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Globular clusters are gravitationally bound concentrations of approximately
ten thousand to one million stars. They populate the halo or bulge of the
Milky Way and other
galaxies with a significant concentration toward
the Galactic Center.
Spectroscopic study of globular clusters shows that they are much lower in
heavy element abundance than stars such as the Sun that form in the disks of
galaxies. Thus, globular clusters are believed to be very old and formed from
an earlier generation of stars (Population II).
More recent estimates yield an age of 12 to 20 billion years; the best value
for observation is perhaps 14 to 16 billion (see e.g. the discussion at
As their age is crucial as a lower limit for the age of our universe, it was
subject to vivid and continuous discussion since decades.
The age of globular clusters is determined by investigating their H-R
diagrams, as discussed in our globular cluster page.
The disk stars, by contrast, have evolved through many cycles of starbirth
and supernovae, which enrich the heavy element concentration in
star-forming clouds and may also trigger their collapse.
Our galaxy has about 200 globular clusters, most in highly eccentric orbits
that take them far outside the Milky Way. Most other galaxies have globular
cluster systems as well, in some cases (e.g., for M87)
containing several thousands of globulars!
Open (or galactic) clusters are physically related groups of stars
held together by mutual gravitational attraction. They are believed to
originate from large cosmic gas/dust clouds in the Milky Way, and to
continue to orbit the galaxy through the disk. In many clouds visible as
diffuse nebulae star formation takes still place at
this moment, so that we can observe the formation of new young open star
clusters (composed of young Population I stars).
Open clusters populate about the same regions of the
Milky Way and other
galaxies as diffuse nebulae, notably spiral arms
in disk galaxies, and irregular galaxies, and are thus found along the band
of the Milky Way in the sky.
Most open clusters have only a short life as stellar swarms. As they drift
along their orbits, some of their members escape the cluster, due to velocity
changes in mutual closer encounters, tidal forces in the galactic gravitational
field, and encounters with field stars and interstellar clouds crossing their
way. An average open cluster has spread most of its member stars along its
path after several 100 million years; only few of them have an age counted by
billions of years. The escaped individual stars continue to orbit the Galaxy
on their own as field stars: All field stars in our and the external galaxies
are thought to have their origin in clusters.
Binary and Multiple Star Systems
Star formation leads to the formation of multiple star systems at least as
often as it does single star systems, such as our own Solar System. In
fact, if the mass of the planet Jupiter were a few times larger, it would
become a star.
One should keep in mind that almost all Messier clusters are members of our
Milky Way Galaxy (the only probable exception
is globular cluster M54 which apparently belongs
to the Sagittarius Dwarf Elliptical Galaxy,
a closely neighbored dwarf spheroidal galaxy which was discovered in 1994).
Other galaxies contain clusters of any type, too,
which can be detected with sufficiently sensitive instruments.
Last Modification: 21 Nov 1999, 23:25 MET