Science with Paul Scowen: Gaining Insight

into Star Formation Using Images of Nebulae

Paul Scowen

Editor's Note: It is astonishing to me to reflect that we human beings had no idea of the extent of our universe before about one hundred years ago. Not until the early 1920’s, less than twenty years before I was born, was it known that many of the fuzzy little patches between stars in the night sky, called nebulae and catalogued by Charles Messier in 1781, are themselves collections of hundreds of millions of stars: galaxies, much like the Milky Way that we have named our own galaxy. Now we know that there are at least as many galaxies in the observable universe as there are stars in a single average galaxy. This awareness grew from Edwin Hubble’s initial findings in 1919 based on observations through the then revolutionary 100-inch Hooker telescope at the Mount Wilson Observatory, through increasing observational powers in observatories around the world, to the knowledge of the universe we now have, thanks to telescopes of many varieties and capabilities surrounding us in space, most especially the Hubble telescope, launched in 1990 and still in use today.

In addition to the cloud-like objects that are recognized as galaxies in and of themselves, the Messier catalog also includes nebulae that actually exist within the Milky Way. These are primarily local accumulations of gas in the process of new star formation.

Of the elect astronomers and astrophysicists who have been privileged to pursue their research with the aid of modern instruments, including the Hubble telescope, one is our own Paul Scowen. A member of the Emeritus College, Paul is Emeritus Research Professor, ASU School of Earth & Space Exploration, and is now Senior Research Astrophysicist at the NASA Goddard Space Flight Center. He lives in Maryland with his wife, Linda, long a staff member of the Department of Physics. In this pictorial of Milky Way nebulae, Paul Scowen introduces us to his science: the birth of stars. R. J.

When we look out at the night sky, from a sufficiently dark location, the majesty of the heavens can be awe inspiring, especially for those of us who grew up under the lights of a major city—in my case, London. Scattered across the sky there are patches of “fuzz” that are termed nebulae. Astronomers are interested in these objects for a variety of reasons that go beyond their photogenic nature—many representing a region of recent star formation. It is only star formation events of sufficient size that produce massive stars, and it is these massive stars that light up the nebulae we see on a clear night from Earth. The emission we see from such nebulae is a suite of radiation emitted when ionized elements such as hydrogen, oxygen, sulfur and other materials recombine with electrons in a gaseous phase to emit the discrete colors of light we see as the structure of the nebulae.

Since only the most massive stars are capable of generating enough ionizing radiation to be seen, and because these massive stars live only a few million years before exploding in a supernova event, we can use the existence of visible nebulae to mark the locations of recent star formation. As such a map of nebulae is a map of recent star formation. This is a very useful tool and has allowed me to study some of these objects to gain further insight into the star formation process itself. Here, I will present some images of nebulae that I have been involved in acquiring and presenting, with some discussion about what we are learning in each case. I hope you will enjoy this journey.

Ironically, we start our journey with an example of the aftermath of a supernova explosion: a supernova remnant, in this case the famous Crab Nebula, the first object in Messier’s catalogue, M1. This object formed very recently, being observed as a bright star for a short period by Chinese astronomers in the year 1054, after which it dimmed. A supernova is the explosive end to a massive star’s life that happens only 10-20 million years after it forms, as massive stars burn their available fuel far faster than smaller stars, such as the Sun. When such a star explodes, outer layers of gas—oxygen, carbon and nitrogen formed during their life cycle—are then driven away at great speed.

At the heart of the nebula is the remnant of the star, generally a compressed neutron star, whose material is so dense that just a teaspoonful of material weighs over 5.5×1012 kg, but it is small—only about ten km across (about the size of Tempe, AZ.) And such stars spin quickly—in the case of the Crab neutron star, thirty-two times a second. Such an object can be hard to imagine within our Earth frame of reference. The Crab neutron star is actually called a pulsar since it has a strong magnetic field that is not aligned with its rotation axis, and it beams pulses of energetic radiation towards the Earth as it spins—when first observed, this was initially thought to be evidence of extraterrestrials because it was so precise and regular.

The images of the nebula presented here were originally taken back in the late 1980s by ASU Emeritus Professor Jeff Hester at the Palomar 60-inch telescope using a focal reducer camera—a device that shortens the focal length of the telescope, thereby increasing its field of view and yielding a brighter image for short exposure times. Using this device, images were taken both in narrow-band emission lines from ionized hydrogen, oxygen and sulfur, as well as broad-band continuum emission. Each tells a very different story.

