globular cluster

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globular cluster:

see star clusterstar 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.
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Color-magnitude diagram for typical globular clusterclick for a larger image
Color-magnitude diagram for typical globular cluster

globular cluster

A spherically symmetrical compact cluster of stars, containing from several tens of thousands to maybe a million stars that are thought to share a common origin. An example is the Great Cluster in Hercules. A few globular clusters, such as Omega Centauri, appear to be slightly flattened. The concentration of stars increases greatly toward the center of the cluster, where the density may be as much as 1000 stars per cubic parsec. Globular clusters occur in our Galaxy and in other galaxies. About 150 are known in the Galaxy. Most appear to move in giant and highly eccentric elliptical orbits about the galactic center, and, unlike open clusters, are not concentrated toward the galactic plane; instead they show a roughly spherical distribution in the galactic halo. About 20% are found in the galactic disk, moving in more circular orbits.

Globular clusters are population II systems (˜80% halo population II): all the stars within them are relatively old (older than the Sun) and have a very low metal content; the metallicity varies from cluster to cluster, but in most clusters all stars have very similar chemical compositions. The galactic disk clusters are younger and more metal-rich than the halo objects. The stars that dominate the visual output of globular clusters are red giants, the bluer horizontal-branch giants becoming dominant at shorter wavelengths. Although very few ordinary binary stars are observed in globular clusters, many contain strong X-ray sources typical of X-ray binaries or cataclysmic variables, i.e. systems containing a neutron star or a white dwarf, respectively.

The distribution and other characteristics of globular clusters suggest that they were formed early in the life of the Galaxy. The oldest formed possibly some 12 to 16 billion years ago, before the main body of the galactic disk had evolved. Because most of the member stars will have evolved away from the main sequence, the Hertzsprung–Russell diagram for stars of a globular cluster differs greatly from the conventional H–R diagram (see illustration). The luminosity at the turnoff point from the main sequence gives a measure of the age of a cluster, given the distance. Distances to globular clusters are usually calculated from the apparent magnitudes of the RR Lyrae stars within them. Although the age of the oldest globular clusters is disputed, the difference between the ages of clusters can be measured more precisely. Evidence is mounting of a spread in ages of several billion years.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Globular Cluster

 

a tightly packed group of stars that can be seen in a small section of the sky, consisting of stars that are close to one another in space and characterized by the spherical shape of the distribution of the stars. Two of the globular clusters closest to us, with a myriad of stars, are located in the constellations Hercules and Centaurus. (SeeSTAR CLUSTER.)

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
References in periodicals archive ?
Globular clusters are inherently beautiful objects, but the subject of this NASA/ESA Hubble Space Telescope image, Messier 3, is commonly acknowledged to be one of the most beautiful of them all.
It also has a shorter lifespan compared to globular clusters, and scientists believe that M11 will disperse completely in several million years.
Even in the tightly packed centers of globular clusters, where there might be up to 1,000 stars per cubic parsec (a cube 3.26 light-years on a side), the average distance between stars would still be 0.1 parsec, or about 21,000 astronomical units.
Globular clusters orbit most galaxies, including our own Milky Way.
47 Tucanae (NGC 104) - This impressive globular cluster can be found inside the SMC.
One of the most impressive globular clusters in the sky lies a bit more than two degrees northeast of Lambda, right beside the star marked 24 on our chart.
Although large telescopes are really needed to visually saviour the beauty that fully resolved globular clusters can give, if you are travelling south on holiday this summer even a pair of 10x50 binoculars will allow you to enjoy some of the splendour of Omega Centauri.
Observations of the precession of the millisecond pulsar PSR B1516+O2B, located some 25,000 light-years away in the globular cluster M5, indicate that the pulsar most likely has a mass equivalent to about 1.94 suns.
Scientists found the huge, gaseous sphere in globular cluster M4 - a cluster of stars 12.7 billion years old and 5,600 light years away.
Globular clusters are thought to be the oldest radiant objects in the universe and are found in all types of galaxies in large numbers.
A little later the Milky Way, the edges of the northern Milky Way, would be coming up, and we'd see globular clusters, which are on the fringes of the galaxy.