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.

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 ?
We show how the relationship between globular clusters and dark matter depends on the distance from the center of the galaxy grouping," said Karla Alamo-Martinez of the Center for Radio Astronomy and Astrophysics of the National Autonomous University of Mexico in Morelia.
Hubble's data was used to discover three generations of stars in the globular cluster NGC 2808.
The globular clusters that populate the sky there are gargantuan balls or cities of stars--sometimes millions of them--that look like glittering diamonds sprinkled across inky black velvet, one of the most splendiferously delicious sites eyes can gaze upon, I think.
Because of their close interactions with neighboring stars, pulsars in globular clusters have elongated orbits, a shape that makes it much easier to infer the mass of these superdense stars.
However the observed linear correlation between masses of black holes with the masses of the "host" elliptical galaxies or globular clusters suggests that either the formation of these systems involves common evolutionary dynamical processes or that perhaps some new aspect to gravity is being revealed.
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.
TEHRAN (FNA)- Scientists used streams produced by dissolving globular clusters to measure the weight of our galaxy and determine the location of the sun within the Milky Way.
Newcomers to deep-sky observing are warmly invited to observe the "Big 5 of the African Sky", a new observing project highlighting the most splendid example of each type of deep-sky object: a galaxy, a bright nebula, a dark nebula, an open cluster, and a globular cluster.
Lower, but still on the meridian, is Capricornus which contains the globular cluster M30, visible in larger binoculars although a moderate sized telescope is needed to begin resolving individual stars.
In 2007, Hubble researchers found three generations of stars in the massive globular cluster NGC 2808.
Star clusters can either be a group of stars bound together by gravity that soon exit out of the cluster, called open clusters, while other clusters contain groups of ancient stars, called globular clusters.