globular cluster


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Related to globular cluster: Open cluster

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 ?
John Herschel, son of William, observing with the 18-inch f/3 speculum at the Cape of Good Hope, records the object as "a globular cluster, bright, 4' long by 3' broad; all resolved into stars, gradually more compressed in the middle.
Unlike globular clusters, open clusters usually show plenty of open space between their stars.
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.
It made exacting all-sky measurements that include many globular clusters. Hubble has a smaller field of view, but it can measure fainter stars and therefore reach more distant clusters.
But is G1 a true globular cluster? With more than twice the mass of Omega Centauri (the Milky Way's heftiest globular), G1 may be the remnant core of a stripped dwarf galaxy that was digested by M31 earlier in its history.
Globular clusters are relics of the early universe and usually contain thousands of stars crammed into a ball a few dozen light-years across.
NGC 6254 is a lovely star-rich globular cluster which displays a flimsy edge.
Messier likely also miscategorized the globular cluster Messier 28 because there was no Hubble Space Telescope to provide a clear view of the celestial object at the time.
15 ( ANI ): The Hubble Space Telescope has clicked the best ever image of the globular cluster Messier 15, a gathering of very old stars that orbits the Milky Way's center.
Using a spectrometer at the Very Large Telescope in Paranal, Chile, Luca Pasquini of the European Southern Observatory in Garching, Germany, and his colleagues have for the first time detected beryllium in two of the Milky Way's oldest stars, residing in the globular cluster NGC 6397.