The first image is the narrow-band image. It needs to be realized that colors you see in this image are not true color; this is not how the nebula would appear if you were floating in space nearby. The science is in the color separation as it indicates areas where one element is denser and thereby emitting brighter than others. What you see here is radiation from the outer layers of the original massive star stimulated by strong radiation from the central pulsar. You can clearly see the filament structure of the outer layers of the nebula formed by instabilities in the gas as it expands and is pushed on by the magnetic field of the pulsar.

The second image is a broad-band image that captures radiation directly from the interior of the nebula, which is dominated by synchrotron radiation from electrons orbiting around the pulsar’s strong magnetic fields.

Structure

in the emission reflects structure in the magnetic fields as they are wound so very tightly by the rapid spinning of the central star from which they originate. This emission is highly polarized and it is possible to generate maps showing variation of the polarization variation with position.

structure columns poking into to capture differences in gas recombination energies as testified by different atomic species: green is ionized hydrogen, blue is doubly ionized oxygen, and red is ionized sulfur. used representory color, as

The Crab Nebula is a special object because it is so close and is relatively isolated. When many supernovae explode, their remnant is driven into the nearby interstellar medium, but in this case the Crab resides in a void and so all the light you see originates with the progenitor star itself. One unique feature from the very deep color images we obtained is the so-called “chimney”—the tube-like structure that sticks vertically up from the nebula. Few images of the Crab are deep enough to capture this wispy structure. Its origin has been debated and theories have been offered, but a full self-consistent explanation has not been settled upon.

The Eagle Nebula is the 16th object in Charles Messier’s catalogue and is a cloud of ionized gas surrounding a cluster of stars. In that regard it is unremarkable, but it became the object of some interest because of the columnar structure that poke out into the ionized volume of the nebula. Most nebula are hollowed out cavities, and the study of the walls of those cavities is complicated because you have to look through the material at the interface to see the details and this can make interpretation difficult. The Eagle, as seen above, has three columns of darker gas and dust that noticeably protrude into the interior, effectively inverting the problem—now we can look at the details of the nebula wall structure in section as we look at the edges of those columns. What we found when Jeff Hester and I made the observations with Hubble was that the interface between the tenuous interior of the nebula and the denser walls was very narrow indeed—only about the width of our solar system—which placed very tight constraints on how radiative energy could be transported through that interface. Why did we care? Exterior to these boundaries the gas is piled up, compressed by the expansion of the nebula—it is not a static structure—the radiation and strong stellar winds from the massive stars actively erode the walls and grow the size of the nebula—that force of erosion compresses the gas behind the wall and triggers the formation of new, second-generation stars. Our observations confirmed, for the first time, that this process is happening, and allowed us to constrain models of how the expansion works. You can see the erosion at the top of the left-hand pil- lar in the Hubble image—the blue streamers of material are gas being radiatively heated and then stripped by the stellar winds.

There is a need for some clarity on the nature of the columns we see in these images. They are big—the left-hand column is three light years long—it takes light three years to go from the top to the bottom. They look solid, but this is the effect of integrated line-of-sight absorption by interstellar dust—they are still so tenuous that the space densities of material are more akin to the best vacuums we can pull in laboratories here on the Earth. The dark columns are there because the gas at the top is denser than the surroundings and the ionizing stars, off the top of the image are in the plane of the page and those caps shadow the gas and dust below them leaving them unaffected by the erosion of the stellar winds, so while the gas between the columns was swept away, the columns remain.

As with the Crab, a lot of the science in these images is captured by the color separation we can see in the images—the fact that each of the ionized species portrayed have very different ionization energies means that color maps to energy (and density) levels in the nebula and this tells us a lot about the dynamics of such structures. As an aside, we can use this as an excellent example of how we create such images, computationally. Hubble does not take color pictures as you and I would term them—they are black-and-white images taken through a particular transmission filter, or colored piece of glass, that isolate the light in a particular narrow wavelength or color band. We then use the RGB color channels on a computer to represent 3 images together to produce the color image we are familiar with. This process is captured in the image sequence above.

Arrayed around the Milky Way Galaxy we call home are a set of smaller galaxies—two of them are known as the Large and Small Magellanic Clouds after the explorer who noted them on his travels into the southern hemisphere and around Cape Horn. These smaller galaxies are excellent nearby microcosms of star formation and allow astronomers a remote view of the processes that govern and shape galaxies—the Milky Way is tough to use for this since we are so deeply embedded in that galaxy ourselves. The Large Magellanic Cloud (LMC) is host to a particularly large region of star formation, by our local standards, named 30 Doradus (after the bright “star” at its center) or the Tarantula Nebula. This complex of star formation is so large when compared so say the Orion or Eagle Nebulae that if placed at the same distance from the Earth as those objects, the Tarantula would cover more than 30% of the night sky. At the heart of this nebula is a complex of stars that are so tightly packed that there were originally thought to be a single object, but the advent of the Hubble Space Telescope, as well as optical interferometric techniques, revealed the heart of the nebulae to be a cluster of many hundreds of massive stars. The Eagle by comparison has 6.

