
that our Solar System will drop towards the centre of our Galaxy, where the neighbourhood is bright, but very violent. Researchers also estimated a 2% probability that our Sun would, during the collision, switch allegiance to Andromeda, becoming a sun in that galaxy. Ultimately, however, in a few billion years time, the two will have merged into one giant galaxy, the ‘Milkomeda’, which would resemble today’s spheroidal and elliptical galaxies.
Right: When the Milky Way and Andromeda collide in the future, they will produce an intriguing pair of a galactic dance, such as these captured with the Hubble Space Telescope. The pair will eventually merge into one single galaxy, Milkomeda. Image: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University), K. Noll (STScI), and J. Westphal (Caltech)
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Above: This cluster of hundreds of galaxies, Abell 2218, is 2 billion light years away. But we can also see arcs of galaxies, that look like blue and red streaks of light, in the picture. These are galaxies much further away, behind the Abell cluster, and their images are distorted and amplified because the foreground cluster acts like a gigantic lens, called a gravitational lens. Image: NASA, ESA, and Johan Richard (Caltech, USA) Acknowledgement: Davide de Martin & James Long (ESA/Hubble)
Above right: The picture shows a frame simulating the structure of the present-day Universe from the ‘Millennium Simulation’. It is seen that matter (in this case dark matter) is found in a web-like structure, and the largest clusters of galaxies are at the intersections of the numerous filaments, as at the centre of this picture. The scale line shows a size of about 400 million light years. Image: Max-Planck-Institute for Astrophysics, Garching, Germany Right (top): The spiral galaxy NGC 1300. Most new blue stars are in the spiral arms, while older yellow stars are in the bar and central bulge. Image: NASA, ESA, and The Hubble Heritage Team STScI/AURA Right: The famous Sombrero galaxy, or M104, has a disk like the spiral galaxies do, but also a very large smooth bulge resembling elliptical galaxies. Image: NASA/ESA and The Hubble Heritage Team STScI/AURA
even M31 and the Milky Way are falling towards each other, which might have catastrophic consequences in the far distant future (see ‘Milkomeda’). The Local Group moves as a whole towards the constellation of Virgo, where there is a large cluster of galaxies. Groups of galaxies have dozens of galaxies in them, however, and the clusters contain hundreds or even thousands of galaxies. Most galaxies in the Universe occur in groups and/or clusters. Clusters in turn form super-clusters, and it was realised in the 1980s that all these galaxy structures form a foamy or bubbly network. Galaxies are located in great the filaments and in the walls of this universal foam, and the largest clusters and super-clusters are at the intersections of these bubbles. Only a few galaxies live in the giant voids inside the bubbles. Mapping these structures and understanding the origins and evolution of galaxies and clusters have become important fields of astronomy and astrophysics.
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Types of galaxies Galaxies differ a great deal among themselves in shape, size and mass, as already seen with our giant Galaxy and the dwarf Magellanic clouds. However, the majority of large and (optically) bright galaxies can be divided into two groups called spirals and ellipticals. And most of the left-over ones are irregulars. In addition, there are the dwarf galaxies, that also come in different shapes, from dwarf spheroidals to dwarf irregulars.
The spiral galaxies are the ones most people immediately recognise as ‘galaxies’, with a flat disk of young stars, a smooth central part of older stars and often striking and beautiful pairs of spiral arms. The origin and progression of the spiral arms is particularly interesting. Because of the rotation of the galaxies, they should quickly wind tightly and disappear. However, it is now commonly
Right: This galaxy, called the Bird because of its appearance, is actually a triple collision at a distance of 650 million light years. It is producing a huge amount of new stars per year. The SALT telescope revealed that the ‘head’ of the Bird is actually on a very highspeed encounter with other galaxies, and is the major reason for the spectacular starburst effect in the system. The ‘wings’ and the ‘tail’ are outflying stars and gas from the two red galaxies in the middle. Image: Vaisanen/ESO/HST
thought that there is a spiral ‘density wave’ that goes around the disk of a galaxy. This wave then compresses the gas and dust at a given location and results in new star-birth. So we see these new and young stars in those locations as a spiral arm. The wave moves on, the stars get old and fade, but new bright ones are born in a new location, where the wave is at a particular point in time. This is similar to a ‘Mexican wave’ at a sports stadium! The wave goes around, though individual people just stand up and sit down. Elliptical galaxies
The elliptical galaxies, in contrast to spirals, do not contain much gas and dust at all, and often none. The Sombrero galaxy on the previous page, without the horizontal dust lane and disk, would resemble a typical elliptical galaxy. Elliptical galaxies consist of old stars, and there is a huge range in their sizes, from giants much more massive than the Milky Way to tiny satellite dwarf spheroidals. The stars of ellipticals do not move around their galactic centre together as do the disk-like galaxies, but essentially all the stars have their own random orbits around the centre. It is thought that most ellipticals, especially the most massive ones, formed quite quickly in the early Universe from collapsing gas clouds, which ‘burned away’ their gas fuel in spectacular bursts of star formation many billions of years ago. The most massive stars have long since died away and what is left is just a smooth spherical or ellipsoidal distribution of long-lived fainter red stars, with no gas and dust left to make new generations of stars. Interacting galaxies
Artist’s conception of an active galactic nucleus (AGN). AGNs are thought to consist of a massive black hole right in the centre of a galaxy, with in-falling material forming an accretion disk (red in the image) around it and emitting very strong radiation. There is also a dust torus around the AGN (the blue in the picture) and radio jets shooting out along the axis of rotation. Image: NASA/Goddard Space Center
where large quantities of gas flow to fuel star formation and probably also to feed a growing and massive black hole. Active galaxies
In the 1960s strong radio sources in the sky were matched up with starlike optical sources with mysterious spectral properties. It took a while to realise that the spectra indicated redshifted, very distant, sources, and hence they had to be intrinsically ▲ ▲
Stars inside a galaxy are very far from each other. For example, the distribution of stars in the Solar neighbourhood would be similar to oranges that were distributed so that only half of dozen of them would be in the whole continent of Africa. Stars essentially never collide, except
in very close binary systems. In contrast, galaxies are quite close together. The distance to the Magellanic Clouds is only two or three times the diameter of the Milky Way, and the next really major galaxy (M31) is only some 30 Milky Way diameters away. Galaxies are far more likely to collide than stars and do so more often. In fact, our current understanding of how galaxies formed suggests that they arose from the merging and colliding of smaller sub-galactic clumps. Fully fledged galaxies may also collide, and perhaps the most spectacular of these are the interactions and mergers of gas-rich spiral galaxies. As the two or more progenitor galaxies pass the first time, they fling out massive quantities of stars and gas, called tidal tails. As tens and hundreds of millions of years go by, the cores of the original galaxies perform an intricate dance around each other, ever spiralling together until at last they from one single nucleus. Meanwhile, the surrounding arms and tails appear very chaotic, and as massive gas clouds collide, they compress, heat up and fragment to produce new stars. Many of these new stars will be massive and short-lived, exploding as supernovae, which compress the surrounding gas even more, resulting in starbursts, a sort of cosmic fireworks. This happens especially in the centre,
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Distances to galaxies Right: Light spreads out with the square of the distance. Through a sphere twice as large the energy covers an area four times larger. Through a sphere three times as large, the energy covers an area nine times larger. Below: Inverse square law: the energy we receive is inversely proportional to the square of the distance.
Cluster nebula in
Distance in light years
78 000 000 1 200 km s–1
Virgo 1 000 000 000 Light spreads out with the square of the distance. Through a sphere twice as large, the energy covers an area four times larger. Through a sphere three times as large, the energy covers an area nine times larger.
15 000 km s–1
Ursa Major 1 400 000 000
22 000 km s–1
Inverse square law: the energy we receive is inversely proportional to the square of distance
2 500 000 000 10 parsecs
39 000 km s–1
100 photons 3 960 000 000
20 parsecs 2 × farther so it’s 22 = 4 times dimmer 10 2 = 100 × __ 1 100 × ___ 4 20
61 000 km s–1
To find distances to faraway galaxies, astronomers use properties of the expanding universe, the Hubble Law. The further away something is, the more its spectrum is shifted towards the red. On the right side of the picture you can see a pair of dark lines on the white horizontal spectrum moving towards the right (red) the further away the object is.
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radar ranging parallax
surface temp. (K) Tully-Fisher relation
main-sequence fitting Cepheids
V Hubble’s law: d = __ Hg
Right: Astronomers use a variety of methods to measure distances to ever larger scales. They call it the ‘distance ladder’. Many of the methods are based on the principle of ‘standard candles’, which means that if you know that a certain candle, or an object, at a distance of one metre has a certain brightness, it will be exactly 100 times fainter when seen from 10 metres away.
How do we know how large the Universe is? How do we know how far away the galaxies are? These are questions that astronomers have struggled with for a long time and have found creative ways to answer. Distance is an important parameter, since many other properties exploding stars such as novae or supernovae. These can be seen even essential to our understanding of galaxies depend on it. If the distance further away than can Cepheids. scale is wrong, our estimates of galaxy sizes and brightness will be wrong And going even further out, whole galaxies may be used as the as well. standard candles. For example, the brightest galaxy in a given cluster, At the nearest Galactic scales, distances to stars are measured by a giant elliptical, has a relatively constant brightness. However, the triangulation using the Earth’s orbit as the baseline. The effect is called uncertainties in these calculations start to become significant because parallax. of factors such as the unknown history and evolution of that particular To go further into the Universe to look for far distant galaxies, we use a galaxy and cluster. system based on ‘the inverse square-law of brightness’. Nevertheless, distances up to few billion light years can be estimated All that remains is to find the perfect torch to compare with: if you this way. This illustration shows some of the methods that astronomers know that a certain type of astronomical object always has the same use to find distances from the nearest objects to the furthest ones. intrinsic brightness, it’s measured brightness will tell its distance. Finally, the method most used in extragalactic astronomy and cosmology, There has been a significant effort in astronomical research over the is the redshift. This is related to the expansion of the Universe, and the effect past hundred years to define these so-called ‘standard candles’. that this expansion has on the spectra of all the objects we see. Perhaps the most famous standard candles are the Cepheid variables. Essentially spectra shift (move), and if that shift (movement) is These are very bright stars that pulsate, so that their brightness varies. interpreted as a velocity, that velocity can be translated into a distance, Crucially, it was discovered that their light-variation period is related to depending on the exact cosmological parameters of the Universe. their intrinsic brightness. Since the variation is easy to measure, and relation to absolute galaxy brightness was calibrated using nearby examples clusters nearby for which the distance was determined with (1010 ly) galaxies other methods, the Cepheid distances can (107 ly) Milky Way be determined in other galaxies up to tens of 5 ly) nearby (10 millions of light years distant. stars solar Planetary nebulae, the end results of (102 ly) system certain kind of stars, also have quite uniform (10–4 ly) white dwarf brightnesses, as do different classes of Venus
Fg = Fc 2 GMm GMm = mv _____ ____ 2 R R 2 R 2R M = v___ G
Mass of a galaxy and a rotation curve How does one measure the mass of the galaxy? We do this in the same way that we measure the mass of the Sun or the Earth: we observe something that goes around it! Objects in an orbit around some other mass behave according to the gravitational field, or pull, of that object. The orbit will tell you the mass affecting it. This means that the mass can easily be calculated using Newton’s law of gravity. Of course we have to assume that Newton’s laws still apply on galactic scales, and most astronomers and physicists do believe that this is the case. The diagram (top right) illustrates a point of mass going around the centre of a galaxy. The point of mass experiences the gravitational force Fg towards the centre, while, on the other hand, a centripetal force Fc would be required to keep it at a circular orbit. Equating these, you can solve the mass M inside the orbit. Try it, for example for the case of the Sun and our Galaxy! We are 8 500 parsecs away from the centre, going around at a speed of 220 km/s. 1 parsec = 3.1 x 1016 m, and the gravitational constant is G = 6.7 x 10(-11) m3/kg/s2. You could express the answer in solar masses, Msun = 2 x 1030 kg, to know approximately how many stars there are in our Galaxy!
Top right: By using the distance and speed of a point of mass going around the centre of a galaxy, we can calculate the mass inside its orbit.
Right: To find the velocity of stars orbiting a galaxy centre (needed to calculate mass, for example), we can use redshifts. The spectrum of the star is moved towards the red if it is going away from us, and towards the blue if it is coming towards us. How much it moves, gives us its speed.
has one, with a mass of about 4 million Solar masses. Some galaxies have material streaming into their BH. The material, stars, gas and dust, spirals in toward the BH and forms an accretion disk around it. This then heats up to millions of degrees as a result of compression, as it accelerates further and further into the dense nuclear region. It is the radiation from this process that we see as an AGN or QSO. Often this central source emits more light and other radiation than the whole galaxy. It is as if a single torch placed in the central square of a major large city could out-shine all other light sources in the city! ■ Petri Vaisanen, who is originally from Finland, is a SALT astronomer doing research on the evolution of galaxies.
w w w. o l w a z i n i . o r g . z a
extremely bright and much smaller than galaxies. They came to be known as quasars, or QSO for Quasi-stellar objects. It was only in the 1990s that faint galaxies were discovered around quasars, although their presence had been suspected for some time. The quasars are the brightest members of a family of objects known as active galaxy nuclei, or AGN. AGN have been the subject of intensive research for decades, and many details of their structure, behaviour, and evolution, and especially their link to their host galaxies, remain under lively discussion. However, most people agree that the AGN are super-massive black holes (BH) in the centres of galaxies. It is possible that every galaxy has a BH in its centre. Even the Milky Way
Many galaxies have an active nucleus. Here, the giant elliptical galaxy M87 is seen to shoot out a radio jet right from its bright core. Image: The Hubble Heritage Team STScI/AURA) and NASA/ESA
Olwazini Discovery Centre is a world-class interactive centre which highlights many interesting aspects of social and natural science, mathematics, career guidance, technology and culture. It is situated under the pavilion at the Scottsville racecourse.The centre offers edutainment for grade 7-12 learners through interactive and interesting exhibits. If you want to be part of this edutainment, call us now on 033 395 8230 or email us on email@example.com. Open Monday to Friday from 9am - 4pm and its free!
d i s c o v e r y centre
Quest 5(1) 2009 27
Enrico Olivier and Patricia Whitelock show us the exciting world of stars.
t sunset hundreds upon thousands of twinkling lights come out to dazzle us. It is a familiar sight, and one that has evoked emotions and prompted questions for thousands of years. But only over the last few hundred years have we realised that each of these sparkling jewels is a furnace of unimaginable heat, like the one in our own backyard, the Sun. The Sun and other stars The Sun is the nearest star to us, being only about 150 million kilometres away. It consists mainly of very hot gas (74% hydrogen, 24% helium, and 2% heavier elements in mass) in which the electrons have been stripped from their atoms, forming a soup of energetic charged particles that is also known as a plasma. Its enormous size (its diameter is about 100 times that of the Earth) makes the Sun large enough to occupy the same volume as a million Earths. The temperature, which is a relatively cool 6 000°C at the Sun’s surface, increases towards its centre to an unimaginably hot 15 million °C. It contains more than 98% of the total mass in the entire solar system, and keeps all the planets, asteroids and comets in orbit around it through its gravitational pull on them. The average pressure in the surface layers of the Sun is a few hundredths of an atmosphere (one atmosphere is the typical pressure at sea level here on Earth). Moving inwards, the pressure increases to a staggering 340 billion atmospheres at the Sun’s centre, while the density there is only about 160 times that of water. The strong increase of pressure with depth inside the Sun is what prevents it from collapsing under its own gravity. If there were no pressure force, gravity would just collapse all the material to a very small volume, forming a black hole (see later). It is mainly the high temperature of the plasma that leads to this large pressure force. We can calculate what the central pressure and temperature of the Sun are using physical laws and the observed fact that the Sun is in balance (i.e. it shows almost no change in size or shape and its energy output has been
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Luminosity versus the temperature of stars. This is the so-called Hertzsprung-Russell diagram, after the two astronomers who first plotted the properties of stars in this way.
nearly constant over long periods of time). The surface layers of the Sun are in continual motion and change and dark spots, known as sunspots, can often be seen. Sunspots are cooler surface regions and have strong magnetic fields. They can be a few tens of thousands of kilometres across, comparable to the diameter of the Earth. They are not permanent features, but last anything from a few hours to a few months. The number of sunspots seen at any given time changes periodically on a time scale of about 11 years and is produced by a 22-year cycle in the Sun’s magnetic field. Very violent eruptive events occur near complex groups of sunspots. During a solar flare the temperature can rise to a few million degrees centigrade in a small region, and vast quantities of particles and radiation are blasted out into space. The most energetic solar flare is equivalent to the explosion of a 100 thousand billion megaton nuclear weapon. Even larger and more energetic events, called coronal mass ejections, also occur; these seem to be related to large-scale changes in the Sun’s magnetic field. From time to time a solar flare, or coronal mass ejection, is aimed in the direction of the Earth. The high energy charged particles ejected during such an event will reach the Earth a few days after leaving the Sun. This plasma poses a serious risk to
astronauts in orbit, can interfere with man-made satellites and can even disrupt electrical and communications equipment on the Earth’s surface. It is important to study the Sun as a typical example of a star, and such observations are helpful as a standard with which to compare other stars. Because it is nearby we can observe it in a detailed way that would be impossible for the distant stars. However, stars are not all alike. They all have different masses when they are formed, and this is the main reason for the large range of physical properties observed between different stars. The surfaces temperatures of stars range from under 3 000°C to over 30 000°C, and they have a corresponding range in colour from reddish-yellow (cool) to white-blue (hot). Their sizes range from around 0.5 to 1 000 times the size of the Sun and their energy output varies from 0.001 to 10 000 times that of the Sun. Observations of other stars also indicate the presence of star spots, flares and magnetic activity as seen in the Sun; the cooler types of star are particularly active in this way. Astronomers have a classification scheme for stars that has distinctly quirky names: dwarfs are stars very much like our Sun in the sense that they derive energy from the nuclear Luminosity is energy output per unit time.
