GLOBULAR CLUSTERS - FACT & THEORY
By Don Clouse
Globular clusters (GC’s hereafter). What are they? What is known about them? How did they come to be? They’ve always seemed rather singular objects to me. These huge gravitationally bound, spherical collections of stars seem to exhibit traits of both star clusters and small galaxies, but are really not quite either. Nonetheless, they are ubiquitous in the universe. From dwarf galaxies (the Fornax dwarf has five), to modest sized galaxies (the Large Magellanic Cloud has several), to large galaxies (the Milky Way has perhaps 180, the Andromeda galaxy about 300), to the giant ellipticals (M87 has over 3,000!), they’re everywhere. GC’s have even been resolved as far away (and farther for all I know) as 375 million light years around NGC6166 (a magnitude 12.8 elliptical in Hercules), a member of the Abell 2199 cluster. To address the first two questions above, we’ll discuss the age of globulars and examine what is known and theorized about their internal dynamics. We’ll follow that by taking a look at some external dynamics. Then we’ll end with a survey of theories on the origins of GC’s. A couple of these theories are suggested by exciting recent observations. But first, let’s get an idea of the general physical parameters of globulars.
Globular clusters consist almost entirely of stars. There is very little, if any, interstellar gas and dust in GC’s. GC’s generally have from 100,000 to 1,000,000 stars. However, the giant Omega Centauri cluster, which is probably the largest and most massive in the galaxy, is believed to have several million stars. It is thought that stars within its central core may average as little as one tenth of a light year apart. GC diameters range from twenty or thirty light years up to well over a hundred light years. There seems to be little doubt that the extreme density of GC’s is what holds them together in the face of the vast tidal forces of the galaxy. However, as we shall see, this is not always the case.
Globular clusters in the Milky Way, although generally old, do seem to have a fair range of ages – from 10 to 16 billion years old. (Here we could delve into how these age estimates play into the controversy over the age of the universe, but mercifully, that is beyond the scope of this article.) Age estimates are based on an analysis of the temperature and luminosity distribution of stars within the cluster, its estimated distance, as well as theories/models of stellar evolution. Detailed HR diagrams (plots of temperature/color versus luminosity/magnitude) of the stars in a number of clusters show conclusively that globulars consist largely of mature yellow stars along with some orange and red giants. Recent deeper studies (e.g., with the Hubble Space Telescope) indicate that a significant portion of globular populations consist of red dwarves. Indeed, in most globulars the number of stars of a given mass increases as mass decreases. You will no doubt recall having heard estimates of our Sun’s life expectancy given as about 10 billion years. These estimates result from stellar evolutionary models. These models tell us that the most massive stars ‘burn out’ (consume their supply of nuclear fuel) the quickest ending their brief careers rather spectacularly as supernovae – the live fast and die hard members of the stellar population. While at the other end of the mass spectrum are the small (only a fraction of the Sun’s mass) red dwarves. They burn slowly and are thought to be extremely stable enduring for billions of years beyond the Sun’s expected life. Since globulars contain none of the massive, short lived blue and white giants and super giants, these heavier stars must have long since burned themselves out. Since the stars remaining in globulars are all (well, almost all – there are the ‘blue stragglers’, but we won’t get into that) less massive than the Sun, then globulars must indeed be ancient objects. However, in practice, things are not quite so clear cut. Other factors like intervening gas and dust and variations in metallicity among stars and clusters tend to muddy the waters a bit, making age estimates somewhat uncertain (to put it mildly). Nonetheless, astronomers seem to have the temperature distribution part of the equation pretty much nailed. However, distance estimates seem to be quite a bit more problematic which in turn introduces a degree of uncertainty into the age estimates. Delving into the somewhat arcane realm of astronomical distance estimates is, once again mercifully, beyond the scope of this article. In any case, there is little doubt that even the youngest GC’s in our galaxy are extremely ancient structures.
Milky Way GC’s seem to fall into two populations. About 25% reside within the disk of the galaxy. These globulars have a mean metallicity of 30% of the solar value. (In astronomer speak, any element heavier than hydrogen and helium is a metal.) Most of the members of this group lie within 30,000 light years or so of the galactic center. These GC’s orbit the galactic center more or less within the disk of the galaxy (within about 3,500 light years of the plane) in what is called the "thick disk". These disk GC’s are about 10 billion years old (see previous caveats). The older, larger population (about 75% of the total) form a spherical distribution roughly 100,000 light years in radius centered on the core of the Milky Way. These ‘halo’ globulars tend to have metallicities of only 0.3% to 10% solar. Some also have orbits that periodically take them plunging through the disk of the Galaxy.
