Kyle M. Cudworth
Globular clusters were originally defined as rich, compact, nearly
spherical, groups of hundreds of thousands (or even millions) of
stars. Work in the past few decades has shown that the stars in
globular clusters are among the oldest stars in the Galaxy, with ages
greater than 1010 years. As a consequence, for most astronomers the
working definition of a globular cluster now concentrates more on the
age of a cluster than on its richness, and a few are quite
sparse. About 140 of these clusters are now known in our Milky Way
galaxy. The brightness and distinctive appearance of globulars make
them relatively easy to detect at large distances (except in
directions where dust very severely absorbs starlight), so it is
likely that most that exist in our Galaxy have been
discovered. Furthermore, globular clusters are found in the galactic
halo, well above and below the thin disk of the Galaxy that contains
most stars and the younger open clusters. (The galactic halo should
not be thought of as a shell, but rather as a roughly spherical volume
of space within which globular clusters and some old stars are found.)
While globulars are strongly concentrated toward the center of the
Galaxy, some are found at very large distances from the galactic
center. These characteristics are major reasons why globulars are key
objects for the study of distant parts of the Galaxy. Not
surprisingly, globular clusters are also seen in and around other
galaxies.
The stars in globular clusters have been found to differ in chemical
composition from most stars in the galactic disk in that globular
cluster stars are depleted in heavy elements (metal poor) by factors
ranging from at least 2 up to 200. In most clusters all stars have
very similar chemical compositions, but the composition differs from
cluster to cluster.
Because the spatial distribution and chemical composition of the
globulars are thus distinctly different from those of most stars,
these clusters reveal a different aspect of galactic structure than
ordinary stars, and because the clusters are the oldest identifiable
objects in the Galaxy, these differences contain information regarding
the formation and early evolution of the Galaxy. Globular clusters
thus provide much, probably most, of the basic observational data on
which any understanding of the early epochs of our Galaxy must be
based. We seek to learn more about these early stages in our Galaxy's
history in order to understand how the Galaxy came to have its current
structure and other characteristics, and we expect that much of what
we learn about our Galaxy's history will also be applicable to other
galaxies as well.
In an early (1915-1919) use of globular clusters in what we would now
call a study of galactic structure, Harlow Shapley derived distances
for many globulars, and found that the distribution of globulars was
centered at about 15 kpc (kiloparsecs, 1 kpc = 1000 pc = approximately
3260 light-years) away from the Sun in the direction of the
constellation Sagittarius. Shapley based his cluster distances upon
the brightnesses of individual stars in each cluster when possible,
and on the size and brightness of each cluster as a whole when
individual stars could not be studied. Since many of the globulars
that Shapley studied are out of the dusty plane of the Galaxy, the
distances that he found were not too severely affected by the lack of
a correction for the absorbing affects of dust. Shapley went on to
argue that such massive objects as globular clusters were most likely
to be centered around the galactic center, and that thus the center of
the Milky Way was 15 kpc away from the Sun toward Sagittarius. Similar
studies in the 1970s and 1980s with absorption corrections and with
much better data from which to determine cluster distances have
yielded distances to the center of the distribution of about 8 instead
of 15 kpc.
While Shapley's conclusions remained controversial for a few years,
they were eventually accepted by essentially all astronomers, and this
technique is still considered one of the primary means of determining
the distance to the center of the Galaxy. This classic work by Shapley
was crucial in that it was the first study of the structure of our
Galaxy to indicate a center well away from the Sun. This shift away
from a heliocentric (Sun-centered) Galaxy may even be likened to the
Copernican shift away from a geocentric (Earth-centered) solar system
(and universe) that had occurred a few centuries earlier, but the
shock from Shapley's studies was significantly less severe from either
an astronomical or a philosophical/religious point of view.
In the 1940s, Walter Baade developed the concept of stellar
populations to describe the differences between the stars commonly
found in the thin Galactic disk (Population I) and those distributed
spherically about the galactic center (Population II). Globular
clusters were and are the primary example of Population II, or the
halo population. We now understand the fundamental difference between
these populations to be age, and that the differences in the types of
stars, their chemical composition, and their distribution in the
Galaxy, are all related to their ages. Further work has broken the
division down into more populations, including a distinction between
extreme Population II and intermediate Population II that was noted in
the 1950s and has been reemphasized in the 1980s.
Most globular clusters are in the extreme Population II or halo class,
having stars metal-poor relative to the Sun by factors of at least 6
or 7 (and in most cases greater than 10), and being distributed
essentially spherically about the galactic center. Halo clusters are
often found as far as 10 kpc or more above or below the galactic
plane. These clusters, like everything else in the Galaxy, are in
orbit around the galactic center. Although many millions of years are
required for a cluster to complete one orbit, we can learn a good deal
about the shapes of the orbits from the current locations and
velocities of the clusters. Such investigations have revealed that
the orbits of halo globulars are not at all circular, but are
generally quite elongated, and are oriented essentially at
random. Clusters in such orbits participate only slightly, if at all,
in the general rotation of the Galaxy that is the dominant motion of
Population I stars in the galactic disk Some halo globulars even show
a motion opposite to galactic rotation.