Above: An example of a open cluster, the lovely Pleiades or Seven Sisters which is only about 400 light-years away. The striking blue reflection nebula associated with this cluster is also clear in this picture. Image: NASA; Tony Hallas Right: An image taken with the Southern African Large Telescope (SALT) of 47 Tuc (NGC 104), one of the largest and best-known globular clusters. The cluster is about 20, 000 light years away. Image: South African Astronomical Observatory Right below: Where do stars come from? This picture was taken with the Hubble Space Telescope using the Wide Field and Planetary Camera on board. It shows the giant pillars of gas seen in the Eagle Nebula (M16). At the end of the pillars, star forming regions known as EGGs, short for evaporating gaseous globules, can be seen. These EGGs are dense regions of mostly molecular hydrogen gas that fragment and gravitationally collapse to form stars. Light from the hottest and brightest new stars heats the end of the pillar which then causes further evaporation of gas, revealing yet more EGGs and more young stars. Image: J. Hester & P. Scowen (Arizona State U.), HST, NASA. Taken from Astronomy Picture of the Day
Over the last several decades astrophysicist have worked hard at trying to understand this variety among stars. This is the story of stellar evolution, the story of the life and death cycle of stars. The formation of stars Stars are often seen together in groups called clusters. Open clusters contain of the order of a few hundred to a thousand young and massive stars. Globular clusters are older and contain many more stars, mainly lowmass stars like the Sun. These pictures are of well-known open and globular clusters. Where do these stars come from? And why do they tend to be in groups? It turns out that the space between stars is not empty, but is filled with very low-density gas and dust, with the
fusion (see later) of hydrogen to helium. Dwarfs can have different masses, luminosities and surface temperatures. The more massive a dwarf, the more luminous it is and the higher its surface temperature. There are yellow dwarfs like our sun, and the cooler red dwarfs. Red giants are very large, luminous and cool (low surface temperature) stars. They are among the brightest of the low-mass stars in any galaxy. Blue supergiants are young, massive, luminous, large and very hot stars; typically the brightest among all the stars in a galaxy. White dwarfs are known to be very hot at the surface (up to 30 000°C typically), relatively small (about the size of the Earth) and very dense. One teaspoon of white dwarf material weighs about five tons!
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Right: A Hubble Space Telescope image of the very stunning planetary nebula called the Cat-Eye Nebula (NGC6543). The dying central star possibly produced the simple, outer pattern of dusty concentric shells by ejecting its outer layers in a series of regular convulsions. The formation of the beautiful, more complex inner structures is not well understood. Image: NASA, ESA, HEIC, and The Hubble Heritage Team (STScI/AURA). Taken from Astronomy Picture of the Day
occasional dense cloud, called a giant molecular cloud. These large clouds are enormous, with diameters ranging from 50 to 300 light years and masses between 100 thousand and 2 million times that of the Sun. Inside the clouds the temperatures are low enough for the constituent atoms to form molecules, hence the name molecular cloud. It is in regions such as these that we believe new stars are born. This image shows molecular cloud complexes in a region named the Eagle Nebula. Using special infrared cameras, light can be seen coming from newly forming stars, called protostars, that are inside very dense gas clumps. Theory shows that when a gas cloud is massive enough and cold enough, it can start to collapse under its own gravity. As a very large cloud of gas starts this collapse it will fragment into smaller clumps of gas, each in turn collapsing and fragmenting. Each of these small fragments will eventually collapse enough to start a new star, a protostar. This also explains why stars tend to form in groups, rather than in isolation. This whole process of collapse can be triggered when the cloud is compressed, for example by a supernova explosion (see later) or when it passes through a spiral arm in the Galaxy. As the protostar forms, it accretes mass from gas and dust that is falling inwards under the force of gravity. As its mass grows the pressure and temperature get so high that nuclear fusion will start in its centre. Nuclear fusion is the process whereby light elements fuse together to form heavier
elements, and the most important of these nuclear reactions is the fusion of hydrogen to produce helium. At this point the temperature and pressure inside the star become high enough to halt the star’s collapse under gravity, and the remaining gas and dust around the star are blown away by the pressure from the starlight itself. Once the fusion of hydrogen to helium starts the object ceases to be a protostar and we can say that a new star has been ‘born’. The humdrum life of a star The process of nuclear fusion in stars liberates huge amounts of energy; indeed it is the dominant source of energy during most of the life of a star and the reason why stars shine. For instance, a star like the Sun gives out the same amount of energy every second as 100 thousand million million tons of TNT (dynamite). Unfortunately this process cannot be duplicated, in a controlled manner, on Earth at the moment because of the extremely high pressures and temperatures needed. The Sun actually converts 700 million metric tons of its hydrogen to helium every
Properties of dwarf stars. Masses, radii and luminosities are in solar units. Surface temperatures are in Kelvin. Lifetimes are only a rough estimate and are in millions of years. For the very low-mass stars the lifetime estimates might be significantly too low.
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second to achieve this rate of energy output. The table below shows some basic properties of dwarf stars, that is stars fusing hydrogen in their cores. Stars cannot shine forever; eventually they will run out of nuclear fuel. A typical star will spend most of its life converting hydrogen into helium in its core (where it is hot and dense enough for fusion to occur). Interestingly, hydrogen will yield more energy via nuclear fusion than any other element and this is the reason that this process is so important. The Sun has already been fusing hydrogen to helium for almost 5 billion years, and will continue to do so for another 5 billion years. The more massive a star, the higher the pressure and temperature in its core, and the faster it will use up its fuel. So, contrary to expectation, stars that are more massive than the Sun will live much shorter lives, perhaps only a few tens to a hundred million years. As the hydrogen in the centre of the star runs out, the structure of the star begins to change. In particular the core contracts and heats up, generating even more energy and using up its hydrogen fuel even faster. In response to the now hotter core, the outer parts of the star expand. The star becomes larger, more luminous and its surface temperature drops so that it turns into a red giant. Eventually the central parts of the star will become hot enough for the next nuclear reaction to start: helium fuses into heavier elements such as carbon and oxygen. After some time, the helium in the core will also be used up, and the core then contracts further and heats up even more. From now on the fate of the star is determined by its mass. The hydrogen and helium nuclear fusion phases in a star’s life are the two longest stages. As mentioned previously, a dwarf is a star fusing hydrogen in its core, and this phase of evolution is by
far the longest in a star’s entire life. Thus most of the stars in any galaxy at any given time will be dwarfs.
When the fuel runs out Stars with masses less than about six to eight times the mass of the Sun don’t evolve much further after depleting the helium in their cores. This is because the temperature in their centres will never get high enough to fuse the newly formed carbon and oxygen into heavier elements. During the last phases of their life they develop large-scale oscillations in their outer layers and gradually shed their outer hydrogen layers into space. At some stage they may catastrophically eject their remaining outer layers. As these gaseous layers move away from the remnant core, they start to fluoresce due to the strong radiation coming from that central hot core, and a planetary nebula is formed. The photograph above shows an example of a planetary nebula. These beautiful stellar remnants come in a wide variety of different shapes, and astronomers are trying to understand why they have the particular shapes they do. The very dense and hot central core that has been left behind contains carbon and oxygen produced by the fusion of helium and will eventually become a white dwarf. More massive stars develop cores that are hot enough to fuse carbon to heavier elements, all the way up to iron in a sequence of fusion reactions. Once iron has been created no more energy can be liberated from the next possible nuclear reaction – that of fusing iron. In fact, energy needs to be added to fuse the iron to create heavier elements – this is a very dramatic time in the life of a massive star. At this point the core starts to collapse under its own gravity on a timescale of seconds. During the core collapse the
density reaches the same value as that found in atomic nuclei, and the material inside this core region is converted into neutrons. Eventually the collapse of the core stops, since gravity cannot overcome the extremely strong internal pressure of the neutron-rich core. The outer layers, in the meantime, also collapse down on to this very dense and rigid core, and then bounce back in a spectacular event called a supernova. During this event some of the kinetic energy of the collapse of the outer layers will be converted into light and a supernova can outshine, for a brief time, a whole galaxy with as many as 100 billion stars. The extremely dense remaining core of the star eventually cools down and becomes a neutron star. A pinhead of neutron star material would weigh the same as two of the world’s largest ships put together! For some very massive stars, not even the internal pressure of the neutron-rich core is strong enough to halt its gravitational collapse and the star becomes what is known as a black hole. The gravity is so strong around such an object, that nothing can escape, not even light! An example of one such very massive star in our own Galaxy is Eta Carinae. It is about 100 times more massive than our sun and it could explode at any moment, though no-one is sure as to exactly when. It is very unstable, showing lots of variation in its brightness. Observations made by South African astronomers helped prove that it is not actually a single star, but a binary (double star). Binary stars orbit each other due to their mutual gravitational attraction. During its orbit one star will obscure the other, and we will see a slight decrease in the total light coming from the system. This occurs about every 5.5 years in the case of Eta Carinae.
Top left: A false-colour image of the Crab Nebula, a supernova remnant. This image was created by combining data taken by space-based observatories, Chandra, Hubble, and Spitzer, to explore the debris cloud in X-rays (blue-purple), optical (green), and infrared (red) light. One of the most exotic objects known to modern astronomers, the Crab Pulsar, a neutron star spinning 30 times a second, is the bright spot near the picture’s centre. This collapsed remnant of the stellar core powers the Crab’s emission at all these different wavelengths. The Crab Nebula is about 12 light-years across, and about 6 500 light-years away. Image: NASA - X-ray: CXC, J.Hester (ASU) et al.;Optical: ESA, J.Hester and A.Loll (ASU); Infrared: JPL-Caltech, R.Gehrz (U. Minn). Taken from Astronomy Picture of the Day
Top: Eta Carinae is a star that may be about to explode. But no one is sure when. It may be next year, it may be one million years from now. Eta Carinae’s mass, estimated to be about 100 times that of our Sun, makes it a candidate for supernova. About 150 years ago Eta Carinae underwent an unusual outburst that made it one of the brightest stars in the southern sky. This image, taken in 1996, resulted from sophisticated image-processing procedures designed to bring out new details in the unusual nebula that surrounds this rogue star. Clearly visible are two distinct lobes, a hot central region, and strange radial streaks. The lobes are filled with lanes of gas and dust which absorb the blue and ultraviolet light emitted near the centre. The streaks remain unexplained. Image: J. Morse (Arizona State U.), K. Davidson (U. Minnesota) et al., WFPC2, HST, NASA. Taken from Astronomy Picture of the Day.
Above: This figure illustrates measurements of the infrared light from the star Eta Carinae taken at Sutherland. It shows how its energy output changed over the last 36 years. These particular observations helped to prove that it is a binary star and if you look very carefully you can see dips in the emission every 5.5 years (the binary period) that coincide with the dotted lines. The star is very unstable and shows many variations as well as the periodic changes of the binary.
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Why study stars? Most of the energy sources that we have here on Earth, and which are so crucial for life, derive from the energy (heat and light) that originated from nuclear fusion inside the Sun. This is only possible due to the Sun’s mass. This is large enough to generate the high temperatures and pressures that allow nuclear fusion to occur in its core, but not so large that the nuclear fuel burned up before life could develop and evolve here on Earth. But the Sun is not the only star we must thank for our existence. Most of the elements heavier than hydrogen and helium were manufactured in cauldrons of extreme heat and pressure at the centres of stars and ejected into space during the death throes of those stars. As one group of stars die they make it possible for an entirely new batch of stars to form. The enriched material is recycled into the interstellar gas, from where it will form not only new stars, but also planets, including planets like the Earth and everything that is found on or in those planets. These elements include the oxygen and carbon that are critical for human beings, the calcium in our bones, the iron in our blood and all the trace elements that make life possible. The main difference between one generation of stars and the next is the abundance of planetarymaking (and life-making) elements that very c_Science_Quest 2/12/09 12:15 PM Page 1 slowly increases from one generation to the next. So we see that our environment is critically dependent on stars and if we want
to understand how this environment came to be the way it is we must also understand the lives and deaths of stars. These are startling and significant facts that only came to light during the middle part of the 20th century. Indeed the application of basic physical laws to the study of astronomical objects has had an enormous impact on how we view our world. Much has also been learned about stars since the development of the electronic computer several decades ago. With the aid of such computers accurate physical models of stars can be made and compared with observations of real stars. Such comparisons show that the basic picture of stellar structure and evolution is essentially correct, although there are still many details we cannot explain. Much of stellar astrophysical research today involves understanding the latter phases of stellar evolution in more depth and detail, e.g. the transport of energy via convection, which so dominates the structure of the cooler stars, close pairs of stars that affect each other’s evolution via their gravitational interaction and the very low-mass stars that dominate the total stellar mass budget of any galaxy. Since much of the light we see in the universe today comes from stars, a better understanding of them is of importance to other areas of astrophysical research such as galactic and cosmological studies. C M Y CM MY CY It has been known for hundreds of years
that the light from some stars changes periodically on timescales between a fraction of a second up to several years. With the advent of modern observational instrumentation it has become clear that almost all stars seem to be variable, at least at some small level. Some of these variable stars are important as tools to determine distances to other galaxies, while the variations themselves give astronomers the opportunity to refine their understanding of the structure of the stars themselves. It is remarkable that the variations of stars can be used to probe their interiors in the same way as earthquakes can be used to study the interior of the Earth. The environments in and around stars are among the most extreme imaginable. Stars provide us with a laboratory to test the laws of physics under conditions that we could never reproduce here on Earth. For example, we have learned a great deal about the neutrino, a very important elementary particle produced during nuclear fusion, through studies of the Sun. ■ Enrico Oliver is an astronomer at the SAAO. His research interest is in stellar astrophysics and he is also involved in public science education and outreach. Patricia Whitelock is currently head of SAAO’s Astronomy Division and holds a joint position with UCT’s Astronomy Dept where she helps run NASSP, the National Astrophysics and Space Science Programme, which aims to train South Africa’s astronomers of the future. Her research CMY K interests cover the late stages of stellar evolution, galactic structure and the Local Group of galaxies.
