Globular cluster

From Free net encyclopedia

A globular cluster (sometimes known more simply as a globular) is a spherical collection of stars that orbits a galaxy as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shape, and relatively high stellar density towards their core.

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1 Composition
2 Ubiquitousness
3 Significance
4 See also
5 References
6 External links

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Composition

Globular clusters are generally composed of hundreds of thousands of old stars, similar to the bulge of a spiral galaxy but confined to a volume of only a few cubic parsecs. Some globular clusters (like Omega Centauri in our Milky Way, and G1 in M31) are extraordinarily massive clusters, weighing as many as several million solar masses. Some globular clusters (like M15) have extremely massive cores which are expected to harbor black holes.

With a few notable exceptions, each globular cluster appears to have a definite age. That is, all the stars in a cluster are at the same stage in stellar evolution, suggesting that they formed at the same time. It was the recognition of this fact, studying Hertzsprung-Russell diagrams of globulars, that led to the earliest understanding of stellar evolution. Globular clusters are typically the oldest objects in the Galaxy, and were among the first collections of stars to form.

Globular clusters have a very high star density, and therefore close interactions and near-collisions of stars occur relatively often. Some exotic classes of stars, such as blue stragglers, millisecond pulsars and low-mass X-ray binaries are much more common in globular clusters.

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Ubiquitousness

Globular clusters are fairly numerous; there are about 150 currently known globular clusters in the Milky Way (with perhaps 10-20 more undiscovered), and larger galaxies such as Andromeda tend to have more (Andromeda may have as many as 500). Some giant elliptical galaxies, such as M87, may have as many as 10,000 globular clusters. These globular clusters orbit the galaxy out to large radii, 100 kiloparsecs or more.

In many galaxies (especially massive elliptical galaxies) there appear to be two populations of globular clusters, which appear to be of similar ages (nearly as old as the universe itself) but of different metal abundances. These subpopulations are generally known as "metal-poor" and "metal-rich", although the metalicities of the metal-rich clusters are generally less than that of the Sun. Many scenarios have been suggested to explain these subpopulations, including violent gas-rich galaxy mergers, the accretion of dwarf galaxies, and multiple phases of star formation in a single galaxy. In our Milky Way, the metal-poor clusters are associated with the halo and the metal-rich clusters with the Bulge.

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Significance

It was through the study of globular clusters that the Sun's position in the Milky Way became known. Until the 1930s, it was thought that the Sun was near the middle of the galaxy because the distribution of stars in the observable Milky Way appeared uniform. However, the distribution of globular clusters was strongly asymmetric. Assuming a roughly spherical distribution of globular clusters around the galaxy's center, one can use the positions of the clusters to estimate the position of the sun relative to the galactic center. By further estimating the distances to those clusters, the distance of the Sun to the galactic center can be estimated as well. It thus became clear that the part of the Milky Way seen from Earth was only a small part of the total galaxy, most of which was obscured by gas and dust.

Since Globular cluster stars are the oldest objects in the Galaxy, their ages place a lower limit on the age of the universe. This lower limit has been a significant constraint in Cosmology.

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References

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General resources

NASA Astrophysics Data System has a collection of past articles, from all major astrophysics journals and many conference proceedings.
SCYON is a newsletter dedicated to star clusters.
MODEST is a loose collaboration of scientists working on star clusters.

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Books

Binney, James; Tremaine, Scott (1987). Galactic Dynamics, Princeton University Press, Princeton, New Jersey.
Heggie, Douglas; Hut, Piet (2003). The Gravitational Million-Body Problem: A Multidisciplinary Approach to Star Cluster Dynamics, Cambridge University Press.
Spitzer, Lyman (1987). Dynamical Evolution of Globular Clusters, Princeton University Press, Princeton, New Jersey.

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Review Articles

Elson, Rebecca; Hut, Piet; Inagaki, Shogo (1987). Dynamical evolution of globular clusters. Annual review of astronomy and astrophysics 25 565. NASA ADS
Meylan, G.; Heggie, D. C. (1997). Internal dynamics of globular clusters. The Astronomy and Astrophysics Review 8 1. NASA ADS

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