CLUSTERS.TXT


C L U S T E R S

cluster, star  
-------------------------------- 
Of the stars in the Milky Way galaxy, about 1 in 100,000 exist in
stable, dense groupings called star clusters. For the most part, the
star clusters can be divided into two types, galactic and globular. The
distinction is based primarily on the location of a cluster and its
orbital motion in the Galaxy; galactic, or open, clusters are found in
the relatively thin disk of the Milky Way orbiting about its center in
nearly circular orbits, whereas globular clusters inhabit a spherical
halo (see HALO, GALACTIC) around the Galaxy and have highly elliptical
orbits.


cluster, galactic 
-------------------------------- 

In astronomy, a cluster is a group of stars that are close to one
another in space and therefore appear as a group in the sky.
Measurements show that all the stars in a given cluster have very nearly
the same velocity and thus are moving together through space; therefore,
they must have originated together.  There are two general types of
clusters, globular clusters and open, or galactic, clusters. Globular
clusters are dense groups that contain large numbers of stars. They are
distributed throughout our galaxy. Their distribution is strongly
concentrated toward its center, but not toward its principal plane.
Galactic clusters are considerably less dense, contain fewer stars per
cluster, and on the average are strongly concentrated toward the
galactic plane. The Pleiades and Hyades are familiar examples.  The age
of a cluster can be determined by obtaining its color magnitude, or its
HERTZSPRUNG-RUSSELL DIAGRAM, and comparing it with theoretical
calculations. It has been found that galactic clusters range from 10
million to several billion years old and are therefore much younger than
typical globular clusters. DONALD E.  OSTERBROCK

Bibliography: Inglis, Stuart, Stars and Galaxies, 4th ed. (1976);
Shapley, Harlow, Galaxies, 3d ed. (1972).


Galactic Clusters. 
------------------

Galactic clusters are younger (almost 10 billion years for the oldest,
but most are much younger) than globular clusters and thus are made up
of stars built from material that has been enriched in heavy elements by
generations of stellar evolution. Galactic clusters are therefore
conspicuously different in chemical composition, with up to 100 times
the amount of heavy elements found in globular clusters. Galactic
clusters are small, averaging only 15 light-years in radius, and most
have between 100 and 1,000 stellar members.  The HYADES is one of the
nearest and most important galactic clusters. Its distance of 160
light-years can be measured by a variety of means and is one of the
fundamental yardsticks in the Galaxy. It is easily visible in the night
sky, forming the head of Taurus, the Bull. Nearby in the sky is another
important cluster, the PLEIADES, 490 light-years distant and made up of
between 250 and 500 stars. PAUL W. HODGE


clusters of galaxies 
-------------------------------- 

Clusters of galaxies (see EXTRAGALACTIC SYSTEMS) are groups of tens to
thousands of galaxies that are bound together gravitationally. The Milky
Way galaxy is a member of a small cluster known as the LOCAL GROUP OF
GALAXIES.  Regular clusters, such as the COMA CLUSTER, exhibit symmetry
and a high central density; irregular clusters, such as the VIRGO
CLUSTER OF GALAXIES, are unsymmetrical and are not centrally
concentrated.  Thousands of clusters of galaxies have been identified
and cataloged by Fritz Zwicky and others.

Bibliography: Chincarni, Guido, and Rood, Herbert J., "The Cosmic
Tapestry," Sky and Telescope, May 1980; Gorenstein, Paul, and Tucker,
Wallace, "Rich Clusters of Galaxies," Scientific American, November
1978; Groth, E. J., et al., "The Clustering of Galaxies," Scientific
American, November 1977.


Globular Clusters.  
------------------

Globular clusters usually consist of several hundred thousand to more
than a million stars and have a full, round appearance. The clusters are
concentrated toward the center of the Galaxy but extend outward at least
50,000 light-years from the galactic center.  Globular clusters, which
are about 10 to 15 billion years old, are among the oldest objects in
the Galaxy. Because of their age, the massive stars in globular clusters
have evolved into white dwarfs and are too faint to be seen with
present-day telescopes. The brightest visible stars are the huge,
evolving red giants, which have masses approximately 1.5 times that of
the Sun. Most globular clusters contain a few hundred red giants, but
most of the stars are much fainter, of lower mass, and are still in the
long-lived state of equilibrium represented by the main sequence.  Most
globular clusters contain at least one variable star, usually an RR
LYRAE STAR. Some contain more than 100 of these stars, whose well-known
true luminosities are useful in determining the distances to the
clusters (see DISTANCE, ASTRONOMICAL). Small differences in the
properties of the RR Lyrae stars, as well as in the colors and
luminosities of the red giant stars, result from differences in their
ages and chemical compositions. Despite these minor differences,
however, all globular clusters have remarkably similar properties.