GATEWAY TO YOUR FUTURE
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Into infinity Starwise: A beginner’s guide to the universe. By Anthony Fairall. (Cape Town. Struik. 2008) This slim volume contains a mind-boggling amount of information, put together by the late Professor Anthony Fairall. Beginning with an outline of the importance of South Africa in the astronomical world, Fairall describes the two types of telescopes that we have in South Africa; SALT – a giant optical telescope and MeerKAT – a radio telescope. As Fairall points out, the original peoples of southern Africa were familiar with the night sky even though they had no actual knowledge of the stars and the Universe around them. The Bantu people used the Moon and the changing night sky as a calendar and could tell the time of year from the star patterns that appeared in the early morning sky. Most of the brighter stars were named. However, fascinating as our astronomical heritage is, it is the research over the past 90 years or so that has revealed some of the secrets of the Universe, and it is these that Fairall explores in detail, starting with our own Earth and its Moon. The explanation of our atmosphere and the phenomena that we observe within and slightly beyond it provide a good introduction to the principles of astronomy. The Solar System is clearly set out – allowing an overview of the planets it contains and our place within this vast region of space. And then to the planets and what a wealth
of information is contained in a compact and accessible format. Fairall takes us through the characteristics of each planet and the research that has been going on over the years, including ongoing projects such as the Cassini spacecraft that took six-and-a-half years to reach Saturn and is still orbiting the ringed planet. The Sun, our nearest star and the reason why life exists on Earth, is shown in scale against the size of Earth – emphasising our relative insignificance in the general scheme of things! The Sun’s energy is described in detail, but provides a concise explanation of the reactions within the star that provide heat and light. The rest of the book outlines the major discoveries in astronomy over the past few decades, using photographs from distant space taken by, for example, the Hubble telescope – showing the enormity and awesome extent of our Universe and its contents. By the time you finish this beautifully put together and illustrated book, you will have a very good understanding of the principles of modern astronomy and an overview of our Universe and its contents. A must for every budding star-gazer. Anthony Fairall was Professor of Astronomy at the University of Cape Town and Planetarium Associate at Iziko Museums of Cape Town. His research career spanned more than 40 years, during which time he produced some 200 research papers and authored several books. He was an Associate of the Royal Astronomical Society (UK) and a Fellow of the University of Cape Town and the International Planetarium Society.
Each topic starts with a short introduction to the subject and includes a question and answer box, extra bits and pieces of knowledge, and numbered facts about each topic. As an example, Plants and the Food Chain introduces the subject by explaining that plants and animals are linked together in their ecosystems – easily fitting the ecosystem concept into the page – illustrates a food chain, talks about the unusual ice plant that collects dew and uses questions and answers to introduce the concept of ‘producers’. The section on Natural Disasters is an interesting one to include in a book of facts about nature and will provide plenty of fun facts, such as ‘which was the deadliest earthquake?’, ‘how do we measure storms?’ and ‘what does “epidemic” actually mean?’ The book finishes with three games, all played by questions and answers. They can be played alone or by two or more people taking turns by rolling a die. The answers are given at the end of the book, along with a comprehensive glossary of terms used in the book. Although the format of the book makes it appear to be aimed at younger children, the sheer volume of information it contains should make it a useful resource for most school grades.
Just about all you need to know about nature 365 Awesome facts and records about Nature. (Cape Town. Struik. 2008) This book could be used like a calendar – a fact for every day of the year. But it is so packed with information, all colourfully illustrated, that I will guarantee that it is more likely to be read much faster than that. The book is divided into five major sections and each double page spread then digs deeper into a particular subject, to explore it in more depth. Starting with Origins, the book takes you through The Plant Kingdom, The Animal Kingdom, Animal Lives and Natural Disasters.
MIND-BOGGLING MATHS PUZZLE FOR Q UEST READERS Q UEST Maths Puzzle no. 9
Win a prize!
The following multiplication problem uses all the digits from 0 to 9 once and once only. Fill in the missing digits to complete the problem. One digit has been filled in to get you started. --- x –5=----(3 Digit number) x (2 digit number ending in 5) = 5 digit number [might be useful to use blocks instead of the hyphens].
Send us your answer (fax, e-mail or snail-mail) together with your name and contact details by 15:00 on Friday, 8 May 2009. The first correct entry that we open will be the lucky winner. We’ll send you a cool Truly Scientific calculator! Mark your answer “QUEST Maths Puzzle no. 9” and send it to: QUEST Maths Puzzle, Living Maths, P.O. Box 478, Green Point 8051. Fax: 0866 710 953. E-mail: firstname.lastname@example.org. For more on Living Maths, phone (083) 308 3883 and visit www.livingmaths.com.
Solution to Q UEST Maths Puzzle no. 8 The two solutions are: 6÷((5÷4)-1) and 4÷ (1-(5÷6)).
Quest 5(1) 2009 33
Big eyes on the skies David Buckley takes us on a tour around the world of telescopes.
hen SALT – the Southern African Large Telescope – was completed in 2005, it joined the big league of the largest telescopes ever built. Today there are dozen or so ‘large telescopes’ ranging from 8 to 10 m in diameter, while future plans are afoot to build several ‘extremely large’ (approximately 20 to 40 m) telescopes. The first of the giants: the Keck telescopes In 1993 the first of the 9.8-m twin Keck telescopes was completed on the island of Hawaii, atop the 4 200 m dormant volcanic peak of Mauna Kea. This became the largest telescope on the planet, increasing the lightcollecting power by a factor of three to four times over the previous largest telescope, the 6-m BTA telescope in the Russian Caucasus Mountains. For about 30 years before the BTA, the ‘world’s largest’ tag belonged to the 5-m Hale telescope on Mt Palomar, in California, completed in 1948. Just as the Hale telescope was a challenging technical feat in the 1940s, so too were the Keck telescopes. For the first time a telescope’s mirror was not made from a single monolithic piece of glass, but rather a mosaic of 36 individual hexagonal mirrors, held in precise alignment. This innovative segmented mirror design allowed astronomers to create a telescope mirror far bigger than anything that could ever be cast in a glass furnace. The success of the first of the Keck telescopes led to funding of its twin In astronomy, an interferometer is an array of telescopes, or dishes for radio telescopes, that act together with higher resolution than a single telescope or dish.
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– Keck II – which began operations in 1996, and has led to other large segmented mirror telescope being built and plans for the next generation of extremely large telescopes (ELTs). Both Keck telescopes stand next to each other and are now linked by an ingenious system of moving mirrors on mini-railway tracks, which allows the telescope to be used as an interferometer, essentially mimicking a telescope with an effective resolution of an 85-m telescope – the distance separating the two telescopes – for determining the sizes of stars. The superb optical quality of the telescope, coupled with the excellent observing conditions of the site, has allowed the Keck Observatory to exploit a technique known as adaptive optics, originally developed by the US military during the ‘Star Wars’ era for ground reconnaissance (i.e. spy satellites) and increasingly used in modern astronomical telescopes. This corrects for the blurring effects of the Earth’s atmosphere, caused by air motion and turbulence, allowing the telescopes to obtain images hundreds of times sharper and more detailed than previously possible. The enormous light-gathering power of the Keck telescopes and the additional use of adaptive optics have allowed astronomers to observe some of the faintest and most distant objects in the Universe. These small, faint, blue galaxies were the first assemblies of glowing matter to condense after the Big Bang, which happened about 14 billion years ago. They have provided clues about the early conditions of the Universe, which we are familiar with, now populated by hundred of billions of galaxies each
containing a similar number of stars, but back at a time when the Universe was only about 10% of its current size and age – barely a billion years old. Other important discoveries made by the Keck Observatory have followed from observing the most violent explosions in the cosmos: massive stars that rip themselves apart called supernovae, and stars that collapse and merge catastrophically, probably into black holes, called gamma ray bursters. These violent explosions had been observed for decades by gamma ray satellites, but it was only after Keck’s observations of their faint optical ‘after-glows’ that they were realised to be extremely distant, rather than within our own galaxy, and therefore incredibly energetic. The observations of the supernovae have resulted in probably the most amazing discovery in astronomy in the last decade or so: dark energy. These distant supernovae showed that not only was the Universe expanding – which was known from before the time the Hale telescope started operating in the 1940s – but that the expansion was accelerating. This accelerated expansion is driven by an unseen dark energy, which in fact fills about 70% of the Universe, and astronomers still have little idea what it is! Finally, over the 16 years since the first of the Keck telescopes began operating, astronomers have discovered over 470 planets orbiting other stars: extrasolar planets. Many have been discovered using instrumentation on the Keck telescopes by accurately measuring the wobble of the parent stars produced by the gravitational influence of their planets. The quest is
now on for discovering systems like our own Solar System and planets like our Earth.
Above left (opposite page): The twin Keck telescopes on the summit of Mauna Kea, Hawaii. Image: NASA
Joining the ‘8-m Club’: the Subaru and Gemini Telescopes At the end of 2000, some two years after the completion of the first telescope of the VLT, the 8.3-m Subaru telescope of the National Astronomical Observatory of Japan (NAOJ) began operating on Mauna Kea. This is one of the most expensive single telescopes ever built and features an array of seven instruments together with an adaptive optics system. Unusual for 8–10-m telescopes, it also has a wider field imaging system, which has allowed astronomers to conduct deep and wide surveys for faint galaxies. Also constructed on Mauna Kea, the first of the pair of 8.1-m Gemini telescopes began science operations in the same year as Subaru. The telescope, known as the Frederick C. Gillett Gemini North telescope, was joined about a year later by its southern hemisphere twin, Gemini South, situated in Chile. Both telescopes are funded by a consortium of countries: the USA, Canada, UK, Argentina, Brazil and Australia. Like VLT and Subaru, both telescopes use meniscus mirrors made from a low expansion glass, but unlike the others, the mirrors are coated with a protected silver coating, rather than the usual aluminium. The reason for this was the better performance achieved in the infrared part of the spectrum using silver, which is fully exploited by the three to four instruments currently available on the telescopes. In addition, both telescopes also use adaptive optics, which are optimal for observations at the longer infrared wavelengths.
Above right: The 8.3-m Subaru telescope of the National Astronomical Observatory of Japan in Hawaii.
Above right (opposite page): Three of the VLT telescopes in the turret-shaped enclosures plus the two domes of the VLT interferometer. Image: Wikipedia commons Above left: The Gemini North telescope inside its dome. Image: Gemini Observatory/Association of Universities for Research in Astronomy
Image: Subaru Telescope, National Astronomical Observatory of Japan (NAOJ)
Bangs for your buck: the HET and SALT Following on the proven success of the Keck telescopes, which pioneered the use of segmented mirrors, an innovative and cost-saving telescope design was conceived for a telescope built in Texas, named the HobbyEberly Telescope (HET), built by a consortium of US and German institutions. Although the HET primary mirror was the same size as Keck, the cost-saving design meant that the telescope cost only about a quarter of Keck I. The reason for this was a trade-off between performance and cost, and the use of clever engineering. Unlike a conventional telescope, the HET design makes use of a fixed steel support structure, which does not change with respect to gravity, simplifying the engineering considerably. Likewise, the mirrors’ surfaces are polished into a simpler spherical shape, rather than the usual, but more difficult, parabolic figure, making them cheaper to produce. All of this comes at a cost, however, namely the inability to observe individual objects for more than an hour or two at a time during a given night. However, this restriction is not so great as it might seem and in fact lends itself to a more flexible observing model, whereby many different targets from many different observing programmes are observed in a single night. Similarly, regular observations at varying time intervals (days, weeks, months) can easily be
The first southern giant: the Very Large Telescope In 1998 the first of four 8.2-m telescopes comprising the Very Large Telescope (VLT) began operating in Paranal, Chile, a high and dry region in the Atacama desert. Operated by the European Southern Observatory (ESO), which comprises partners from most of Europe, these four telescopes (the last was completed in 2001) now boast an impressive suite of 11 instruments and include adaptive optics capabilities and an interferometer. The latter – VLTI – is the first generally available optical/ infrared facility interferometer, and will allow astronomers unprecedented sensitivity, using all four 8.2-m telescopes (making them equivalent to a single 16.4-m telescope), plus four smaller 1.8-m telescopes. The VLT telescopes use circular monolithic meniscus mirrors made from a low expansion glass ceramic and are much thinner, relative to their diameter, than the plethora of 3–4-m telescope mirrors produced in the 1960s to 1980s. Because of this the mirrors have to be well supported on a system of small actuating ‘pistons’, which can push and pull on the back of the mirror, changing its shape slightly. This is necessary to compensate for both temperature and gravity changes experienced by the mirrors during operation, a system known as active optics. This was pioneered on another ESO telescope in the 1980s, the 3.5-m ‘technology demonstrator’ for the VLT, known as the New Technology Telescope (NTT). Active optics is now routinely applied in all modern telescopes. Many scientific results have followed from VLT observations throughout all the fields of astronomy. One of
the most significant was observations of the centre of our Milky Way, where star positions were seen to change significantly over a period of several years. This led to the positive identification of a massive black hole at the centre of our Galaxy.
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Above left: The Hobby-Eberly Telescope (HET) in West Texas. Image: PennState University Above: The Large Binocular Telescope in Arizona, featuring twin 8.4-m mirrors. Image: Large Binocular Telescope Corporation
Left: The primary mirror array of SALT, consisting of 91 hexagonal 1.2-m mirrors. Image: SALT
catered for, which is more difficult for conventionally scheduled telescopes. This is all achieved by adopting a queue scheduled service observing mode, which avoids the necessity of individual astronomers having to travel to the telescope to conduct their observations. This model is in fact starting to be increasingly used at other telescopes too, which both reduces travel costs and has positive carbon credits. HET began science operations in 1997, reaching full maturity some three years later. By the end of 1998, South Africa had decided to duplicate the HET design for SALT – the Southern African Large Telescope, to be built for its partners from South Africa, Poland,
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US, Germany, UK and New Zealand, and to be joined by an Indian partner in 2008. Construction of SALT began in 2000 and the telescope was inaugurated in November 2005 following completion of the major construction phase. Technology advances and innovations, coupled with the lessons learned from the pioneering HET design, meant that most aspects of SALT were re-optimised, resulting in a more capable telescope with a wider field of view and enhanced sensitivity at shorter wavelengths, down to the ultraviolet cut-off of the Earth’s atmosphere (sunburn territory!). Since 2006 SALT has been undergoing commissioning, together with some science observing, and is expected to be fully operational later in 2009.
The newest and the biggest: LBT and GTC The most recently completed large telescope, which is still undergoing commissioning, is the Large Binocular Telescope (LBT), built by a US, German and Italian consortium in Arizona. This strange looking telescope, as the name suggests, is actually two 8.4-m telescopes, mounted side by side on a single enormous steel structure. Light from each telescope can either be sent to separate instruments, or combined together, effectively creating a single 11.8-m telescope – the largest ever built. In addition, they will eventually be used as an interferometer, giving an effective resolution of a 22.8-m telescope. LBT features a third type of mirror technology, namely spun cast lightweighted glass mirrors. These were created using a novel approach whereby the entire furnace used to melt the glass is rotated slowly to produce a natural parabolic shape. Relatively cheap borosilicate glass is used, but the mirror is built in a honeycomb structure, which allows it to be actively ventilated to ensure a uniform temperature during operation. This technology was pioneered in earlier smaller telescopes, such as the twin 6.5-m Magellan telescopes, built in Chile, and another Arizona telescope, the 6.5-m MMT. The next telescope to join the big league will be the Gran Telescopio Canarias (GTC), or Grantecan, a Spanish built 10.4-m segmented mirror telescope, very similar to the Kecks. This has been built on the island of La Palma, off the coast of Morocco. It achieved ‘first light’ in 2007 and is expected to be fully operational soon.
