1 November 2016 |The Clouds of Magellan
A new bright Nova in Sagittarius flared up on 20 October to Mag 10 before slowly fading from view, and more recently on 28 October a Mag 11 nova was discovered in the Small Magallenic Cloud. This discovery image is from the MASTER-OAFA auto-detection system. November nights are an ideal time to observe and photograph the Magellanic Clouds. The Small cloud transits at 9:30pm and the large cloud transits due at about 2 am, so now is a good time to take a closer look at them in detail. In the southern hemisphere of the sky, the famous sixteenth-century Portuguese explorer Magellan, who died in the Philippines in 1541, noticed two strange luminous patches. They look almost like two detached portions of the Milky Way, and both are easily visible to the naked eye. Today, these objects are known as the Magallenic Clouds or Nubeculae, even though Magellan himself was not their actual discoverer. The first preserved mention of the Large Magellanic Cloud is by the Persian astronomer Al Sufi. In 964 BC, in his Book of Fixed Stars, he called it al-Bakr ("the Sheep") "of the southern Arabs"; he noted that the Cloud is not visible from northern Arabia and Baghdad, but can be seen at the strait of Bab el Mandeb (12°15' N), which is the southernmost point of Arabia. The Clouds never rise in our own latitudes. They are therefore permanently on view for astronomers to study at the European Southern Observatory in Chile. They are of great
importance in astronomical study, and intensive research into their nature and characteristics has been going on during the past half-century or so. The NASA spacecraft Chandra, orbiting high above the Earth, has the best vantage point. It confirmed the first detection of X-ray emission from young stars in the small Magellanic cloud, with masses similar to our Sun, outside our Milky Way galaxy. The Chandra observations of these low-mass stars were made in the region known as the "Wing" of the SMC. In this composite image of the Wing, the Chandra data are shown in purple, optical data from the Hubble Space Telescope are shown in red, green, and blue, and infrared data from the Spitzer Space Telescope are shown in red. You can see more images from Chandra at the end of the program. Here on the ground the visual work must naturally be done from a southern station, but much of the theoretical work is carried out in the north, and part of the investigations has been played by the Observatory of Armagh. The Armagh Observatory, in Northern Ireland, has a long and honourable history. It is the oldest British observatory still operating from its original site; Greenwich Observatory is now at Cambridge University in the UK. The founder was Archbishop Robinson, a great man in the history of Armagh; memorials to him are to be found all over the city. During the closing years of his life, Archbishop Robinson was anxious to set up an Ulster university. This project was never realized, but the founding of the observatory, due entirely to Robinson, was probably connected with the university scheme. The Archbishop's generosity led to the setting-up of the observatory in 1790, and in July of that year the Rev. Dr James Hamilton was appointed Director. Since that time Armagh has maintained a fine record of observational and theoretical work. There have been a number of Directors, including Mark Bailey the present holder of the office; special mention should be made of Dr Romney Robinson (no relation to the Archbishop) whose regime extended from 1823 to his death in 1882, and Dr John Louis Dreyer, who is remembered for his work in connection with the New General Catalogue of star-clusters and nebulae. There are many historical exhibits in the present observatory, there is, for instance, a mirror made by Sir William Herschel, discoverer of the planet Uranus and 'explorer of the heavens', together with letters written by his sister Caroline. Herschel was well acquainted with Archbishop Robinson, and the two used to visit Bath in the UK together from time to time, even after Herschel had ceased to be organist there, and had begun to devote all of his time to astronomy. Equally interesting is a small reflector which was owned and used by no less a person than King George III, who was keenly interested in astronomical Science. Historic instruments at Armagh include the Troughton equatorial, which is 2-inch aperture, and 3-feet focal length, and arrived there in December 1795. It was mounted in the English style, under the south dome on two stone piers.