30 Doradus as seen with the Hubble Space Telescope. The larger nebula has a whole range of astronomically interesting objects such as high proper motion stars, standalone stars blowing ionized bubbles, but two sets of physical analogs to the Eagle Nebula structures, proving this phenomenon occurs commonly in massive star forming regions.

In the wake of our work on the Eagle, we decided to go after the Tarantula with Hubble to see what was going on there, not in the stars, but in the gas and dust surrounding those stars. The structure around the central cluster is riddled with a menagerie of astronomical objects such as supernova remnants, individual stellar clusters in their own right, and stars in the process of forming, as well as those well along into their middle age blowing extended bubbles from their stellar winds. What we did find, though, was a physical analog to the columns we saw in the Eagle, proving that these kinds of structures, and the mechanisms that form them, are universal and are therefore part of the equation to be used when considering how stars and planets form in the nebula environment around massive stars.

Another nearby galaxy to our own is the Pinwheel Galaxy – visibly large enough to be named as a Messier object (M101) in the night sky, but in reality, more than ten times farther away than the LMC and several times larger than our own Milky Way. This object is classified as a across all radii. The large regions of star formation in the outer disk are large enough and bright enough to have their own classification number separate from now adding hydrogen colored in yellowthe “J” shaped pattern is visible in the image of the entire galaxy above directly south of the nucleus. (lower left) the same field with the stars removed and the light of ionized oxygen and sulfur added—the color scheme here is the same as that used in the Eagle nebula pictures. (lower right) the lower right part of the previous panel enlarged for greater detail—note the arc-like structures typical of supernova remnants, as well as adjacent bright structures that appear to support the idea of triggered star formation where age and size map to physical location and translation from site to site.

“grand design spiral” as it has very well-defined spiral arms threaded by active star formation from the interior all the way out to the outer reaches. It is also seen face-on making analysis and exploration much less complicated. This object is personally special to me as it was the subject of my PhD thesis work at Rice University back in the late 80s.

The excellent viewing angles and uncluttered and uncompromised aspect to this galaxy make it very attractive for star formation studies. We took images with the Hubble Space Telescope of a region in the southern disk to “look around” and look for evidence of triggered star formation: the idea that star formation occurs in a particular place because of either existing star formation nearby—the macroscopic embodiment of the principle we saw in the Eagle—or because of cloud-cloud collisions or a supernova event. Star formation as a process requires some kind of energetic trigger to overcome the balance in interstellar clouds between gravity and heat to cause collapse to make stars. The images we acquired are below.

What we see in these images is a comprehensive sampling of the types of structures associated with star formation and the propagation of star formation in a typical example of a spiral galactic disk. The emission from ionized hydrogen is pervasive and shows a large number of ongoing sites of star formation coupled with older structures that are more related to the later stages of stellar evolution and development. The addition of emission from ionized oxygen and sulfur in the third panel provides additional morphological and radiation information and allows us to see where some structures are more likely to be supernova remnants or planetary nebulae. The fourth panel shows enlarged detail from the third panel providing some clear and very well-defined examples of potential triggered star formation in addition to the types of structures mentioned above.

Our final example is from our own backyard, back in the Milky Way Galaxy. This is not a region of new star formation, but instead a windblown bubble around a single star, well into its middle age. The bubble is visible because the wind from the star has run into a pre-existing gradient of interstellar gas and dust, which is more concentrated to one side than the other, which is why the central star is not at the center of the bubble. The Hubble image shows a bright ridge of material seen to one side of the star coupled with a set of illuminated structures well outside the bubble. The former is a pre-existing interstellar cloud that the expanding bubble has run into and has now wrapped around. The latter are a similar set of pre-existing structures that the bubble has yet to reach. Wispy structure in the walls of the bubble itself can be seen in blue indicating the non-smooth nature of the medium into which the bubble is expanding. Measuring the brightness of the gas emission in this image allows us to estimate the specific luminosity of the star and any variance in transparency of the medium inside the bubble with position angle around the star.

Ray Bradbury