The interiors of globular clusters seem to be rather dynamic areas since they harbor a number of energetic processes. Given the great age of GC’s, it is reasonable to expect that a significant fraction of their stellar population will have evolved (or, I suppose, devolved – depending on your point of view) into various types of stellar remnants – white dwarves, neutron stars, and perhaps black holes. Firm evidence has been gathered for accreting white dwarves (cataclysmic variables or CV’s), accreting neutron stars, and millisecond pulsars (also neutron stars). Given the extreme density of the core areas, it is easy to speculate (which is exactly what I’m doing in this instance) that all possible combinations of red giants, normal stars, red dwarves, white dwarves, and neutron stars exist in binary and multiple star systems. Black holes are another possibility.
Before the Hubble Space Telescope’s high-resolution photographs of the interiors of GC’s, some astronomers thought that GC’s might hide black holes at their center. However, even though a few astronomers still support the black hole notion, no convincing data has been collected. A process known as core-collapse is now thought to control or strongly influence the internal dynamics of GC’s. It is suggested that stars at the center form rapidly orbiting contact binary pairs. These act as a sort of energy reservoir which other stars draw on to increase their velocities. These and other types of gravitational interactions ultimately lead to a situation wherein all stars have an equal amount of energy. The least massive stars end up with the highest velocities and tend to migrate to the outer regions of clusters while more massive stars move more slowly and ‘sink’ toward the center of the cluster. This process, known as mass segregation, results in a net loss of energy in the core, hence the term core-collapse. Approximately one-fifth of the globulars in the Milky Way appear to have undergone core-collapse. This process has some rather dramatic implications for globulars whose orbits take them repeatedly through the disk of the Galaxy.
In what are, apparently, the first conclusive results, a long suspected mechanism for the disruption of globular clusters has been confirmed. Astronomers from the European Southern Observatory using the first of the four planned 8.2 meter VLT telescopes have completed a study of the halo globular cluster NGC 6712. NGC 6712, located in Scutum at a distance of 23,000 light years from the Sun, contains somewhat less than 1,000,000 stars. It is currently about 11,000 light years from the center of the Galaxy. This places the globular rather close to the large central bulge of the Milky Way. Indeed, it appears that NGC 6712 has passed through the disk of the Galaxy within the past few million years and has done so repeatedly as it has a rather short orbital period. (Somewhat frustratingly, the source for this info failed to state the estimated orbital period – alas. However, another source did state that GC’s tend to have orbits whose perigalacticon is one-third their apogalcticon distance – pretty eccentic orbits.) The data from the VLT show a deficit of light stars when compared to most other globular clusters. Recall the mass segregation process described above. This preferential migration of lighter stars to the outer regions of globulars makes them more susceptible to being gravitationally ‘stripped’ from globulars when passing through the disk of the Galaxy, especially the denser central regions near the bulge - hence the paucity of low mass stars in NGC 6712. This erosion of globular clusters is almost certainly responsible, at least partially, for the population of halo stars. (This halo is roughly spherical and centered on the Galaxy’s central bulge with a diameter of at least 100,000 light years.) Indeed, there is even some evidence suggesting that the number of globular clusters was much greater in the past – possibly even as many as one hundred times what we see today. It seems quite possible that the disruption of NGC 6712, which is on-going today, is simply the latest chapter in a process which has been at work for billions of years.
Theoreticians have also been addressing the question of globular cluster origins. There seems to be essentially two or three major lines of thought and none are necessarily mutually exclusive. One theory holds that the Milky Way was formed by the accumulation of millions of small gas clouds. As these clouds fell into the nascent galaxy some collided at high speed. The resultant shock compressed the gas thus beginning the star formation process. Perhaps as much as 50% of the gas in these clouds formed stars as opposed to only a few percent consumed in the formation of normal galactic (open) clusters. This very high conversion rate resulted in globular clusters. Studies of the metallicity of GC’s and temperature distribution of the stars within GC’s in the 1970’s led to another theory. This study suggested that many GC’s, especially those at distances in excess of 30,000 light years from the galactic center, were brought into the Milky Way by other galaxies that were perhaps similar to the dwarf galaxies which orbit the Milky Way today. It may even be that the current ‘local’ dwarf galaxies are the remnants of that process. Further, recent findings appear to confirm that this process continues (although presumably at a much decreased pace) even today.