The Calver telescope, shown here, was made by the famous George Calver, in 1883, for a Colonel Tupman, of Harrow, who was a wealthy amateur. Tupman spent £800 on the telescope, and a further sum on a suitable dome. There is the 15-inch Newtonian-Cassegrain Reflector by Grubb, constructed in 1835, and there is also the Grubb 10-inch refractor that was mounted on 28th July, 1885. Here is the transit Instrument by Thomas Jones that was installed in 1827. It is of 3-inch aperture, and 63 inches’ focal length. The Armagh observatory also has some rather nice celestial globes. This is the Celestial Globe by L.H. Bardin. The inscription reads: “To the Rev. Nevil Maskelyne, DD, FRS Astronomer Royal. This new British Celestial Globe containing the Positions of nearly 6000 stars, clusters, nebulae, planetary nebulae, &c. correctly laid down to the present period from the latest observations and discoveries by Dr Maskelyne, Dr Herschel, and The Revd. Wollaston…” There are also some nice orreries which you can see here. Excellent though they are, so-far as quality is concerned, the Armagh telescopes are small compared with the European Southern Observatory giants such as the 8.2 meter Very Large Telescope at Chile. Conditions in Ireland are not ideal for astronomical observation, although admittedly they are no worse than in many other parts of Europe (including England). For these reasons, a link was formed with the famous Harvard Observatory in the United States and with the Dunsink Observatory at Dublin. The result was the setting up of the 'A.D.H.' (ArmaghDunsink-Harvard) telescope at the Boyden Station of the Harvard Observatory, near Bloemfontein in South Africa, with the intention of studying objects which could never be seen from northern latitudes. The project dates from 1950. In 1955 the Swedish, Belgian, and West German Observatories Joined with Harvard and the two Irish Observatories to form the Boyden Observatory. The A.D.H. telescope at Boyden is a 36-inch reflector. Use is also made of other instruments at the observatory, notably the 60-inch reflector. The ADH telescope is of the Baker-Schmidt type, being designed by Professor James Baker of Harvard University in the US. It came into operation in 1950, and in 1951 the then world's largest objective prism, 33" in diameter, was provided from funds contributed by Harvard and the Department of Astronomy at Queen's. The telescope was placed on the mounting of the 24" Bruce refractor at the Boyden observatory. The two-mirror and corrector system give a flat field of view and reduced tube length 168" (4.6m) compared with the classical Schmidt telescope. Among the objects studied with photographs from the ADH have been globular star clusters, cluster Cepheid’s, and the Magallenic Clouds. The ADH optics is currently at the Dunsink Observatory, though still jointly owned by the two Irish Observatories.
In 2001 the telescope received a major upgrade. This upgrade included a new control system, and a new primary light shield, and today the telescopes are used by the general public on clear nights for observing sessions. The Harvard station was superseded in 1962 by the European Southern Observatory which is a 16-nation intergovernmental research organization for ground-based astronomy. Over the past 64 years ESO has provided astronomers with state-of-the-art research facilities and access to the southern sky; its observatories are located in northern Chile. In 1951 the conditions were excellent, and the programme which has been undertaken is a very full one. Emphasis being placed on the two Clouds of Magellan. There are two clouds; the Large (NubĂŠcula Major) and the Small (NubĂŠcula Minor). They are not, in fact, detached parts of the Milky Way; they are galaxies in their own right. Each is about 180,000 lightyears away, so that the Clouds are considerably nearer than any other external systems. Our Galaxy is made up of perhaps 100,000 million stars, of which the Sun is one. The system is arranged in a flattened form, so that the familiar Milky Way effect is nothing more than an effect of perspective; when we look along the main axis of the Galaxy, we see many stars in much the same direction. Together with stars, the Galaxy contains huge clouds of gas (nebulae), and clusters of various kinds; there are open or loose clusters, of which the Pleiades group is the best known, and globular clusters, such as Messier 13 in Hercules. The two brightest globular clusters, Omega Centauri and 47 Tucanae, are, like the Clouds of Magellan, too far south to be seen in Europe or the northern United States. The closest of the really large outer galaxies is Messier 31 in Andromeda, known as the Great Spiral, which is dimly visible to the naked eye on