The next generation of giants: TMT, E-ELT and GMT While the lifetimes of the 8–10-m large telescope will last many decades, astronomers’ seemingly insatiable appetite for bigger telescopes continues. Driving these desires are the huge astronomy questions of today: unravelling the mysteries of dark matter and dark energy – the unseen and seemingly unknowable 95% of the Universe, and understanding the beginnings of the Big Bang; ever more detailed studies of galaxies and their constituent stars; the study of exoplanets and particularly the search for Earth-like planets and, with it, surely the biggest prize of all – finding evidence of life elsewhere in the Universe. To answer these questions requires telescopes of even greater light-gathering power than currently available in the 8–10-m club. So astronomers and engineers are developing designs and raising funding to build several Extremely Large Telescopes (ELTs), telescopes that are 20-, 30- or 40-m in diameter, eventually maybe even larger, with 10 to 20 times the collecting area of a 10-m telescope. This means being able to see things equivalently fainter and further away. Two groups – one in Europe and one in the USA – are developing ELTs based on segmented mirrors, with 500 to 1 000 of them. A North American consortium from California and Canada are building the Thirty Meter Telescope (TMT), while the Europeans, through ESO, are building the European Extremely Large Telescope (E-ELT). Both are midway through their design phases and neither have yet decided on a final site selection, although both Hawaii and Chile are likely choices. A third ELT is being built by a predominantly US consortium, but also with Australian and Korean participation, called the Giant Magellan Telescope (GMT), to be built in Chile. Unlike the other two ELTs, GMT will not be a massively segmented
telescope, but will consist of just seven 8.4-m mirrors, spun cast just like the ones used in the LBT pair. All of the telescopes will take years to construct, with completion expected in the latter part of the next decade, and will cost anything from around $700 million to more than $1 billion. Finale In 2008 astronomers celebrated the 400th anniversary of the invention of the telescope and this year will celebrate the first documented astronomical observations conducted with a telescope, by Galileo Galilei in July 1609. Since those observations we have seen an increase in collecting power of the telescope by a factor of over 100 000! There are now 13 telescopes larger than 8-m in diameter, including South Africa’s own SALT, which is the single largest in the southern hemisphere. In the decade or so to come, the world will see the completion of the next generation of giants – the ELTs – whose impact on the science of astronomy can only be imagined. ■ David Buckley is SALT Project Scientist and Astronomy Operations Manager.
Top left: The Gran Telescopio Canarias (GTC) during construction on La Palma in the Canary Islands. Image: GTC Top right: Artist’s impression of the Thirty Meter Telescope (TMT). Image: TMT Observatory Corporation Top: Conceptual design of the European Extremely Large Telescope (E-ELT) with an Airbus A340 for scale. Image: ESO Above: What started it all: a replica of Galileo’s original telescope used for the first recorded astronomical observations in 1609, compared with a modern telescope. Image: Sheffield University
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Right: In this rock engraving, the lion has its tail wrapped around a disc, which is interpreted as the Sun. Image: Anne Rogers/Cosmic Africa SA
Thebe Medupe and Themba Matamela look at African astronomy.
When a crocodile eats the Sun: indigenous astronomy I
f the first human lived in Africa, then the first astronomer must have been an African. You do not have to live too far from the cities to notice how beautiful the clear moonless night sky is. When you do go to the country you can see just how impossible it is that any culture of the world would not have taken a keen interest in the night sky. But, more seriously, Africa has the longest known records of human activities related to the stars, the Sun and the Moon. Earliest records Rock engravings by the San
The symbolism seen in some rock paintings of the San people record astronomical events. The earliest records may be found in the San rock paintings throughout southern Africa. The San people are Africa’s oldest modern humans, with a history going back several thousands of years. Some of their rock paintings can be interpreted to represent a depiction of astronomical events. The most notable is the engraving of a lion with a long tail that is wrapped around a disc. This is interpreted as being the disc of the Sun. This rock art suggests that the San people understood a solar eclipse as an unnatural event that was caused by evil lions wrapping their tails around the Sun. Rock alignments
The rock alignments at Nabta Playa in the south of Egypt are probably the oldest known rock alignments to the Sun and stars. They predate Stonehenge, an impressive alignment of rocks in England, by about 1 000 years and were erected long before the first pyramids were built in ancient Egypt. The first one is not very large, about three metres in diameter, and thus smaller than Stonehenge. This small arrangement of rocks were aligned both to the rising and setting
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A solstice is an astronomical event that occurs twice a year, when the tilt of the Earth’s axis is most orientated away from or towards the Sun, which causes the Sun to reach its northernmost or southernmost extreme. An equinox occurs twice a year, when the tilt of the Earth’s axis is such that the Sun is directly above the equator.
points of the Sun during the summer solstice about 6 900 years ago. The rocks are aligned in the direction of the points at which the Sun rises along the eastern horizon each day of the year. These rising (and setting) points shift from south-east, through due east and turn back towards the north-east. A year is defined as the time it takes for the Sun to rise in the south-east, move to the north-east and then back to its original rising position. There is a second alignment at Nabta Playa which is much bigger than this and consists of large rocks that radiate from a common centre and are aligned to different stars, Arcturus, Sirius and one of the stars of Orion’s belt. Rock alignments are also found at Namoratunga, Kenya. These are thought to be about 2 000 years old, but there is much debate about whether or not these rocks had an astronomical purpose. African cultural astronomy African societies were enthralled by the beauty of the night sky. They were inspired (like people in other continents) to form constellations out of groups of stars that appear to form a pattern. For example, the ancient Greeks thought that the Orion constellations looked like a hunter with a sword. In southern Africa, the Namaqua people saw different patterns
in the same group of stars: the three zebras (which are the stars of the belt of Orion), the lion (Betelgeuse), and the arrow (The sword of Orion). Next to Orion, there is the Pleiades star cluster which, according to the Namaquas, is a group of the daughters of the sky god. Aldebaran stands between the zebras and his wives (the daughters of the sky god). The story that is told is that Aldebaran, the hunter, wanted to shoot the zebras to bring meat to his wives. But he was a bad hunter and missed the zebras. He was too scared of the lion (Betelgeuse) to fetch his arrow, and too embarrassed to go back to his wives because he had no meat to bring them. But it is not just stories that drew African peoples to the skies. They used the stars and the planets in their daily lives, basing their calendar on the changing phases of the Moon. The length of their month was given by the number of days between one new moon and the next. This is approximately 29.5 days long, which means that the African calendar was 29.5 x 12 = 354 days long. It is short by 11 days when compared with the modern calendar, which is based on the movement of the Earth around the Sun. However, a problem with the 11 missing days is that it means that events occur 11 days earlier in each year. For example, after three years an event that took place in December three years ago, would now take place in November! To avoid this, southern African people added an additional month after every three years. So, some years were 12 months long, and others were 13 months long. The Sesotho language uses the same word for month and for moon, reinforcing the idea that the southern African calendar was based on the phases of the Moon.
Right: An ancient manuscript showing how early astronomers used the stars to determine latitude and longitude. Image: Courtesy of Timbuktu Science Project, Astronomy Department, UCT
African astronomy during medieval times
A common misunderstanding is that the history of Africa is purely an oral history, with written records only starting at the time of arrival of colonialism of the continent. However, we now know that over the last 1 500 years, there was regular contact between several regions of the continent and several other countries, involving knowledge sharing and written records. For example, the Swahili language has a written record spanning 800 years. Even more impressive has been the exchange of knowledge between West Africa and the Islamic world that probably started over a 1 000 years ago when Muslim North Africa started trading with West Africa during the Ghana empire. The Ghana empire covered parts of Mauritania, Mali and Senegal and collapsed in the 11th century. It was during the Ghana empire that the culture of learning was established in West Africa. It is largely from this that cities such as Timbuktu (in the north of the Mali republic) benefited from the later movement of scholars. We do not know how far back astronomy was studied in Timbuktu, but there are astronomical documents that were definitely written by black West Africans in the early 1700s. One such scholar, Mohammed Baghayogo Gurdu, mentions that the teacher of Ahmed Baba (the most celebrated of the 16th century scholars from Timbuktu) studied under a Lybian astronomer in Egypt in the 16th century. It is clear that by searching for more manuscripts, we may be able to extend the times for astronomy scholarship in Timbuktu. West African scholars used astronomy to measure time during the day and at night by using the shadows cast by a gnomon to determine times for prayer during the day. A gnomen is a stick of specified length that is put into the ground to cast a shadow. The length of the shadow is used to mark time.
The scholars also used lunar mansions at night to mark the passing of time. A lunar mansion is one of the 28 divisions of the sky that is marked by the stars in that division. These divisions are called asterisms, which is a pattern of stars seen in the sky that is not an official constellation. Astronomers in Timbuktu also used the Muslim calendar, which is based on the phases of the moon. To find the direction of Mecca, they used Polaris to measure their latitude, and the lunar eclipse to measure their longitude. The altitude of Polaris above the local horizon directly measures your latitude. The timing of the lunar eclipse
observed by two different observers in different longitudes gives the difference in longitude of the two observers. This difference can be used to determine the direction of Mecca. Abul Abbas, one of the astronomers of Timbuktu of the 1700s writes: ‘It (Astronomy) is useful for five things: - people’s guidance in and off sea. As God said: ... so that you might guide yourselves by them in and off the sea. – Knowing estimation of the years, ... – Decorating sky nearest to us. God said: we decorated the nearest sky with stars. – One of the uses of this science is knowing prayer times … – So that the angels can hurl them at the devil’ In other words, astronomy was useful for guiding people on land and on sea, determination of a calendar, decorating the night sky, determining times for prayer, and quite interestingly, some stars are used for throwing at devils. This, of course, refers to shooting stars. Early African astronomers borrowed ancient Greek notions of the Earth-centered universe, with nine orbits. Because they did not have telescopes, they could see only five planets (Mercury, Venus, Mars, Jupiter and Saturn). Here is a quote from one of the 18th century manuscripts that describes such a model: ‘… He said that the orbits that God created in Heavens are nine. Seven of them bear planets. The eighth bears some other stars. The ninth is devoid of planets. The illustration of that is; the moon is in the orbit next to us. There is no other planet next to us if not the moon, except those stars that are said to be destined to hurl the Devil with. The next one is bigger it contains Mercury. He hinted here that an orbit is bigger than the one below it and smaller than that of the planet above it. The third orbit bears Venus.The fourth bears the Sun. The fifth bears Mars. The sixth bears Jupiter. The seventh bears Saturn. The eighth bears other planets rather than these ones. The ninth is devoid of planets. This means that there is no planet there but the eight planets share it. Every one of them rotates round it once every day and night with the might of God. The orbit is where a planet makes a round voyage. God said every one of them swims in an orbit. We are in its center. The orbits are layered on each other like onion shells.’
A page from one of the older manuscripts in a private library in Timbuktu. It shows the orbits of the Sun (the red dot) around the Earth and the orbits of Mercury (black dot).
This quote comes from the scholar Abul Abbas in 1732. This article is only a brief outline of the extent of astronomy in Africa. Further research into the Timbuktu manuscripts will reveal exactly how extensively astronomy was taught at local schools in ancient times. ■ Dr Thebe Medupe is currently a senior lecturer in the Astronomy Department of the University of Cape Town, and is a researcher at the South African Astronomical Observatory. He heads a project to search for astronomy content of the Timbuktu manuscripts. The project is funded by the Department of Science and Technology through the National Research Foundation. Themba Matomela studied astronomy at UCT and is currently an outreach educator at the Iziko Planetarium.
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MeerKAT vision. Image: Jeroen de Boer
MeerK AT coming on t rack during IYA 2009 South Africa is one of the countries that has put in a bid to host the Square Kilometre Array.
he International Year of Astronomy 2009 has special significance for South Africa’s Northern Cape Province, where construction on the Karoo Array Telescope, known as ‘MeerKAT’, has started. A team of engineers and radio astronomers have been developing, testing and fine-tuning the design and layout of this high-precision radio telescope over the last few years. In 2007 the team built a single dish (antenna) at the Hartebeesthoek Radio Astronomy Observatory to test a new kind of dish based on composite materials instead of steel, and to test the design. The single dish is also being used to test all the other components required to operate a radio telescope (the receivers, the software and the digital back-end). Now phase one of MeerKAT – the construction of the first seven dishes (called KAT-7) – is underway on site in the Karoo. The civil engineering team is working hard to complete all the road construction, power lines, workshops and staff accommodation at the site. They are also building a large shed where the dishes will be assembled and they are busy digging trenches for onsite reticulation (power and optic fibre). The optic fibre to Cape Town will be part of the Infraco backbone. The team plans to have the first KAT7 dish on site by mid April 2009 and all seven dishes on site by December 2009. MeerKAT will eventually consist of up to
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SKA South Africa team, Johannesburg office, with project director Dr Bernie Fanaroff in the middle, front row.
The SKA South African engineering team, based in Cape Town.
80 dishes, each 12 m in diameter. The telescope will cost about R1 billion and should be operational by 2012. Aiming for the SKA MeerKAT is part of a hugely more ambitious astronomy project - South Africa’s bid to host the Square Kilometre Array (SKA). The SKA will be a radio telescope so powerful that it will be able to look back in time to the Big Bang. South Africa is competing against Australia for the SKA. The world’s radio astronomers have to
consider the competitive bids from South Africa and Australia and should announce their final decision on where to locate the SKA by 2011. ‘Our site is as good as the Australian site and better in many respects,’ says Dr Bernie Fanaroff, the Project Director. ‘Local construction and infrastructure costs are less than those in Australia.’ In the mean time, MeerKAT serves as a ‘pathfinder’ for the cutting-edge technologies of the SKA. ‘It will prove that South Africa can deliver on a high-tech,
Q Science news The phased array at the heart of the Square Kilometre Array (artist’s impression).