A recent article in "Sky & Telescope" magazine ("Our Galaxy’s Nearest Neighbor", by Ray Jayawardhana, May, 1998) summarizes evidence which strongly suggests that the recently discovered (1994) Sagittarius dwarf galaxy is being assimilated ("resistance is futile" – with apologies to the Borg) by the Milky Way. This includes not only its individual stars but its globular clusters as well. Four globular clusters, once thought to be denizens of our galaxy, are now believed to belong (at least for the time being) to the Sagitarrius dwarf based on studies of their location, distance (on the order of 100,000 light years), and velocity. Visually, Terzan 7, Terzan 8, and Arp 2 (not to be confused with the galaxy of the same name) are dim (magnitude 12.0 to 13.0) and small (1.2’ to 3.5’). M54, however, is relatively large (12.0’) and bright (7.7). Being on the far side of the galaxy, the stars that make up the Sagittarius dwarf, except the GC’s, are too obscured by interstellar dust to be detected visually. Nonetheless, it’s fun to know where it lies in the sky. If you have a copy of the May 1998 S&T, turn to page 45. If you look carefully at the photo of the Milky Way, you can discern the Sagittarius Teapot asterisim in the midst of the superimposed Sagittarius dwarf. Traces of the dwarf begin to the upper left of the Teapot’s lid (Kaus Borealis, Lambda Sgr) and extend through the handle and east side of the pot and then on beyond the handle to the east-southeast for another eight or nine degrees. The three dimmer GC’s are in the portion of the dwarf to the east and southeast of the Teapot. M54, however, lies at the bottom of the Teapot much nearer to the apparent center of the dwarf. In fact, many astronomers now believe that M54 is actually the core of the ‘rapidly’ dissolving Sagittarius dwarf!
A third origin of globular clusters is suggested by studies of the Antennae Galaxies (NGC4038-39) and other interacting galaxies. Dense, bright star birth regions in these galaxies may be GC’s in the making. The short version of the theory is that the collision at high speed of giant molecular clouds of one galaxy with the interstellar gas and dust of the other, results in the creation of hundreds of thousands of stars before any can go supernova. Thus, the giant molecular clouds of both galaxies collapse to form huge clusters of associated stars in a ‘short’ period of time. Hubble Space Telescope photos of the Antennae clearly show these huge, dense, star forming regions that may be the precursors of globular clusters.
I would guess that all of these theories are correct in some degree. Regardless of their origin and exact nature, globular clusters remain fascinating and beautiful objects for observation in our backyard telescopes. In the process of collecting and organizing this information, I have improved my knowledge of globular clusters considerably. Hopefully, you may have picked up a tidbit or two as well that you were not aware of or had perhaps forgotten. Ultimately, having some appreciation of the nature of the objects we view through our telescopes, adds tremendously to the experience. Certainly, this Summer I’ll view M54 with an altered perspective.
Sources (along with a few comments):
- "Globular Cluster Systems", Keith M. Ashman and Stephen E. Zepf, Cambridge University Press, 1998. A difficult, technical treatment, but comprehensive with lots of good info. Also, you begin to get an idea of just how rudimentary and speculative much of our ‘knowledge’ really is. Not that we haven’t learned a lot – we have. But, it’s a big universe. We’re really just beginning to scratch the surface.
- "The Guide to the Galaxy", Nigel Henbest and Heather Couper, Cambridge University Press, 1998. A very readable and completely excellent book. A regular page-turner, crammed full of cool info. Fascinating stuff.
- "The Alchemy of the Heavens", Ken Croswell, Anchor Books, 1995. Another very interesting and very readable book. Wonderful treatment of the evolution of modern knowledge of the Mikly Way and its environs and the astronomers who did it.
- "A Photographic Tour of the Universe", Gabriele Vanin, Firefly Books Ltd., 1996. Pretty pictures.
- "Hubble’s Universe", Simon Goodwin, Penguin Books, 1996. More pretty pictures.
- "Hubble, A New Window to the Universe", Daniel Fischer and Hilmar Duerbeck, Springer-Verlag, 1996. Even more pretty pictures. Hey, pictures are good. I mean, why would we load a bunch of expensive equipment in our cars, drive for an hour, freeze half to death, and go to work on three hours sleep the next day, if we didn’t enjoy the view?
- "Rand McNally Atlas of the Universe", Patrick Moore, Rand McNally, 1994.
- "Our Evolving Universe", Malcolm S Longair, Cambridge University Press, 1996.
- "Galaxies and Other Deep-Sky Objects", Gary Mechler, Alfred A. Knopf, 1995 (a National Audubon Society Pocket Guide). This is a cool little book – and it only costs $5.95!
- "Our Galaxies Nearest Neighbor", Ray Jayawardhana, Sky and Telescope, May 1998 issue
- "When Galaxies Collide", Joshua Roth, Sky and Telescope, March 1998 issue
- http://www.eso.org/outreach/press-rel/pr-1999/pr-04-99.html. Website of the Euopean Southern Observatory. Check this site out occasionally over the next few years as the other three 8.2 meter telescopes come on line.
- http://physun.physics.mcmaster.ca/GC/mwgc.dat. A tabular listing of lots of data on lots of globulars.
- MegaStar 4.0, E.L.B. Software, Willmann-Bell Inc., 1996-1997. A neat charting program that I used to find out exactly where those obscure Sagittarius Dwarf Galaxy globulars are located.
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