a clear night. This system appears to be considerably larger than our Galaxy, but is very remote, since it lies at a distance of more than 2,000,000 light-years. The Clouds of Magellan are seen to be relatively near to us on the cosmical scale. The Clouds are also relatively close to each other. The distance between their centres is about 75,000 light-years, and are actually connected, forming a twin system; for instance, radio astronomers have been able to show that the two Clouds are embedded in a common envelope of hydrogen gas. The Clouds are also satellites of our own Galaxy, and move around it, taking an immensely long time to complete one revolution. There is no doubt that the Clouds are true members of the so-called 'local group' of galaxies. Broadly speaking, there are three main types of galaxies: elliptical, spiral, and irregular. In a spiral, such as Messier 31 in Andromeda, there is a central nucleus, with highly luminous, hot blue stars that are young in their evolution. In the spiral arms, there are a high proportion of red giant stars, which are well advanced in their development, together with clouds of gas and dust. Parts of the Large Magallenic Cloud are seen to be of the 'spiral-arm' type, and there is inconclusive evidence of an incipient spiral structure; there is certainly a main axis with a great concentration of stars. The smaller Cloud is different; there is no nucleus, no symmetry, and nothing in the nature of a spiral arm, so that the Small Magallenic Cloud is best classified as an irregular galaxy. There is one marked extension, but this is probably due
to tidal effects caused by the Large Cloud. All things considered, efforts to find order and structure in the two Nubeculae have not so far proved very rewarding. What is far more important is the fact that the Clouds contain objects of the same kind as are found in our own Galaxy, and we may assume that all objects in the Clouds are at the same distance from us. It was by studying the short-period variables in the Small Cloud, 100 years ago, that Miss Henrietta Leavitt, at Harvard - using photographs taken from southern stations - made the discoveries that led on to the 'period-luminosity law' of Cepheid stars. These Cepheid’s brighten and fade regularly, and their behaviour can always be predicted. Miss Leavitt found that the longer the period of a Cepheid, the brighter it looked; since she could assume that all the Cloud Cepheid’s lay at the same distance, it followed that the longer-period stars were genuinely the more luminous. Without the Clouds, this farreaching discovery would have been very difficult to make. At Armagh, from 1953-1974, Dr Lindsay and his assistants compiled a survey of various types of objects photographed in the Clouds with the A.D.H. telescope. There are clusters, both open and globular; there are planetary nebulae, and also gaseous nebulae. A little earlier I mentioned NASA’s Chandra spacecraft which made X-ray observations of low-mass stars in the region known as the "Wing" of the SMC. Most star formation near the tip of the Wing is occurring in a small region known as NGC 602, which contains a collection of at least three star clusters. One of them, NGC 602a, is similar in age, mass, and size to the famous Orion Nebula Cluster theta Orionis. Researchers have studied NGC 602a to see if young stars have different properties when they have low levels of metals. Using Chandra, astronomers also saw extended X-ray emission, from the two most densely populated regions in NGC 602a. The extended X-ray cloud likely comes from the population of young, low-mass stars in the cluster, which have previously been picked out by infrared and optical surveys using Spitzer and Hubble space telescope. This emission is not likely to be hot gas blown away by massive stars, because the low metal content of stars in NGC 602a implies that these stars should have weak stellar winds. The Chandra results imply that the young, metal-poor stars in NGC 602 produce X-rays in a manner similar to stars with much higher metal content found in the Orion cluster in our galaxy. NASA's Hubble Space Telescope has peered deep into the SMC to reveal details of the formation of new stars. Hubble's target was a newborn star cluster catalogued as N81. The new images show young, brilliant stars cradled within a nebula, or glowing cloud of gas. These massive, recently formed stars inside N 81 are losing material at a high rate, sending out strong stellar winds and shock waves and hollowing out a cocoon within the surrounding nebula. The two most luminous stars, seen in the Hubble image as a very close pair near the centre of N 81, emit copious ultraviolet radiation, causing the nebula to glow through fluorescence.