mega instrument and that we have the capability, professionalism, capacity and track record to operate and maintain it optimally’ adds Dr Fanaroff. Balancing cost and cutting-edge technologies According to Anita Loots, one of SKA South Africa’s Associate Directors, the major challenge of the SKA and MeerKAT projects is the balance between generating cutting-edge technologies on the one hand, and the need to keep costs down on the other. ‘We have to develop ultra-fast signal and data processing capacity, combined with sophisticated computing, data mining and archiving abilities, while we also need the technologies to build very sensitive receiver antenna systems, including lownoise amplifiers,’ she explains. ‘However, if costs spiral out of control, it will undermine the feasibility of the telescope itself. This means the engineers must find creative solutions to make the systems more affordable.’ Loots explains that South Africa’s contribution to the SKA design focuses on a number of key areas. Local engineers are working on the antenna design and the materials necessary to construct the antennae. They are also looking at digital signal processing and the development of next-generation correlators that are scalable and flexible, able to process data extremely fast and also store huge amounts of data. South Africa has been welcomed into the international SKA community, based on our excellent track record of delivering top scientists and engineers for the project over the last two to three years, according to Loots. ‘This clearly shows that South African engineering training competes with the best
in the world,’ she adds. ‘Our newly qualified engineers have no problem in joining and even leading SKA and MeerKAT teams.’ People skills to power SKA and MeerKAT The SKA South Africa Project, including the building of MeerKAT, is one of the biggest science and engineering projects currently in South Africa. It presents an incredible variety of challenges and opportunities and requires skills across a wide range of disciplines such as digital signal processing, RF engineering, antenna design and software development. That is why the South African SKA Project is supported by a targeted ‘Youth into Science and Engineering Programme’ to develop highly skilled young scientists and engineers, explains Kim de Boer, who leads this capacity building programme from the SKA project office in Johannesburg. ‘The young people supported by this programme will serve South Africa, and our African partner countries, in key areas of economic development,’ she adds. The programme offers comprehensive bursaries to students in engineering, mathematics, physics and astronomy at undergraduate and postgraduate level. Bursary students benefit from regular workshops and student conferences where they interact with the world’s leading astronomers. To date more than 80 postgraduate students from South Africa and the rest of the African continent and 36 undergraduate students are studying or have studied with SKA bursaries and are on their way to being a part of South Africa’s exciting future in radio astronomy. Find out more at www.ska.ac.za/ studentsupport/
Protecting radio astronomy’s future South Africa’s new Astronomy Geographic Advantage Act will declare Astronomy Advantage Areas for both radio and optical astronomy. According to the Act, at least 12.5 million hectares around the proposed SKA core area can now be protected from future radio frequency interference. ■ About the SKA Following an intensive bidding process, South Africa and Australia are now the only two countries on the shortlist as a possible location for the SKA. The international radio astronomy community will announce their decision on where to build this powerful telescope in 2011. If South Africa wins the SKA bid, the core of this giant telescope will be constructed in the Karoo region of the Northern Cape Province. However, the SKA is so huge that outlying stations will be spread over several countries, including Namibia, Botswana, Mozambique, Zambia, Mauritius, Madagascar, Kenya and Ghana. To obtain the required sensitivity and resolution for the SKA approximately 4 000 antennae will be spread over 3 000 km. The antennae will be grouped in stations of about 30 to 40 each. The combined collecting area of all these antennae will add up to one square kilometre. At about 50 times more sensitive than any other facility on Earth, the SKA will be powerful enough to explore the origins of galaxies and probe the edges of our Universe. It will search for Earth-like planets and potential life elsewhere in the Universe, test theories of gravity and examine the mystery of dark energy. The SKA instrument is estimated to cost about 1.5 billion euro (about R20 billion). Construction is likely to start in 2014 and will take place in phases over several years. Find out more at www.ska.ac.za
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The physic s behind
ast rophysics Lisa Crause, Kevin Govender and Nicola Loaring explain that physics is not just a dry, academic subject, but the exciting backbone of astrophysics.
he laws of physics that we have uncovered so far appear to hold throughout the Universe and are believed to apply for all past and future time. This means we have an incredibly powerful tool-kit at our disposal when we set out to explore the world – whether we choose to focus on a drop of water or on a cluster of galaxies. Looking around we can see physics everywhere – often hidden behind other names like biology, chemistry, geology, engineering, technology ... But let’s think about what’s really going on, fundamentally, and marvel at how it all comes down to physics! alpha particle
repulsive electromagnetic force attractive nuclear force gamma ray
Above: The electromagnetic force causes like charges to repel, more so the closer they are to one another. Given the concentration of positive charges within the incredibly tiny volume of an atomic nucleus, it is clear that some kind of attractive force must overcome this repulsion in order to hold the nuclei together. This is known as the strong nuclear force.
What is physics? Physics itself can be broken down into four fundamental interactions. In order of increasing strength, they are gravitation, the weak force, the electromagnetic force and the strong force. Gravity has infinite range and acts on all matter, while the weak force is responsible for radioactive decay and neutrino interactions. The electromagnetic force governs the behaviour of charged particles and hence describes most macroscopic phenomena and determines the atomic-level properties of elements, while the strong force holds atomic nuclei together. One of the main reasons for astronomy’s enormous popular appeal is that it showcases physics on the ‘grandest’ scale. Phenomena and processes that we are familiar with on Earth are always hugely more extreme, and hence more mindboggling, when encountered in the context of astronomy. Temperatures, pressures, magnetic fields, velocities, densities – in the science of astronomy, all of these may be many orders of magnitude larger, or sometimes smaller, than those experienced on Earth. Almost more remarkable is the fact that we can actually make measurements and infer this kind of information, given that in astronomy we never have direct access to our experiments. We can only collect and analyse the trickle of light that
The four fundamental forces in Nature Interaction
W & Z bosons
10-18 Infinite 10-15
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finally reaches us from impossibly distant sources. To do this, we again rely on physics – to design and build the types of instruments we need to detect the signals, and then also to interrogate the data, interpret the results and refine our theoretical models to ultimately advance our understanding. The light we refer to is not just the visible sort that our eyes are optimised to detect, but also all the other types of electromagnetic radiation that astronomical objects emit across the entire spectrum. We encounter these other forms in every aspect of daily life – ranging from the low frequency transmissions used to broadcast radio, television and cellphone signals, to microwave ovens, to infrared/heat radiation, to the ultraviolet rays that cause sunburn, to airport security and medical applications of X-rays and gamma rays, including diagnostic imaging and radiation therapy to name just a few. The physics of an evolving star To compare the manifestations of physics in ordinary and astrophysical situations, we briefly consider some of the physical principles that govern the evolution of a massive star – from its gravitationally induced birth inside an enormous cloud of molecular hydrogen, through to the extraordinary supernova remnant known as a neutron star. Descriptions of the individual stages could each fill this entire article, so we restrict ourselves to the basics so that we can cover a number of examples. Gravity is by far the weakest of the four fundamental forces in nature, but the one we are most familiar with, since it tends to keep our world ‘in order’. You put this magazine down on a desk and sit down to read it: neither you, the book, nor the
Q Science in action
Above: The electromagnetic spectrum – including the relative sizes of the various wavelengths and examples of the phenomena and applications associated with the different frequency domains. We observe astronomical objects at as many wavelengths as possible to study the variety of physical processes that give rise to the radiation. Right: The proton-proton chain is the main fusion reaction by which hydrogen is converted into helium within the cores of stars the size of the Sun or smaller. This requires core temperatures greater than 4 million degrees and takes place at about 15 million degrees in the case of the Sun. Only 0.7% of the mass of the four protons is transformed into energy (via Einstein’s famous E=mc2 equation) in producing a helium atom and this is released in the form of gamma rays and neutrinos.
a tug-of-war between gravity trying to squeeze the atmosphere and gas pressure trying to expand it. Most of the time these competing forces maintain hydrostatic equilibrium – whenever either gains the ascendency, interesting things happen, but that’s another story! This type of equilibrium is just another example of balanced forces, the likes of which we rely on daily to support things. Think of the tension in a rope suspending a crate being loaded onto a ship, or the effect of buoyancy that keeps the crate afloat after the rope snaps and the crate falls into the sea. In both situations, the upward force needs to exactly balance the downward-
proton gamma ray neutrino
furniture floats arbitrarily about the room. We take this for granted, but just imagine the alternative! Slightly further afield – gravity ensures that the Earth keeps its Moon and remains in orbit about the Sun. Likewise, the Sun continues to revolve around the Milky Way, and so the list goes on as we expand our view. Gravity also triggers star formation by causing ‘dense’ regions within cold molecular clouds to collapse into balls of plasma as they contract and heat up. The transition from a cloud core to a star involves a staggering increase in density, by a factor of roughly 1020 – one hundred billion billion times! The working life of a star involves
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The Pauli Exclusion Principle γ
ν ν 15O
γ neutron ν proton
gamma ray neutrino
neutron Top: The CNO cycle dominates the fusion of hydrogen into helium in stars heavier than about 1.5 times the mass of the Sun and starts at about 13 million degrees. Above: The triple alpha process is the fusion reaction by which helium nuclei (also known as alpha particles) are transformed into carbon. This set of reactions requires a core temperature of the order of 100 million degrees.
acting weight of the crate to keep it stationary. What do stars do? They spend most of the time fusing hydrogen to produce helium, a process that releases a huge amount of energy and thereby makes them shine. This may not seem terribly important to us, until we realise that every form of energy that living organisms rely on can ultimately be traced back to its origin within the Sun, which after all is just our nearest star! Furthermore, stars more massive than the Sun achieve higher core temperatures, allowing them to perform more demanding fusion reactions. These yield heavier products such as carbon, oxygen and many other elements in the Periodic Table, up to and including iron. The formation of elements heavier than iron will be mentioned later, but in the meantime we should contemplate
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Wolfgang Pauli formulated his Quantum Exclusion Principle in 1925. Essentially, what he said was that only one electron can occupy a given ‘quantum state’. So what is a ‘quantum state’? Simply put, the quantum state of a particle provides a description of the particle’s orbital position, angular momentum and energy in terms of ‘quantum numbers’. For example, for an electron in orbit around a nucleus its principle quantum number, n, describes the radius of its orbit around the nucleus, its orbital angular momentum quantum number, l, descibes its orbital angular momentum and its spin projection quantum number, ms, decsribes its spin direction (which is either +1/2 or -1/2 for an electron). According to Pauli no two electrons can have the same combination of quantum numbers. It’s like saying that only one key can fit into a particular keyhole. We now know that his principle holds not just for electrons, but for electrons, protons, neutrons, muons and many more particles that are called fermions.
the humbling fact that were it not for stellar nucleosynthesis, we could not exist! An interesting possibility during the course of a star’s lifetime is that it may become unstable to pulsation. In the simplest case, pressure (sound) waves confined within the star produce periodic radius variations that cause the brightness to oscillate. Again, this seems far removed from our earthly pursuits – but looking at the physics of vibrations and waves, we recognise the same principles that account for the behaviour of musical instruments. On a larger scale, seismology allows us to probe the interior structure and composition of the Earth by studying the propagation of the elastic waves produced by an earthquake. The field of helioseismology gives us similar insights into solar oscillations and astroseismology is the extension of these techniques to investigate pulsating stars in general. The core of a star Let’s now get back to what happens to a massive star once it has developed an iron core. Up to this point, the various fusion processes released more energy than they required and so it was energetically viable for the star to proceed in this way – but not so for fusing iron. Without fusion to provide the outward gas pressure to balance the effect of gravity, only the unwillingness of electrons to occupy the same quantum states (according to the Pauli Exclusion Principle) prevents the core from collapsing. Incidentally, it is this apparently whimsical property of electrons, also exhibited by protons and neutrons, that actually underpins many of the characteristics of matter, from its largescale stability to the existence of the Periodic Table. Eventually, as the mass of the core continues to increase, even electron degeneracy pressure fails to counteract gravity and a catastrophic collapse follows. Protons and electrons
then merge via the weak interaction to form neutrons and neutrinos, at which point the short-ranged strong nuclear force between the neutrons suddenly halts the collapse. This causes the implosion to bounce outward and the resulting shock wave tears through the star, forging heavier elements and flinging the outer layers of the star out into space at more than 10 000 km/s. More energy is released during a supernova explosion than the Sun will emit in its entire 10 billion year lifetime and, like the snapping of a spring-loaded seed pod, this cataclysm scatters the nuclear-processed material that will later get incorporated into subsequent generations of stars. Gravity, neutron stars and black holes All that remains of the core after this incredibly violent episode is a neutron star, a thoroughly bizarre object even by astronomical standards! Conservation of angular momentum (the law that makes an ice skater spin faster when she pulls her arms in) results in the neutron star spinning around hundreds of times per second and its tiny (approximately 10 km) diameter leads to a staggering density of 5x1017 kg/m3 – about 44 thousand billion times denser than lead and comparable to the density of an atomic nucleus. The gravitational field of the neutron star is so great that it behaves like a lens, bending light from the star by enough to even allow parts of the rear surface to be seen. If we imagine trying to launch a space probe from the surface of the neutron star, how fast would it have to travel in order to escape the star’s enormous gravitational pull? To launch something from Earth, it needs to travel at 11.2 km/s – more than 10 times the speed of a rifle bullet. Due to the neutron star’s much larger mass and remarkably small radius, the corresponding escape speed is about 100 000 km/s! This is a third of the
Q Science in action H, He
Left: After a lifetime of fusion reactions and neutron capture processes have built up heavy elements inside a massive star, gravity eventually causes a catastrophic collapse. The resulting shockwave tears the star apart and disperses the heavy elements into the interstellar medium.
He, N He, C, 22Ne O, C O, Ne, Mg
Massive star near the end of its lifetime has an “onion-like” structure just prior to exploding as a supernova
Si, S Fe, Ni core
In one of his Caltech lectures on the relation of physics to other sciences, Feynman said ‘Poets say science takes away from the beauty of the stars – mere globs of gas atoms. Nothing is ‘mere’. I too can see the stars on a desert night, and feel them. But do I see less or more?’ … ‘It does not do harm to the mystery to know a little about it. For far more marvelous is the truth than any artists of the past imagined!’
Red giant star
Nuclear burning occurs at the boundaries between zones
Example of nuclear reactions that build neutron-rich isotopes
speed of light, the ultimate speed limit in the Universe. Black holes are even more enigmatic than neutron stars, but the escape velocity concept offers an intuitive way to think of these intriguing objects. They are simply bodies for which the escape speed is greater than the speed of light, and hence we can receive no information directly from a black hole. This example illustrates how even one of the more esoteric astrophysical concepts can be understood in terms of familiar, basic physics. For Richard Feynman, one of the greatest physicists of the twentieth century, the most remarkable discovery in all of astronomy was that ‘the stars are made of atoms of
the same kind as those on the Earth’. That may seem simple to us now, but the implications remain extraordinary when you really think about it! ■ Lisa Cruse is an astronomer at SAAO. She did her honours degree at the University of Cape Town and her PhD at SAAO. Her postdoctoral research was on stellar astronomy and she now works on instrumentation. Kevin Govender is a physicist and the chairperson of the South African steering committee for the International Year of Astronomy. Nicola Loaring studied Astronomy and Physics at University College London before completing her PhD in Astrophysics at the University of Oxford. Her thesis investigated quasar clustering using the 2dF QSO Redshift Survey. She now works at SALT and does research in extra-galactic astronomy.
The paths labelled A–E indicate projectiles launched from the Earth with increasing speeds. A and B fall back to the ground, C achieves a circular orbit, D an elliptical one and projectile E, launched with the required escape velocity, has enough energy to overcome the Earth’s gravitational pull.
This year the South African Institute of Aquatic Biodiversity celebrates 10 years as one of the National Research Foundation’s family of research facilities. Situated in Grahamstown in the Eastern Cape, SAIAB houses world-famous collections of marine and freshwater fishes from African inland water systems and surrounding seas. Recognised internationally as a hub for the study of aquatic biodiversity, SAIAB reseach involves: Discovery – Exploring African Aquatic Biodiversity Systematics and Taxonomy, Phylogenetics, Phylogeography SAIAB Somerset Street Grahamstown http://www.saiab.ac.za Tel: +27 (0)46 6035800 Email: email@example.com
Conservation Biology – Coastal and freshwater conservation biology and Invasion biology Ocean Exploration – African Coelacanth Ecosystem Programme – SAIAB’s flagship programme Biodiversity Informatics and the National Fish Collection
Quest 5(1) 2009 45
When the light goes out:
Pluto & Charon
Artist’s conception of Pluto and Charon, with Pluto in the foreground. Pluto and Charon are locked in a synchronous orbit, which means that the same side of each object always faces the other. Pluto’s surface is covered in methane and nitrogen ices, with light and dark areas having surface temperatures of approximately –230º C and –210º C respectively. There is water ice on Charon’s surface, and its temperature is approximately –220º C.
Number of moons as of end 2008 1 2 0 63 60 27 13 3 2 0 Charon
0.4 0.3 0.2
Fractional size (satellite diameter/planet diameter)
Image: Fahad Sulehria, http://novacelestia.com
The currently known moons for each planet (labelled in black) and dwarf planet (labelled in grey). The satellites of each planet are plotted as a function of their size with respect to the object around which they revolve, and the total number is shown at the top. Pluto’s moon Charon is unique in that it is the largest moon relative to the object it orbits. For many of the moons, sizes are approximate because observations can only determine the amount of sunlight reflected. The size then depends on the reflective properties of the surface. Stellar occultation observations, as described in the article, are a highly accurate method of determining the sizes of these small objects.
