Star Clusters

Click the icon to view Star Clusters of the Messier Catalog

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Globular

Globular clusters are gravitationally bound concentrations of approximately

ten thousand to one million stars. 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

M92).

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

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 star clusters.

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

which was discovered in 1994).

Other galaxies contain clusters of any type, too,

which can be detected with considerably sensitive instruments.

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Last Modification: 25 Jan 1998, 15:50 MET