GLOBULAR CLUSTERS - FACT & THEORY
By Don Clouse
Globular clusters (GCs hereafter). What are they? What is known
about them? How did they come to be? Theyve 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!), theyre everywhere. GCs 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, well discuss the age of globulars and examine
what is known and theorized about their internal dynamics. Well follow that by
taking a look at some external dynamics. Then well end with a survey of theories on
the origins of GCs. A couple of these theories are suggested by exciting recent
observations. But first, lets get an idea of the general physical parameters of
Globular clusters consist almost entirely of stars. There is very
little, if any, interstellar gas and dust in GCs. GCs 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 GCs
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 Suns 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 Suns mass) red dwarves. They burn slowly and are thought to be extremely stable
enduring for billions of years beyond the Suns 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 wont 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
GCs in our galaxy are extremely ancient structures.
Milky Way GCs 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 GCs 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 GCs 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 GCs, 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
CVs), 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
Im 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 Telescopes high-resolution photographs of
the interiors of GCs, some astronomers thought that GCs 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 GCs. 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 GCs 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 Galaxys 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 GCs and temperature distribution of the stars within GCs in the
1970s led to another theory. This study suggested that many GCs, 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
Galaxys 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 GCs, are too obscured by interstellar dust
to be detected visually. Nonetheless, its 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
Teapots 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 GCs 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 GCs 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 Ill 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 havent learned a lot we
have. But, its a big universe. Were really just beginning to scratch the
- "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.
- "Hubbles Universe", Simon Goodwin, Penguin Books, 1996. More pretty
- "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 didnt enjoy the
- "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
- "Our Galaxies Nearest Neighbor", Ray Jayawardhana, Sky and Telescope, May 1998
- "When Galaxies Collide", Joshua Roth, Sky and Telescope, March 1998 issue
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.
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
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