star cluster

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star cluster,

a group of stars near each other in space and resembling each other in certain characteristics that suggest a common origin for the group. Stars in the same cluster move at the same rate and in the same direction. Two types of clusters can be distinguished—open clusters, also called galactic clusters because of their wide distribution in our galaxygalaxy,
large aggregation of stars, gas, and dust, typically containing billions of stars. Recognition that galaxies are independent star systems outside the Milky Way came from a study of the Andromeda Galaxy (1926–29) by Edwin P.
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 (the Milky Way), and globular clusters. More than one thousand open clusters have been cataloged in the Milky Way, most of which are found in the spiral arms of the galaxy. Typically, an open cluster contains from a few dozen to a thousand loosely scattered stars and exists in a region rich in gas and dust. Among those which can be detected with the unaided eye are the HyadesHyades
, in astronomy, open star cluster in the constellation Taurus, located immediately to the right of the bright star Aldebaran. The cluster is about 130 light-years from the earth. It consists of about 100 stars all moving in the same direction.
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 cluster in the constellation Taurus, the Coma Berenices cluster, the PleiadesPleiades
, in astronomy, famous open star cluster in the constellation Taurus; cataloged as M45. The cluster consists of some 500 stars, has a diameter of 35 light-years, and is 400 light-years distant from the earth.
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 cluster, and the PraesepePraesepe
[Lat.,=manger], open star cluster in the constellation Cancer; cataloged as M44 or NGC 2632. It was first recorded by Hipparchus (c.150 B.C.). The cluster is often called the Beehive because of its shape. It contains several hundred stars, many of which are doubles.
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 cluster. Globular clusters are spherical aggregates of from thousands to hundreds of thousands of densely concentrated stars. Rather than lying on the galactic plane, these clusters are members of the outer halo, moving around the nucleus of our galaxy in highly inclined orbits. Because of their distribution around the galaxy, they provide an outline of its shape. About 150 globular clusters are known in the Milky Way galaxy, and others have been found in nearby galaxies. Visible to the unaided eye are Omega Centauri and 47 Tucanae, both in the southern skies, and M13 in the northern sky. Star clusters are cosmologically important as a first step to understanding the distance scale of the universe (see HyadesHyades
, in astronomy, open star cluster in the constellation Taurus, located immediately to the right of the bright star Aldebaran. The cluster is about 130 light-years from the earth. It consists of about 100 stars all moving in the same direction.
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); and theoretical astronomers use observations of globular clusters to investigate the evolution and life span of stars. Because all the stars in a particular cluster are coeval (the same age), astronomers can infer that massive stars change more rapidly over time than less massive ones. X-ray sources have been detected recently in some globular clusters. Millisecond pulsarspulsar,
in astronomy, a neutron star that emits brief, sharp pulses of energy instead of the steady radiation associated with other natural sources. The study of pulsars began when Antony Hewish and his students at Cambridge built a primitive radio telescope to study a
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 have also been found.

star cluster

See cluster.

Star Cluster

 

a grouping of stars that are bound to each other by forces of mutual attraction and that have a common origin and nearly the same age and chemical composition. Star clusters usually have a dense central concentration, the core, surrounded by a significantly less dense coronal region. The diameters of star clusters range from several parsecs to 150 parsecs, and the radii of the coronal regions exceed the radii of the cores by several (sometimes dozens of) times. Star clusters are divided in open (sometimes called galactic) clusters and globular clusters. The distinction is basically determined by the mass and age of the formations. As a rule, open clusters have dozens or hundreds, rarely thousands, of stars, while globular clusters have tens or hundreds of thousands of stars. The Pleiades, Praesepe, and the Hyades are open clusters; examples of globular clusters are the cluster M3 in the constellation Canes Venatici and the cluster M13 in Hercules.

In our galaxy open clusters are concentrated in the plane of symmetry of the Milky Way (the galactic plane) and possess a small velocity relative to the sun (20 km/sec on the average). Among these it is possible to distinguish clusters that have emerged comparatively recently (less than 100 million years ago) and are associated with the spiral arms and clusters of intermediate age, or disk clusters, which exhibit no link with the spiral arms and are more weakly concentrated toward the galactic plane. All open clusters have the normal metal content inherent in Population I stars. Globular clusters in our galaxy are distributed in a spheroidal volume the center of which coincides with the center of the galaxy. They are strongly concentrated toward this center and are characterized by large velocities relative to the sun (170 km/sec on the average). They are usually poor in metals, but the objects observable in the regions around the center of the galaxy are richer in metals than those observed on the periphery of our stellar system. The study of the Hertzsprung-Russell diagram or magnitude-color diagram yields important information about the evolution of star clusters. Diagrams of the magnitude-color relation of stars in typical open and globular clusters of our galaxy are essentially different. The interpretation of these diagrams from the viewpoint of the modern theory of stellar evolution enables one to conclude that stars in typical globular clusters are 100–1,000 times older than stars in open clusters.

The kinematic characteristics and spatial distribution of globular clusters in our galaxy reflect the distinctive features of the initial distribution of matter, out of which these formations arose at an early stage of the galaxy’s existence. The magnitude-color diagrams of the stars in the globular clusters of that epoch should bring to mind corresponding diagrams in contemporary open clusters. Similar young globular clusters are observed in neighboring galaxies (for example, NGC 1866 in the Large Magellanic Cloud). At present, star clusters in our galaxy originate only near the galactic plane in the regions of the gas and dust spiral arms.

The dynamic evolution of a star cluster occurs simultaneously with the change in the physical characteristics of its members. Close encounters between stars in the cores of star clusters lead to a mutual exchange of the energy of their motion. As a result, certain star cluster members receive excess energy and proceed into the region of the corona or escape from the cluster entirely. During this process the core contracts. The dissipation of the core occurs particularly rapidly in clusters with a small number of members—that is, open clusters. Thus, out of the old clusters in our galaxy only the most massive ones—the globular clusters—have been preserved. Nonperiodic variables and flare variable stars are usually observed among the weak members of young open clusters. Certain globular clusters contain RR Lyrae and W Virginis variables, and cepheids are sometimes encountered in open clusters. Star clusters very close to the sun (for example, the Hyades), in whose members’ proper motion the phenomena of perspective (the directions of their proper motions upon their continuation on the celestial sphere intersect at one point) are observed, are called moving clusters. These clusters play a special role in the problem of determining stellar distances, since the distances to them can be reliably determined by simple geometric methods.

REFERENCES

Parenago, P. P. Kurs zvezdnoi astronomii, 3rd ed. Moscow, 1954.
Sawyer-Hogg, E. “Zvezdnye skopleniia.” In the collection Stroenie zvezdnykh sis tern. Moscow, 1962.

P. N. KHOLOPOV

star cluster

[′stär ‚kləs·tər]
(astronomy)
A group of stars held together by gravitational attraction; the two chief types are open clusters (composed of from 12 to hundreds of stars) and globular clusters (composed of thousands to hundreds of thousands of stars). Also known as cluster.
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