Outside the hot, glowing gas is cooler material consisting of hydrogen and dust. Normally this material is invisible, but some of it can be seen in silhouette against the nebular background, as long dust lanes and a small, dark, elliptical-shaped knot. It is believed that the young stars have formed from this cold matter through gravitational contraction. Few features can be seen in N 81 from ground-based telescopes, earning it the informal nick-name "The Blob." Astronomers were not sure if just one or a few hot stars were embedded in the cloud, or if it was a stellar nursery containing a large number of less massive stars. Hubble's high-resolution images shows the latter to be the case, revealing that numerous young, white-hot stars within N 81. A planetary nebula consists of a central star surrounded by a ring or shell of gas, and is not appropriately named, since it is not, strictly speaking, a nebula - and is certainly not a planet. The brightest example in our own Galaxy, the Ring Nebula in Lyre Messier 57 (not far from the brilliant bluish Vega) is visible with a moderate telescope. Planetary nebulae in the Clouds are too remote for their structure to be seen, but their spectra, as obtained with the A.D.H. telescope, reveal their true nature, and about 150 have now been found. Also, to be seen are a few objects which look very much like the wrecks of supernovae, or stars which exploded long ago, and never returned to their original state. Another nova flared up a few days ago on 28 October 2016, which will leave behind a remnant of the dying star. The Magellanic Clouds have also been studied by NASA’s Swift ultraviolet space telescope, which has shown a lot of fine detail in our satellite galaxies. Video: 4 Minutes In 1987 a supernova was seen in the outskirts of the Tarantula Nebula in the Large Magellanic Cloud, and it was bright enough to be seen with the unaided eye. It was the closest observed supernova since Kepler’s Nova of 1604, which occurred in the Milky Way itself. It was discovered by Ian Shelton and Oscar Duhalde at the Las Campanas Observatory in Chile on February 24, 1987. As it was the first supernova discovered in 1987, it was labelled “1987A”. Its brightness peaked in May with an apparent magnitude of about 3 and slowly declined in the following months. It was the first opportunity for modern astronomers to study the development of a supernova in detail, and observations have provided much insight into core-collapse supernovae. Of special importance, 1987A provided the first chance to confirm by direct observation the radioactive source of the energy for visible light emissions by the detection of predicted gamma-ray line radiation from two of its abundant radioactive nucleus. This proved the radioactive nature of the long-duration post-explosion glow of the supernovae. 1987A appears to be a classical type-II supernova, which should have resulted in a neutron star. The neutrino data indicate that a compact object did form at the nebula’s core. However, since the supernova first became visible, astronomers have been searching for the collapsed core but have not detected it. The Hubble Space Telescope has taken images of the supernova regularly since August 1990, and, so far, the images have shown no evidence of a neutron star.
The three bright rings around 1987A are material from the stellar wind of the original star that exploded. These rings were ionized by the ultraviolet flash from the supernova explosion, and began emitting in various emission lines. These rings did not illuminate until several months after the supernova. The rings are large enough that their angular size can be measured accurately: the inner ring is 0.8 arc seconds in radius. The material from the explosion is catching up with the material expelled during both its red and blue supergiant phases, and heating it up, so we can observe the ring structures of the nova remnant. About two to three hours before the visible light from 1987A reached Earth, a burst of neutrinos was observed at three separate neutrino observatories. Transmission of visible light is a slower process that occurs only after the shock wave reaches the stellar surface. Of the gaseous nebulae in the LMC, the most striking is known as the 'The Tarantula'. It is of the same type as the celebrated Orion Nebula in our own Galaxy, but is much larger. The Orion Nebula, which may be seen with the naked eye in the Hunter's Sword, is 1,600 lightyears away and about 25 light-years in diameter; its mass is about 100 times as great as that of the Sun, and it is one of the sites where fresh stars are being created from interstellar material. The Tarantula Nebula in the Large Cloud of Magellan is 800 light-years across. If it lay in our own system, it would be a magnificent object. Embedded in it are Clusters of hot, young, very luminous stars. The Tarantula Nebula, also known as 30 Doradus, shines at Mag 8 and covers an area of 40’ by 25’ arc minutes. 