In contrast to the halo objects, about 20% of the globular clusters
in our Galaxy are less metal poor and are found within about 1 or 2
kpc of the galactic plane (compared to most Population I stars lying
within 0.4 kpc of the plane), and thus belong to intermediate
Population II, often called the thick-disk population. These clusters
are also in orbit around the galactic center, of course, but their
orbits are more nearly circular and are oriented near the galactic
plane. These clusters are moving in the same direction as galactic
rotation, though at a rate that is slightly slower than the rotation
of the thin disk of Population I stars.
Nearly all of the thick-disk globulars lie closer to the galactic
center than does the Sun (i.e., within about 8 kpc of the center), but
halo clusters are found out to much larger distances, though also with
a strong concentration toward the center. Globular clusters of both
types are thus found in large numbers in the general direction of the
galactic center, but only halo clusters are found well away from the
center. An individual globular can usually be assigned unambiguously
to one population or the other on the basis of its chemical
composition or velocity.
All of these differences presumably reflect changes that happened in the
Galaxy very early in its history, as stars and star clusters formed
first from a large, nearly spherical protogalactic cloud of gas.
This cloud would have contracted
under its own gravitational pull and its small initial rotation would
have sped up as the cloud contracted and caused the contraction to end
in a disk rather than a small sphere. According to a commonly accepted
picture, stars and clusters that formed a little later would have been
formed in a thick disk and would have a higher metal abundance, as the
chemical composition of the gas from which stars could form was
enriched by the products of nuclear reactions in the most massive of
the earliest stars.
A major issue in current research on globular clusters is the
question of whether there are significant age differences from cluster
to cluster, and especially between the two populations of
globulars. Such age differences could tell us how long it took the
Galaxy to contract from the original spherical cloud to a thick disk,
and how long the thick-disk phase lasted. It presently appears that
the difference in ages is no more than a few billion years (out of a
total age of about 15 billion years), implying fairly rapid changes in
the early history of the Milky Way. Our instruments for obtaining
appropriate data, and our ability to infer reliable ages from those
data, are both improving rapidly, however, so these conclusions
regarding ages and age differences could change in the next few years.
As noted earlier, halo globulars exist much further from the galactic
center than the Sun's position. The outer limit for normal halo
clusters seems to be about 30-40 kpc from the Galactic center, though
a few globulars are also found between about 60 and 100 kpc from the
center. Most of these outer halo clusters are less dense than typical
globulars and most show some peculiarities in their constituent stars
that may indicate slightly younger ages. A few dwarf spheroidal
galaxies are also found at distances similar to those of the outer
halo globulars. These systems contain old metal-poor stars similar to
those in globular clusters, and have masses comparable to the most
massive globulars, but are different in that the star density within
each system is very low compared even to the low density outer halo
globulars.
It is generally believed that the outer-halo globulars and the dwarf
spheroidal galaxies are in orbit around the Milky Way, and are not
merely passing by. As they are the most distant objects associated
with our Galaxy, their motions should reflect the effects of the
gravitational pull of the entire mass of the Milky Way. Observations
of their velocities in recent years have thus yielded some of the best
available derivations of the mass of our Galaxy: about 5 x
1011 times
the mass of the Sun. Since this is considerably more mass than can be
accounted for by adding up the masses of all the stars and gas clouds
we observe directly, this is part of the evidence for unseen dark
matter (sometimes called missing mass) in the Galaxy.
Because of their importance in questions of galactic evolution and
the study of the outer parts of the Galaxy, globular clusters continue
to be investigated intensively by many astronomers today. Questions
such as the distance scale, chemical compositions, and ages of
globulars are all interrelated and are receiving considerable
attention from both observational and theoretical perspectives. We can
thus look forward to increasing knowledge and, one hopes, a better
understanding of these issues in the coming years.
GALATIC STRUCTURE, GLOBULAR CLUSTERS
CLASSIC STUDIES
TWO POPULATIONS OF GLOBULAR CLUSTERS:
GALACTIC EVOLUTION
THE OUTER GALACTIC HALO
Armandroff, T.E. (1989). The properties of the disk system of
globular clusters. Astron. J. 97 375.
Bok, B.J. (1981). The Milky Way galaxy. Scientific American
244 (No. 3) 92.
Freeman, K.C. and Norris, J. (1981). The chemical composition,
structure, and dynamics of globular clusters. Ann. Rev. Astron.
Ap. 19 319.
Harris, W.E. and Racine, R. (1979). Globular clusters in galaxies.
Ann. Rev. Astron. Ap. 17 241.
King, I.R. (1985). Globular clusters. Scientific American
252 (No. 6) 78.
Zinn, R. (1985). The globular cluster system of the Galaxy. IV.
The halo and disk subsystems. Ap. J. 293 424.
See also Galactic Structure, Large Scale; Galactic Structure,
Stellar Populations; Galaxy, Chemical Evolution; Star Clusters,
Globular; Star Clusters, Globular, Chemical Composition; Star Cluster,
Globular, Extragalactic; Star Clusters, Globular, Variable Stars; Stars,
RR Lyrae Type; Stellar Orbits, Galactic.