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 ?
Astronomers have used white dwarfs in globular clusters as a measure of the universe's age.
The globular star cluster in Abell 1689 is roughly twice as large as any other population found in previous globular cluster surveys - in comparison, our Milky Way galaxy hosts about 150 - and constitutes the most distant such systems ever studied, at 2.
Another surprise to be found is the globular cluster NGC 5634 taking shelter at Virgo's feet half way between magnitude 3.
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.
Also, because the ages of both of these globular clusters are known--12 billion years for M15 and 10 billion for G1--these studies mark the first time astronomers have determined the age of black holes, Rich adds.
The study also raises questions about how and when globular clusters, dense groupings of the oldest stars in our galaxy, were formed.
0) Dec NGC 1711 Open Cluster 04 (h) 50 (m) 6 -69[degrees]50' NGC 1737 Emission Nebula 04 54 0 -69 10 NGC 1743 Diffuse Nebula 04 54 5 -69 12 NGC 1782 Open Cluster/Neb 04 57 8 -69 23 NGC 1809 Galaxy 05 02 5 -69 34 NGC 1854 Globular Cluster 05 09 3 -68 51 NGC 1910 Open Cluster 05 18 7 -69 14 NGC 2070 Bright Nebula 05 38 6 -69 06 NGC 2074 Bright Nebula 05 39 3 -69 30 NGC 2100 Open Cluster 05 42 2 -69 13 Object Type Mag.
In the list below, "Type" refers to the object type: Galaxy (Glxy), Open Cluster (OpCI) Globular Cluster (Glcl), Planetary Nebula (Plnb), and Bright Nebula (BrtN).
Hurley, also of the museum, have used the dome to project their computer simulations of dense clouds of stars known as globular clusters.
Globular clusters, which contain about a million stars, are some of the oldest structures in the Universe, having formed shortly after the Big Bang around 14 billion years ago.
If you have to look for globular clusters in just about all the categories then Ophiuchus offers quite a variety.
Unlike most stars in the Milky Way, globular clusters don't reside within the galaxy's plane.