30 Doradus has at its centre the star cluster NGC 2070 which includes the compact concentration of stars known as R136 that produces most of the energy that makes the nebula visible. The estimated mass of the cluster is 450,000 solar masses, suggesting it will likely become a globular cluster in the future. In addition to NGC 2070, the Tarantula Nebula contains a number of other star clusters including the much older Hodge 301. The most massive stars of Hodge 301 have already exploded as supernovae. The Hubble Space Telescopes’ ‘Hubble Tarantula Treasury Project’ survey, or HTTP, has revealed new insights on the star formation history of Hodge 301. According to a study published on 19 October 2016, this star cluster could be several million years older than previously estimated. The cluster is known for its high supernovae activity as scientists have calculated that at least 38 stars within it have already exploded as supernovae. Previous studies have estimated that the cluster's age ranges from 20 to 25 million years. Now, an international team of astronomers, led by Michele Cignoni of the University of Pisa in Italy, have used the photometric capabilities of the Hubble survey to better understand Hodge 301's formation history and measure its age more accurately. For their study, they focused on the optical colour-magnitude diagram of H301, the only one deep enough to reach 30-million-year-old pre-main sequence stars. In effect, they found that this cluster could be over 29 million years old.
Taking the new calculations into account, they added that Hodge 301 is much older than the bulk of the stars in NGC 2070, the most active region of the Tarantula Nebula. However, it is only slightly older than its oldest stars, which are approximately 20 million years old. Besides estimating the cluster's age more accurately, the team managed to calculate Hodge 301's total stellar mass and its total reddening. According to the paper, the cluster has a mass of about 8,800 solar masses. It is also interesting to note that the Large Cloud contains S Doradus, which has the distinction of being the most luminous star known. It is equal to about 1 million Suns - and yet is so far away from us that without a telescope it cannot be seen at all. It is variable, and is using up its 'nuclear fuel' at a fantastic rate, so that it must have a relatively short expectation of life in its present form. The Sun will not change much for 5 thousand million of years; S Doradus can hardly continue squandering energy at its current rate for more than a million years to come. Great attention is being paid to the star-clusters in the Clouds, and by now many have been detected; about 120 in SMC, almost 1,000 in the Large Magallenic Cloud. Most of these are of the open type, though there are also numbers of globular Clusters. From their distribution, it is possible to estimate the maximum dimensions of the systems, and it has been found that the diameters are 7,000 light-years for the Small Cloud, and double this for the Large Cloud. Even the senior of the two is much smaller than our Galaxy, which is about 100,000 light-years across: even so, the Clouds are certainly not dwarfs. The LMC lies about 160,000 light years away from us, while the SMC is around 200,000. Astronomers consider that we are fortunate in having the two Magallenic clouds near at hand. Studies of them give us considerable insight into the anatomy or structure of galaxies in general, and also yield information about the evolutionary processes of the stars and starsystems. It is probably true to say that without the Clouds our knowledge of the universe today would be markedly less than it actually is. There are no other comparable systems within range; the Andromeda and Triangulum spirals are much more remote, so that they cannot be examined so easily, and the remaining galaxies of the local group are much less informative. At Armagh, the 'old' meets the 'new'; the observatory itself is a charming old building set in picturesque surroundings, and the modem domes housing the Schmidt telescope and the 10-inch refractor fit well into the picture. At the end of the program you can see again all of the deep sky images of the Magallenic clouds & historic science instruments in the Armagh Observatory in Northern Ireland. I have also included all of the images taken by the Chandra X-ray telescope during 2016. The accompanying music named ‘Halley’s comet,’ was composed by Sir Patrick Moore, and is played by the Scottish National Orchestra. Well that is all we have time for this month, thank you for watching. If you are interested in astrophotography I recommend Astrophotography Ezine, which is Free and is full of useful tips and nice images; here is a link to the website.
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Richard Pearson FRAS