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.

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.)

References in periodicals archive ?
By contrast, our Milky Way contains approximately 180 blue and red globular clusters.
I've been studying globular clusters in galaxies for a long time, and this is the first time I've ever seen this," said Michael Beasley, also of the Instituto de Astrofisica de Canarias.
Sagan at least once used the phrase "the citadel of the stars" in his Cosmos series, and if he didn't sometimes apply the phrase to globular clusters, somebody should.
The team, led by MPE's Maximilian Fabricius and including Texas' Eva Noyola, observed 11 globular clusters from the University of Texas at Austin's McDonald Observatory with the Harlan J.
Globular clusters, dense bunches of hundreds of thousands of stars, are the homesteaders of galaxies.
It is densely populated with stars and is one of the few globular clusters visible to the unaided eye.
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.
Ancient globular clusters such as M4 were thought to be the last place to find planets because they contain very little metal, which is considered an essential raw material for planet formation.
Computer simulations of the formation of globular clusters - the oldest star clusters known - have shown how small clouds in young galaxies collided and stuck together.
Open clusters contain small numbers of young stars, while globular clusters are densely packed with old stars.