46 Quest 5(1) 2009
Amanda and Eric Gulbis discuss the use of a technique called a stellar occultation to characterise objects in the Solar System.
hen Galileo turned his telescope to the sky 400 years ago, five of the planets in our Solar System were known. As recently as 150 years ago, all eight of the now-known planets had been observed. A notable standout is Pluto, which was discovered in 1930 and has recently been reclassified into a new category called dwarf planets. The criteria required for an object to be a planet, and the story of Pluto’s reclassification, are discussed in an article by C. Rijsdijk (QUEST 3(2) 2007, pp. 24–27). One key component to the debate surrounding what constitutes a planet was the existence of any moons, or satellites, orbiting the object. In 1610, Galileo discovered the first satellites around a distant body in the Solar System: four moons of Jupiter. This monumental discovery provided evidence against the Solar System being a geocentric system in which all objects were assumed to revolve around the Earth. Today, there are more than 170 known moons orbiting planets and dwarf planets. Most of these moons are small and faint
(relative to the planet); therefore, it is often difficult to characterise their parameters. In this article we describe the discovery and characterisation of the largest of Pluto’s moons, Charon. A strange observation In 1978, James Christy noticed something strange about an image of Pluto that was taken with the 1.55-m telescope at the US Naval Observatory in Flagstaff, Arizona. Rather than seeing a circular image as expected, he saw an image that appeared to be elongated, more like an ellipse than a circle. Christy and his colleagues carefully investigated possible reasons for this surprise. They considered explanations such as atmospheric conditions and potential defects in the photographic plates before hypothesising that what they saw was an object orbiting Pluto. By comparing their image with previous observations, they were able to confirm their hypothesis: Pluto had a moon! They determined that the moon circled Pluto once every 6.4 days, with the same side always facing Pluto. They named this satellite Charon.
Q Science in action
Since Christy’s discovery over 30 years ago, it has been difficult for astronomers to learn more about the properties of Charon, for three reasons: Charon is far away from Earth (approximately 30 AU), it is close to Pluto, and Pluto is roughly 20 times brighter than Charon. Stellar occultation Just as in its discovery image, Pluto and Charon appear merged as one object when viewed through most telescopes. However, astronomers have developed an indirect technique – called a stellar occultation – which allows them to study objects in the solar system that cannot easily be observed directly, like Charon. A stellar occultation occurs when a foreground object, such as a planet, passes in front of a background star, blocking out (or occulting) the light from the star as seen from the Earth. Stellar occultation observations have proven to be one of the most accurate methods for revealing information about the sizes and the atmospheres of solar system objects, short of sending a spacecraft. Not long after Charon was discovered, it was predicted in 1980 that a star would pass quite close to Pluto and Charon, as viewed from Cape Town. Alistair Walker, an astronomer at the South African Astronomical Observatory (SAAO), decided to observe this possible stellar occultation using the SAAO’s 100-cm telescope at Sutherland. He collected photons of light from Pluto, Charon, and the target star. Near the time of the predicted occultation, the observed light dropped for nearly 50 seconds: something had blocked the starlight! By carefully analysing astrometric, or positional, measurements of Pluto, Charon, and the star before and after the occultation, other astronomers proved that the occulting object was Charon.
bright dark time increasing Discovery image of Charon taken on June 22 1978. This image is a photographic plate taken using the 1.55-m Kaj Strand telescope in Flagstaff, Arizona. Image: U.S. Naval Observatory
During a stellar occultation, the length of time that the object blocks the starlight corresponds to the size of the occulting object. In this case, the star and Charon were moving 24 km/sec with respect to each other. Therefore, a 50 second drop in the light corresponded to a distance of 1 200 km. Walker could not conclude, however, that Charon had a diameter of 1 200 km. An occultation chord refers to a line across the occulting body that joins the point where the star disappears (called immersion) and the point where it reappears (called emersion). The longest chord across Charon would thus be at the centre of the circle, just like the longest chord across a circle is a diameter of that circle. All chords above or below the centre must be shorter. When Walker observed the occultation, he had no way of knowing if the starlight was blocked by a central chord (the full diameter) of Charon, or a shorter chord above or below the centre. His conclusion: Charon had to be at least 1 200 km across, but it could be larger if the observed chord were off-centre. During the 1980 stellar occultation by Charon, the observed light dropped and rose quickly during immersion and emersion rather than exhibiting the more gradual drop and rise that are indicative of an object that has an atmosphere. Nonetheless, these ▲ ▲
An AU is an Astronomical Unit, defined to be the average distance between the Earth and the Sun (1.5 × 107 km). Pluto and Charon have an eccentric orbit around the Sun, which causes their distance from the Earth to range between roughly 28 and 50 AU over their 248-day revolution.
Plot of light signal
Schematic drawing of a stellar occultation by Charon (not to scale).
Observing a stellar occultation When astronomers predict an occultation of a star by an object, they point their telescopes at the target star and record the amount of light. The occultation occurs when the target star appears to pass behind an object such as Pluto’s moon Charon. As the star passes behind the object, the starlight is blocked, and the object casts a shadow on the Earth. In this way, a stellar occultation is similar to a solar eclipse. Just as the moon casts a shadow over a certain region on Earth during a solar eclipse, the shadow path produced by an object during a stellar occultation is cast only over a certain region on the Earth’s surface. To observe a stellar occultation, you must be in the object’s shadow path – otherwise the starlight never appears to be blocked. The record of the light observed during an occultation is called a ‘lightcurve’, a stylised example of which is shown at the bottom of the diagram. When the target star first slips behind the object, the observed light drops from being bright to dark; when the star emerges on the other side of the object, the observed light jumps back up to bright. Stellar occultation lightcurves can reveal much about the object passing in front of the star. They provide one of the most accurate ways to measure the sizes of objects in the solar system and are very sensitive probes of atmospheres. The rings of Uranus were first discovered using the lightcurve from a stellar occultation, as was one of Neptune’s moons (Larissa), and Pluto’s atmosphere.
Collecting light Observational astronomers measure electromagnetic radiation, some of which is in the form of visible light. Photons or particles of electromagnetic radiation, can be collected by different types of telescopes. The more photons an astronomer collects, the more easily he or she can determine the properties of an object or discover new phenomena.
Quest 5(1) 2009 47
Images of the July 11 2005 stellar occultation by Charon from the 6.5 m Clay (Magellan II) telescope at Las Campanas Observatory, Chile. (Left) Less than a minute before the occultation, Pluto is clearly discernable. The star and Charon are so close together that they are merged. (Middle) During the occultation, the starlight is blocked by Charon. Charon is the faintest of the three objects and appears as an elongation in brightness on one side of Pluto. (Right) Less than a minute after the occultation, Pluto and the star (merged with Charon) are once again seen. Images: J.L. Elliot and E.R. Adams (MIT) and D. Osip (LCO)
Table 1. Determinations of Charon’s radius throughout history
Image: Massachusetts Institute of Technology Planetary Astronomy Laboratory
Normalised stellar flux
Photons per two seconds
The path for the shadow cast by Charon for the predicted stellar occultation on July 11 2005. The shaded region of the globe represents nighttime on the Earth, where the sun is below the horizon. The three lines, from top to bottom, indicate the path of Charon’s northern edge, centre, and southern edge. The actual path went slightly south of this prediction by less than half the size of Charon’s shadow.
11 000 9 000 stellar occultation by Charon, as seen from South Africa
7 000 5 000 3 000
Seconds after 23:38:51 UT on 1980 April 06
1000 ± 100
Stellar occultation (single obs.)b
642.5 ± 117.5
593 ± 13
635 ± 13
606.0 ± 1.5
Stellar occultation (multiple obs.)e
Note: This is a representative list, and does not include all measurements that have been made. a The Earth’s atmosphere causes the light from a distant object to be spread into a pattern of small spots. Speckle interferometry is a technique in which very short exposures are taken (to limit atmospheric effects on any given image) and then processed to remove the pattern. b This lower limit on Charon’s radius is based on one observation made by Alistair Walker in South Africa during the 1980 stellar occultation. c Mutual events refers to the period between 1985 and 1990 when Charon’s orbit was oriented such that it appeared from Earth to pass in front of and behind Pluto each time it went around. This alignment occurs once every 124 years. d Direct images of Pluto and Charon, obtained with the Hubble Space Telescope, were fitted to determine their sizes. e The most accurate measurement to date of Charon’s size, based on observations from many different telescopes during the 2005 stellar occultation.
stellar occultation by Charon, as seen from Chile
0.6 0.4 0.2 0 25
Seconds after 03:35:00 UT on 2005 July 11
The amount of observed light versus time during the two stellar occultations by Charon. The swift drop and rise in observed light indicate that Charon has practically no atmosphere. (Top) Data taken by Alistair Walker in 1980, using the 100-cm telescope in Sutherland with the high-speed pulse counting photometer belonging to the University of Cape Town. Each data point represents the number of photons counted in two seconds. (Bottom) Data taken by J.L. Elliot and E.R. Adams in 2005 using the 6.5-m Clay telescope in Chile with a portable camera and global positioning system. Each data point indicates the amount of light detected every 0.1 seconds, divided by the total amount of light from Pluto, Charon, and the star. (Note that UT stands for Universal Time, which is the same as Greenwich Mean Time or two hours less than South African Standard Time.) Credits: A. Walker; A.A.S. Gulbis
48 Quest 5(1) 2009
data could not rule out the possibility that Charon had a thin atmosphere. It wasn’t until 25 years later that another occultation of a star by Charon settled the question of whether or not Charon has an atmosphere and determined its diameter with great precision. In 2005, a predicted stellar occultation indicated that Charon’s shadow path would cross South America. This prompted more than a dozen teams of astronomers to attempt observations using telescopes in countries including Chile, Brazil, and Argentina. Many of these teams were successful, and multiple stellar occultation chords were obtained spanning a range of latitudes on the Earth that corresponded to a range of latitudes on Charon. By analysing five of these occultation chords, Charon’s size was determined to be 601.0 ±
1.5 km in radius. The power of the stellar occultation technique is clear: the size of an object that is located 4.5 × 108 km away from the Earth was measured to the accuracy of a few kilometres. The highest-quality data from the 2005 occultation were taken at the 6.5-m Clay (Magellan II) telescope at Las Campanas Observatory, Chile. The light observed at this telescope not only showed a steep drop and rise during immersion and emersion, but also displayed a diffraction fringe. This fringe, seen as an increase in observed light just before immersion and just after emersion, is an effect that occurs when light passes a sharp edge. This effect could only be witnessed if Charon’s surface acted as a sharp edge, which indicates that it has practically no atmosphere.
Q Science in action An artist’s rendering of the New Horizons spacecraft during its planned encounter with Pluto and Charon. Pluto’s faint atmosphere is shown in this drawing, along with the distant sun. The spacecraft’s most prominent feature is a nearly 2.1-m diameter antenna, which it will use for communications back to Earth from as far as 7.5 billion kilometres away. Image: National Aeronautics and Space Administration, Johns Hopkins University Applied Physics Laboratory, Southwest Research Institute
Table 2. Satellites of Pluto Satellite Pluto I: Charon
Approximate size (relative to Pluto) 0.53a
Orbital period (days)
Pluto II: Nix
Pluto III: Hydra
a The size of Charon is well known compared with the other moons because stellar occultations by Charon have been observed.
New moons In 2005, two new moons of Pluto were discovered using the 2.4-m Hubble Space Telescope. These moons, named Nix and Hydra, had never been seen before because they are so small (approximately 40 to 150 km in diameter) and faint. Then, observations from the 10-m Gemini North telescope in Hawaii in 2007 suggested that there might be cryovolcanism occurring on Charon. Cryovolcanism refers to the process by which liquid trapped beneath an icy surface forms geysers that freeze after eruption and snow back down to the surface. These exciting discoveries reaffirm that we still have much to learn about the Pluto system. To date, the 1980 and 2005 events are the only successfully observed stellar occultations by Charon. These
observations have provided some of the highest quality information that can be obtained from the Earth. In 2006, the USA’s National Aeronautics and Space Administration (NASA) launched the New Horizons spacecraft with the goal of more closely studying Pluto and its moons. New Horizons should reach Pluto in 2015. The intimate study afforded by this spacecraft will certainly reveal more fascinating information about these small, icy objects in our outer solar system. ■ Dr. Amanda Gulbis is a Southern African Large Telescope astronomer working at the South African Astronomical Observatory, Cape Town; firstname.lastname@example.org. Mr. Eric Gulbis is a Mathematics teacher for grades 9–12 working at The LEAP Science and Maths School in Pinelands, Cape Town; email@example.com.
Image of the Pluto system taken on March 02 2006 by the Hubble Space Telescope (HST), using the High Resolution Channel of the Advanced Camera for Surveys. Pluto and its three known satellites are clearly visible. This image was taken using a blue filter – a similar image was taken using a red filter. The amount of light reflected from the surfaces of the objects at different wavelengths (i.e. in the red and blue filters) was used to determine that the surfaces of Nix and Hydra are essentially the same colour as Charon, while Pluto is redder. Image: National Aeronautics and Space Administration, the European Space Organization, H. Weaver, A. Stern, and the HST Pluto Companion Search Team
Quest 5(1) 2009 49
Right: Father Tachard’s observatory. This has deliberately been reversed left to right, because it only then has some resemblance to Cape Town! The engraver of the picture probably did not reverse it from the original sketch. The illustration is taken from the book on Tachard’s voyage, published in 1688.
Latitude and longitude If you want to describe where you are in the world you use latitude and longitude. These two numbers are used to describe every location on Earth. To understand how latitude and longitude work, think of the Earth as a sphere that is divided into lots of different sections. The Equator divides the Earth into a northern and southern section, and the Greenwich meridian divides the Earth into an eastern and western section. Longitude gives the location of a place easte or west of the Greenwich meridian, which is at a 0º W/E. Longitude lines run perpendicular to the Equator, passing through the North Pole and the South Pole. They measure the distance in degrees west or easte of the Green wich meridian. Latitude gives the location of a place north or south of the Equator, which is at a 0º N/S. Latitude lines are horizontal lines parallel to the Equator, running from east to west. They measure the distance in degrees north or south from the Equator. These co-ordinates can be used to mark the position of any place on Earth. Johannesburg is 26º 12’ south and 28º 4’ east.
How are latitude and longitude calculated? Latitude is relatively easily calculated using the height of the sun or stars of accurately known position above the horizon. But longitude is not as simple. Longitude can only be measured at sea, for example, if you know the exact time on board ship and also the time at your home port or another place of the same longitude – at exactly the same moment. The two times allow the navigator to convert the difference in time into a geographical separation – and so work out geographical position. This sounds simple in today’s world of watches and clocks. But before time could be accurately measured at sea, ships sailing around the world’s oceans had only a very poor knowledge of their longitude, and often became lost. A self-taught Yorkshire clockmaker, John Harrison, invented the marine chronometer. This clock was critical to the measurement of longitude and was only developed during the mid-1700s.
50 Quest 5(1) 2009
ASTRONOMY at the Cape Brian Warner sketches the history of astronomy at the Cape, from the 17th century, to the present day.
he earliest astronomical observations at the southern tip of Africa were made by Portuguese and Dutch sailors on their way round the Cape, heading for the east. For them, knowing the latitude and longitude of the Cape was of great importance, even if only to avoid running into it unexpectedly in the middle of the night. But the first very careful measurements were made by a group of French missionaries and mathematicians, headed by Father Guy Tachard, who were heading for the court at Siam (modern-day Thailand). They stopped at the Cape for ten days in June 1685. Their aim was to determine the difference in longitude between Paris and the Cape. They did this by observing the four bright satellites of Jupiter with the aid of a small telescope, and noting the times at which these satellites passed in and out of the shadow cast by Jupiter itself. They then compared these with computations made for Paris time and so found the difference in longitude between Paris and the Cape. These tricky observations could not have been done from the heaving deck of a ship. Latitude was
obtained from the altitudes of stars that had already been measured in France. Along with these observations, made in essence for geographical purposes, Tachard used his telescope for examining the southern stars and discovered that the brightest star in the constellation of Crux is a double star. Although there was an extended visit of a German astronomer, Pieter Kolbe, to the Cape from 1705 to 1713 he published no useful new observations, so it wasn’t until the French Academy of Sciences sent out Nicholas Louis de la Caille (usually abbreviated Lacaille) in 1781 that another professional astronomer contributed to knowledge of the southern sky. Lacaille watched the skies from a house in Strand Street and within a year had measured the positions and brightnesses of nearly 10 000 stars and had discovered several more double stars and nebulae. In plotting the stars on a celestial map he realised that the southern constellations that had been invented by the early navigators were not adequate, so he added a further fourteen of his own. His constellations are the ones with modern-sounding
Q History of science
for instruments to be constructed that were identical to the ones at Greenwich. Like the Greenwich Observatory, the Cape Observatory came under the direction of the British Admiralty, which was the authority most concerned with navigational matters at the time. The main purpose for the new observatory was to measure accurate positions of all the bright southern stars, and to provide accurate time for shipping passing through Table Bay. The person chosen to get the observatory started was Fearon Fallows, a mathematical Fellow of St John’s College, Cambridge. Because of delays in providing the funds building was only started in 1825 and the observatory was officially opened on 29 October 1828 by the then Governor, Sir Lowry Cole. A year later a scientific expedition, under the command of Captain Henry Foster, was passing through the Cape and on board was a talented artist, Lieutenant E. N. Kendall, who made the earliest surviving picture of the new observatory. As can be seen, the observatory was sited on a low hill, which had a direct line of sight to Table Bay, which allowed time signals to be sent to shipping. At first this was done using a flash pistol fired from the roof at a prescribed time after dark; later it was a time ball dropped at 13:00. A time ball from a later period survives at the Waterfront in Cape Town. Fallows had only just started on his work of producing a catalogue of southern stars when he died during an epidemic of scarlet fever on 25 July 1831. The Admiralty replaced him with Thomas Henderson, a leading amateur astronomer from Edinburgh who arrived at the observatory in April 1832. However, he did not like the place and resigned after only 13 months in office, returning to Edinburgh in May 1833,
Above left: The Royal Observatory, Cape of Good Hope, as it appeared in October 1829, by Lieutenant E. N. Kendall. From the Fehr Collection and reproduced by permission of Iziko Museums. Above: Thomas Maclear shown in a watercolour presented to him on his departure for the Cape in 1833.
soon afterwards to become the first Astronomer Royal for Scotland. While at the Cape, Henderson observed nearly 10 000 star positions, including many positions of the bright southern star Alpha Centauri. Only when he was later working through his observations did he notice that Alpha Centauri showed motion in a small ellipse in the sky. He had discovered the effect of parallax, which is the change in position as seen from Earth as it orbits round the Sun. Astronomers had been trying to detect this effect for centuries; it showed that Alpha Centauri is the closest bright star to the Sun. This major discovery established the Cape Observatory’s international reputation. After Henderson had left, the Admiralty appointed Thomas Maclear, another amateur (but, like Henderson, a skilled mathematician, capable of dealing with all astronomical computations), who was a doctor living in Biggleswade, where he ran his own small observatory. Maclear and his family arrived in Cape Town in January 1834. A year after he arrived Maclear appointed the 16-year old Charles Piazzi Smyth to be his First Assistant. Smyth spent ten ▲ ▲
names, rather than the names of classical and mythical beasts that were preferred by ancient observers. Lacaille’s names were telescopium, microscopium and others. Lacaille also measured an Arc of Meridian by obtaining accurate latitudes at two sites along a line of longitude and then finding the linear distance between them by using land survey techniques. The chosen sites were his observatory in Strand Street and the farm Klipfontein in the Picketberg region, over 100 km north. The purpose was to make the first measurement of the radius of the Earth in the southern hemisphere. The result was a significant difference from the radius measured at a similar northern latitude, which meant that the Earth appeared pear-shaped. Lacaille did not believe his own result, thinking that there were errors in his observations, but published it anyway. It wasn’t until 1820, when leading astronomers in England decided that their science needed a boost that a permanent southern observatory with equipment equivalent to that being used at the Royal Greenwich Observatory in London was founded. Choosing the Cape of Good Hope as the site for such an observatory was not difficult: other southern locations considered were the Atlantic islands of St Helena and Ascension, but Edmund Halley and Neville Maskelyne had observed at St Helena, the former in 1677–78 and the latter in 1761, and found it to be too cloudy. The newly formed colonies in Australia and New Zealand were too far away for efficient communication (many months of sailing in each direction), whereas given fair winds the Cape was only two months’ sailing each way! The outcome was a large treasury grant, an architect’s plan, and instruction
Parallax In astronomy, parallax is the only direct method that astronomers can use to measure distances to objects, usually stars, beyond the Solar System. By measuring the change in apparent position of an object on the sky as the Earth travels in orbit around the Sun, and knowing the radius of the Earth’s orbit (149 597 870.66 ± 0.012 km – defined to be one astronomical unit), the distance to the object is found.
Quest 5(1) 2009 51
Top: Camping on the Picketberg during Maclear’s survey to verify Lacaille’s Arc of Meridian. Drawing by Charles Piazzi Smyth. Reproduced by permission of the Director of the South African Astronomical Observatory. Top right: The Cape Observatory in 1835. Pencil drawing by Thomas Bowler. Reproduced by permission of the Director of the South African Astronomical Observatory. Above: The earliest photograph of the Cape Observatory – about 1843. From the Piazzi Smyth collection in the Royal Society of Edinburgh. Above right: Gill’s Reversible Transit instrument, at the Cape Observatory, used for measurement of accurate positions of stars.
years at the observatory, before leaving for Edinburgh as Astronomer Royal for Scotland after Henderson’s death. During his time at the Cape Smyth became the pioneer photographer in South Africa. For the first decade in his new position, Maclear did relatively little astronomical work. Instead he carried out the Admiralty’s instruction to repeat the work that Lacaille had done eighty years earlier, to find out
where the error lay. Maclear found that there were no significant errors; the problem was one of interpretation. Mid-eighteenth century French astronomers did not subscribe to Newton’s theory of gravity, at least as far as all bodies attracting each other. So Lacaille, in determining his latitudes, had overlooked the gravitational pull of Table Mountain in the south, and the Picketberg range in the north on the plumb-bob that gave him the direction of the zenith. Maclear went on to extend Lacaille’s work northwards almost to the Orange River, and south to Cape Agulhas, becoming as a result the ‘father of South African trigonometric survey’. Thomas Maclear retired from the Cape Observatory in 1870, having worked incessantly for his entire life, and leaving a mass of unreduced observations that his successor, Edward James Stone, struggled to complete before he in turn left in 1879. Stone was replaced by David Gill, who had joined his father’s business as a watchmaker but, having established his own small observatory in Aberdeen and been appointed to run a large private observatory in Scotland, had made a reputation as an outstanding observer and instrument designer.
Sir John Herschel – South Africa’s greatest astronomer Herschel and his family arrived in Cape Town 11 days after the Maclear family, and it was no coincidence – Herschel needed the scientific support that he knew Maclear would be able to give. John Frederick William Herschel was born in 1792, the only child of Sir William Herschel, who discovered the planet Uranus in 1781 and who was the world’s greatest astronomer at the end of the eighteenth century. John Herschel had been educated at Cambridge and was an exact contemporary and friend of Fearon Fallows and had started on a career there as all-round theoretical and experimental scientist when his father persuaded him to return home and continue the latter’s survey of the sky. From 1817 to 1831 John Herschel re-surveyed the entire northern sky, finding hundreds of nebulae and double stars that his father had overlooked, and naturally wanted to complete the work by observing the southern sky with the same instrumentation – the ‘20-foot telescope’ (actually a reflector with a mirror 18 inches (45 cm) in diameter), then by far the largest and finest telescope in the world. This is what brought him to Cape Town in 1834. From 1834 to 1838 Herschel and his family lived at Feldhausen, in what is now Claremont, a few kilometers from the Maclear family. Herschel completed his survey of interesting celestial objects, measuring their positions relative to the accurate positions of bright stars given to him by Maclear. His resulting book, which took until 1847 to complete, is one of the great monuments of astronomical publishing, and enabled Herschel in later life to combine his northern and southern observations into a General Catalogue, which is used by all astronomers to this day as the new General Catalogue (NGC) in its updated form.
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It is topical to point out that Herschel was at the Cape when Charles Darwin, on his voyage round the world in the Beagle, visited in June 1836. Their meeting gave the Darwin ‘the most memorable event which, for a long period, I have had the good fortune to enjoy’, saying also that he (Darwin) ‘felt a high reverence for Sir J. Herschel’.
Sir John Herschel, in 1833. from a mezzotint in Museum Africa, Johannesburg.
The 20 foot reflector, installed at Feldhausen, Claremont.
Right (from the top): The staff of the Cape Observatory in 1879 (when David Gill arrived – he is on the extreme left).
Q History of science
Modern photograph of the main building of the old Cape Observatory, now the administration headquarters of the South African Astronomical Observatory. Some of the smaller telescopes at the Sutherland site of the South African Astronomical Observatory. The Southern African Large Telescope, at Sutherland.
During his 28 years at the Cape Gill built the observatory into the greatest in the southern hemisphere and one of the leading observatories in the world. In addition he, like Maclear, directed land surveys, this time for the whole country and into what became Rhodesia. His astronomical work, however, was extensive and innovative. He determined the parallaxes of southern stars that were the best in the pre-photographic era; he introduced wide-field astronomical photography, enabling him to produce star catalogues containing many tens of thousands of star positions; he built up the instrumentation at the observatory, including the 24-inch refractor, then the largest in the southern hemisphere; he designed a position-measuring telescope of novel design that was in use until the 1960s; he determined the mass of the Moon and gave the most accurate measurement of the distance of the Earth from the Sun; he introduced the then new science of astrophysics at the observatory, concentrating on stellar spectroscopy. By his retirement in 19067 he had also greatly increased the staff of the observatory. The Directors of the Cape Observatory who followed in the twentieth century (S. S. Hough, 1907–1923; H. Spencer Jones, 1923–1933; J. Jackson 1933–1950; R. H. Stoy, 1950–1968) largely followed the large-scale astronomical projects started by Gill, producing what was considered to be the proper duty of a government observatory, that is fundamental data on stars. By the 1960s observing conditions were becoming intolerable because of the growth of city lights. Site testing was carried out in remote parts of the Cape, in regions that had reasonably clear skies around the whole of the year, and that were away from light pollution. The site chosen was Sutherland in the Karoo, to which the main telescopes would be moved. But at this time the British Science Research Council (SRC), which had taken over responsibility from the Admiralty, were considering closing the observatory. It was rescued when the SRC and the South African Council for Scientific and Industrial Research decided to run the observatory jointly, and on 1 January 1972 it started a new life as the South African Astronomical Observatory.
However, it was still necessary to move telescopes to Sutherland, and the opportunity was taken also to move the one modern telescope from the Republic Observatory in Johannesburg. The new observatory was opened in early 1973, with the old observatory site in Cape Town remaining as the administrative and technical headquarters. The Sutherland site became a field station, with its own technical support staff, but no permanent resident astronomers. Since that time the observatory, used by its staff, South African universities, and visitors from overseas, has maintained a scientific output of high quality and quantity. The observatory specialises in optical and infrared photometry and spectroscopy applied to stars and galaxies, often pursuing longterm programmess not feasible at other southern hemisphere observatories. These programmes are in areas as diverse as compact stars, binary stars, variable stars, gravitational lensing of stars, studies of the Magellanic Clouds, more distant normal galaxies, active galactic nuclei, and the distance scale of the universe. These and further topics will be covered in the other articles in this issue of QUEST. The Sutherland site continued to add telescopes: the 74-inch (1.9-m) reflector that was the largest telescope in the southern hemisphere when it was established in Pretoria in 1939 was moved and started operation at Sutherland in 1978. Other telescopes, run for or by astronomers from other countries (the UK, Japan, Korea, Germany), including remotely operated and robotic telescopes, have been installed during the past twenty years. But the greatest acquisition has been the Southern African Large Telescope (SALT), an 11 m aperture reflecting telescope owned by an international group. South Africa is a major player and the telescope was completed during the past two years and is still undergoing trials. This is the optical telescope that carries South African astronomy into the twenty-first century and puts us back on the map as a leader in the field. ■ Brian Warner is Emeritus Distinguished Professor of Natural Philosophy at the University of Cape Town. He was head of the Department of Astronomy at UCT from 1972 to 2004. His particular interests are in astrophysics and the history of science.
Quest 5(1) 2009 53
Diary of events Q Shows and exhibitions Maropeng celebrates 200th anniversary of Darwin’s birth In celebration of the 200th anniversary of Charles Darwin’s birth on February 12, 2009, Maropeng will be holding a poster exhibition entitled Darwin, Origins and Africa, in conjunction with the Institute of Human Origins and the Origins Centre at the University of the Witwatersrand. The exhibition, which opened at the Maropeng Visitor Centre on February 12, 2009, runs for most of the year, and focuses on the following facts: n Darwin visited South Africa during his famous voyage aboard the HMS Beagle, anchoring in Simon’s Town from 31 May to 18 June, 1836 – the longest the ship spent at any one place during its voyage around the world, other than the Galapagos Islands. n Darwin predicted that Africa would prove to be the place of origin of humankind, long before the first hominid fossils would be uncovered in the World Heritage Site that is now known as the Cradle of Humankind. n Darwin was the first person to articulate the theory that apes and humans had a common ancestor. Charles Darwin (1809-82) achieved international prominence and academic acclaim, with the publication of his book On the Origins of Species, which also marks a significant anniversary this year – 150 years since its publication. On weekends, pop in at Maropeng’s Darwin Café where birthday cake and coffee will be on sale. Guides will also be on hand to take you through important aspects of the poster display. Later in the year, Maropeng will host a new original fossil display based on the theory of evolution, in line with the year-long Darwin celebrations. Contact Maropeng on 014 577 9000 or firstname.lastname@example.org for more information, or visit www.maropeng.co.za.
Guinea Minnie & the piece of sky One day something fell right onto Guinea Minnie’s head! Bonk! What could it possibly be? A piece of sun? Maybe a piece of moon or was it a piece of star? Join Guinea Minnie and her friends to find out for yourself! 4 April–14 April Monday to Friday – 12:00 & 13:00 Saturday – 12:00 Sunday – 12:00 Especially for children aged 5–12.
The Sky Tonight An interesting live lecture on the current night sky is presented every Saturday and Sunday. You will receive a star map and be shown where to find the constellations and planets that are visible this month. Saturday – 13:00 Sunday – 13:00 27 April – 13:00 Suitable for teenagers & adults.
A basic guide to Stargazing Drawings by the acclaimed cartoonist – Tony Grogan. In this 45-minute presentation we give you a basic understanding of the night sky and how it changes throughout the year. We introduce some easily recognisable constellations, explain the nature of stars and the galaxy in which we live and give basic information in using binoculars and small telescopes. Until 31 March Monday to Friday – 14:00 (excluding 2 March) Tuesday evening – 20:00 (& sky talk) Saturday – 14:30 Sunday – 14:30 Suitable for teenagers & adults.
The Iziko Planetarium is closed for maintenance on the first Monday of the month, excluding school holidays. Iziko Museum
Unconquerable spirit: George Stow and the rock art of the San Extended to May 2009 George Stow was a Victorian man of many parts – poet, historian, ethnographer, artist, cartographer and prolific writer. Stow’s paintings are interpretations of the art of the San, informed by his own understanding of a particularly turbulent time in South African history and his sense of the tragic demise of the San way of life. This exhibition celebrates his pioneering achievement and reminds us, too, of the richness of the imaginative universe of the San. The exhibition brings together works from the Iziko South African Museum, the National Library of South Africa and the of Cape Town (UCT). It is curated by Pippa Skotnes and her team at the Centre for Curating the Archive, Michaelis School of Fine Art, UCT. A new publication on Stow will be launched at the exhibition. Enquiries: Petro Keene, Tel. 021 481 3883, or email email@example.com.
Outings n Botanical Society of South Africa (Bankenveld Branch), Gauteng: Small Mammal Biodiversity Assessment at Melville Koppies with Professor Neville Pillay, School of Animal, Plant and Environmental Sciences, Wits (21 March 2009, meet at the Nestlé Environmental Education Centre Walter Sisulu National Botanical Garden at 07:00); Orchids of Melville Koppies with Alan Abel (18 April 2009, meet at Marks Park Parking area, Judith Road, Emmarentia (Opposite lower MK entrance) at 09:30. Booking essential. Contact Karen by phoning (011) 958 0529 (mornings only) or e-mail firstname.lastname@example.org.
Lectures and courses n 19 March, 2009. Witwatersrand Bird Club: ‘Birds: The inside story‘ An evening meeting with Professor David Grey. Professor Dave Gray spent two years at Rhodes University in Grahamstown, before joining the School of Physiology at Wits University. His research interests have focused on two aspects, namely salt and fluid balance (osmoregulation) in birds and avian thermal biology, specifically the fever response. Venue: Delta Environmental Centre; 19:00 – 21:00
The Southern African Large Telescope Iziko Planetarium, Cape Town
Davy Dragon gets his own weekly Sky Guide Show! Davy Dragon’s guide to the night sky Come and join Davy Dragon while he learns all about the sky above so that he can fulfil his dream of becoming the world’s best flying dragon! This is a playful introduction to astronomy especially for the under 10s. Just right for inquiring young minds. Until 29 March & from 18 April Saturday – 12:00 Sunday – 12:00 27 April – 12:00 Especially for children aged 5–10. For the school holidays!
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Built at a fraction of the normal cost – using a 91segment mirror and an ingenious mobile ‘tracker’ – the Southern African Large Telescope has the largest aperture of any telescope in the world. Sited in the heart of the Karoo, from where the San people built up their rich astronomical heritage, this telescope can see galaxies so far away, and so long ago, that their light has taken many billions of years to reach us. It opens a new era for South African astronomy, and a new window on the cosmos. 1 April – 31 July Monday to Friday – 14:00 (excluding 27 April Tuesday evening – 20:00 (& sky talk) Saturday – 14:30 Sunday – 14:30 27 April – 14:30 Suitable for teenagers & adults.
Diarize n The International Year of Biodiversity (2010). In 2006 the United Nations declared 2010 to be the International Year of Biodiversity. It designated the secretariat of the Convention on Biological Diversity as the focal point for the year and invited the secretariat to cooperate with other relevant UN bodies, multilateral environmental agreements, international organisations and other stakeholders, with a view to bringing greater international attention to the continued loss of biodiversity. Look out for news of local events. n World Water Day, 22 March. This year this World Water Day will focus on transboundary waters. n World Health Day, 7 April. This year World Health Day will focus on the safety of health facilities and the readiness of healthcare workers who provide emergency care.
Q ASSAf News
South African scientists to engage with TWAS South Africa will experience one of the largestever influxes of notable scientists and scholars from the South in October this year when the Academy of Science of South Africa (ASSAf) hosts the meeting and conference of the Academy of Sciences for the Developing World (TWAS). The meeting and conference will be held at the Durban International Convention Centre, South Africa from 19 to 23 October 2009 and more than 400 top scholars and scientists are expected. South Africa’s hosting of the TWAS conference was announced by the Minister of Science and Technology, Dr Mosibudi Mangena in Mexico. Africa was excluded from • Geological, South Geotechnical, Geochemical, Metallogenic and the Marine mapping crucial development phase • Mineralsorganisation’s Development • Construction Materials and Agricultural Minerals during the 1980s, so the event has a special • Water-Resource Assessment and Protection significance. • Environmental Geoscience • EngineeringTWAS Geologyis and Geohazards international anPhysical autonomous • Palaeontology organisation and has as its main mission the • Laboratory Services promotion of scientific excellence and capacity • Geophysics • Seismology in the South for science-based sustainable • Geographic Information Systems (GIS) development. It was founded in 1983 by • Information Databases • NationalPakistani Geosciencephysics Library Nobel prize winner, Abdus Salam, with a small group of distinguished co-founders, and recently celebrated its silver jubilee at a meeting and conference in Mexico City (November 2008) attended by over 250 Fellows. The Trieste-based Academy now has 870 Fellows, of whom all but the 150 North-based Associate Fellows (either born in or have been actively involved in science activities in developing countries) reside and work in developing countries. A majority of them work in India (163), China and Taiwan (148), Brazil (82), Pakistan (32) and Mexico (31), but the list extends to over 70 countries. TWAS has adopted a model of functioning that is essentially similar to that of the Royal Society of London, building on the core notion of a club of the most eminent scientists/scholars to function as a kind of yeast in the sciencedevelopment system of the South. This it has
done by deploying the funds entrusted to it by far-sighted sponsors (most prominently the Italian government, in an unusually effective form of focused development aid) in a highly strategic manner, allowing the bright ideas of its brilliant scholar leaders to flourish independently of bureaucracy, and becoming perhaps the strongest voice of developing countries and, possibly, of truly global science. It must, of course, be remembered that TWAS seeks to serve four-fifths of the world’s population, and its role-model in the UK, around 1% of that number. The Academy of Science of South Africa annually selects a TWAS Young Scientist Awardee and a female scientist (through another off-shoot, the Third World Organisation for Women in Science (TWOWS) focus on women).
Led by the Academy’s Scholarly Publishing initiative, the proposed platform will allow users worldwide to access a wide range of the top peer-reviewed South African academic journals in full on the Internet, at no cost. The project is led by Susan Veldsman (newly appointed as Director of the Scholarly Publishing Unit), a specialist in the field of Open Access. She was previously the project coordinator of the South African Site Licensing Initiative and the project manager at Electronic Information for Libraries (eIFL.) Publication of journals in Open Access format unlocks peer-reviewed scholarly works in their entirety to the end-user. The articles are in digital format, available online at no cost and free from most copyright and licensing restrictions. The project is inspired • Geoscience Museum by a wide-reaching movement towards the • National Core Library implementation of online journals, pioneered by ASSAf leads the way in Open the Scientific Electronic Library Online (SciELO) AccessFOR publishing in MISSION: South To Africa COUNCIL GEOSCIENCE provide expert based in Brazil. This fully indexed information and services to improve the managementproject, of The Academy of Science of South Africa natural resources and the environment for the benefit of the platform has been successfully implemented in (ASSAf) is leading the establishment of an society. eight countries, mostly in Latin America, with Open Access platform for high-quality South others being in the developmental phases. 280 Pretoria Street, Silverton, PRETORIA African scholarly journals. The plan is supported • Private Bag X112, PRETORIA, 0001 SciELO South Africa will be the first site of this and funded by (0)12 the Department of (0)12 Science and Tel: +27 841-1911 • Fax: +27 841-1221 growing system on the African continent. www.geoscience.org.za Technology.
LEADING EARTH-SCIENCE SOLUTIONS
Tianjin science passion echoes around the world One of the Academy’s Editorial Members, Dr Albert Modi, is one of a handful of top young scientists from around the world who released their own statement titled ‘Passion for Science – Passion for a Better World’. Modi, who was recently appointed to the Editorial Board of QUEST – Science for South Africa, was one of 43 young scientists selected by the InterAcademy Panel (IAP) in collaboration with its member academies from around the world. The young scientists represented 32 countries on five continents in the Annual Meeting of the New Champions of the World Economic Forum in Tianjin, China in September 2008. This was part of a pilot venture with the World Economic Forum whereby young scientists were selected to take part in the first IAP Young Scientists Conference held in conjunction with the World Economic Forum's Annual Meeting of the New Champions. The statement emphasises the contribution that young scientists can make to science and that science can make to society. It maintains that public support for science and young scientists, to play the role required in the modern, technological and challenging world, is essential. It calls on young scientists to engage with and educate the public and maintain excellence in science through good governance, the highest standard of ethical conduct in research by all stakeholders, and the freedom to conduct independent research.
LEADING EARTH-SCIENCE SOLUTIONS • Geological, Geotechnical, Geochemical, Metallogenic and Marine mapping • Minerals Development • Construction Materials and Agricultural Minerals • Water-Resource Assessment and Protection • Environmental Geoscience • Engineering Geology and Physical Geohazards • Palaeontology • Laboratory Services • Geophysics • Seismology • Geographic Information Systems (GIS) • Information Databases • National Geoscience Library • Geoscience Museum • National Core Library
COUNCIL FOR GEOSCIENCE MISSION: To provide expert information and services to improve the management of natural resources and the environment for the benefit of the society.
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Q Back page science ‘Comeback' forests rich in biodiversity, say scientists – Eva Aguilar Tropical forests that have re-grown after deforestation are proving to be more biodiverse than previously thought, scientists say. It seems that secondary regrowth of forests is widespread and is leading to areas rich in plant and animal life that can play an important role in conservation efforts in the tropics. In Costa Rica, for example, one study by Robin Chazdon, from the University of Connecticut, found that 176 species (59%) of old-growth tree species were present in second-growth forests. Of 123 species expected to survive only in mature forest, some 94 occur as small stems in second-growth forests. However, William Laurance, of the Smithsonian Tropical Research Institute in Panama, said that though the work was good, Chazdon operates 'mainly in Costa Rica where the situation is pretty positive'. ‘In places like Brazil, regrowth is often burned and cleared again before it has a chance to become very old, so I think that limits its value for tropical species,’ he told SciDev.Net. Source: SciDev.Net
Robot inspects wind energy converters Wind energy converter rotor blades must withstand intense forces – so how can they be checked for any damage? Easy – a robot called RIWEA is ready to help and it’s so good it inspects wind energy converters more precisely than a human ever could. Nothing escapes its sensors, not even the smallest damage – and what’s better it can detect damage even below the surface. It quickly pulls itself up a cable, metre by metre, until it reaches a giant rotor blade, usually made of glass-fibre-reinforced plastics and as long as 60 m. Then it goes to work. Missing nothing, it thoroughly inspects every centimetre of the rotor blade’s surface. It registers any crack or other problem in the material and relays its exact position. In this job, a robot is superior to humans. The researchers at the Fraunhofer Institute for Factory Operation and Automation IFF are experts in robotics and RIWEA is their latest helper. ‘Our robot is not just a good climber,’ says Dr. Norbert Elkmann, Project Manager am Fraunhofer IFF and coordinator of the joint project. ‘It is equipped with a number of advanced sensor systems. This enables it to inspect rotor blades closely.’ Are there cracks in the surface? Are the bonded joints and laminations in order? Is the bond with the central strut damaged? The inspection system comprises an infrared
A robot inspects a wind energy converter’s rotor blades for possible damage. Image: © Fraunhofer IFF
radiator that conducts heat to the surface of the rotor blades, a high-resolution thermal camera to record the temperature pattern and thus register flaws in the material and an ultrasonic system and a high-resolution camera to enable the robot to also detect damage that would remain hidden to the human eye. A specially developed carrier system guides the robot securely and precisely along the surface of a rotor blade, delivering an exact log of the blade’s condition and ensuring humans do not need to risk life and limb on a blade far from the ground. Source: http://www.fraunhofer.de
Medicinal plant extinction 'a quiet disaster' Key medicinal plants used for cancer, malaria and other remedies are being overexploited – potentially putting the health of millions at risk. The warning comes from international conservation group Plantlife. According to their report, almost one-third of medicinal species could become extinct, with losses reported in China, India, Kenya, Nepal, Tanzania and Uganda. Factors in this loss include commercial overharvesting, pollution, competition from invasive species and habitat destruction. The solution, says the report's author, Alan Hamilton, is to 'provide local communities with incentives to protect these plants'. This approach has already proved successful in Uganda, where a sustainable supply of lowcost malaria treatments has been established, and China, which has created a community-run medicinal plant reserve. Ten such grassroots projects are highlighted in the report. ‘Improving health, earning an income and maintaining cultural traditions are important in motivating people to conserve medicinal plants and thus the habitats,’ Hamilton says. Source: New Scientist via SciDev.Net
How ‘puppy-dog eyes’ do their trick: it’s chemistry If you’ve ever wondered how just one doleful look from your dog always makes you forgive that chewed-up shoe – or almost anything else – scientists may have an answer. A dog’s gaze triggers release of the so-called ‘trust hormone’ oxytocin in owners, according to Japanese researchers. Oxytocin, produced by the pituitary gland at the base of your brain, has been implicated in bonding behaviours in animals and humans. Experiments have found that even if you sniff oxytocin it will increase your trust in others. In a new study, Mino Nagasawa of Azabu University in Japan and colleagues placed dog owners together with their pets in a series of half-hour sessions, and the owners were told to look at their pets during the interactions. They measured the levels of oxytocin in owners’ urine before and after the sessions. They found increases in the hormone level that were highly correlated to ‘the frequency of behavioural exchanges initiated by the dog’s gaze,’ they reported. As a check, the researchers arranged another test, except this time the owners were told to not look at their pets during the interactions, and in these experiments, the oxytocin/gaze correlation was absent. In short, it seems that those ‘puppy-dog eyes’ play an important role in how we relate to our pets. Source: World Science via SciDev.Net
Black holes came first, astronomers conclude Astronomers may have solved a cosmic chickenand-egg problem – the question of which formed first, galaxies or the giant black holes found at their cores. Black holes are objects so compact that their gravitational pull is strong enough to suck in everything nearby, including light. Most galaxies have at their centres enormous, or ‘super-massive’, black holes, whose gravity holds the stars in a galaxy together. ‘It looks like the black holes came first. The evidence is piling up,’ said Chris Carilli of the National Radio Astronomy Observatory in Charlottesville, Va. Earlier studies of galaxies and their central black holes in the nearby Universe revealed an intriguing linkage between the masses of the black holes and of the central ‘bulges’ of stars and gas in the galaxies. The black hole consistently weighs about one-thousandth of what the surrounding galactic bulge weighs. The constancy of this relationship ‘indicates that the black hole and the bulge affect each others’ growth’ as a galaxy forms, said Dominik Riechers of the California Institute of Technology, and a member of the research team. ‘The big question has been whether one grows before the other or if they grow together.’ The evidence suggests that the thousand-toone ratio seen nearby may ‘not hold in the early universe. The black holes in these young galaxies are much more massive compared to the bulges than those seen in the nearby universe.’ Source: National Radio Astronomy Observatory and World Science staff via SciDev.Net
Tiny pressure sensor in artery measures blood pressure High blood pressure can be a trial of patience for doctors and for sufferers, whose blood pressure often has to be monitored over a long time until it can be regulated. This will now be made easier by a tiny pressure sensor that is inserted in the femoral artery.
The tiny pressure sensor – depicted here on a finger – measures blood pressure directly in the femoral artery. Image: © Fraunhofer IMS ‘A doctor introduces the pressure sensor directly into the femoral artery in the groin,’ explains head of department Dr. Hoc Khiem Trieu of the Fraunhofer Institute for Microelectronic Circuits and Systems IMS in Duisburg. ‘The sensor, which has a diameter of about one millimetre including its casing, measures the patient's blood pressure 30 times per second and transmits the readings via a flexible micro-cable to a transponder unit, which is likewise implanted in the groin under the skin. This unit digitizes and encodes the data coming from the micro-sensor and transmits them to an external reading device that patients can wear like a cell phone on their belt. From there, the readings can be forwarded to a monitoring station and analyzed by the doctor,’ he says. Source: http://www.fraunhofer.de
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