A Hubble Space Telescope image of the typical globular cluster Messier 80, an object made up of hundreds of thousands of stars and located in the direction of the constellation of Scorpius. The Milky Way galaxy has an estimated 160 globular clusters of which one quarter are thought to be ‘alien’. Image: NASA / The Hubble Heritage Team / STScI / AURA. Click for hi-resolution version.
A globular cluster is a collection of stars that is spherical and orbits a galactic core as a satellite. Globular clusters are tightly bound by gravity. The force of the gravity gives them their spherical shapes and relatively high stellar densities. They are found in the halo of a galaxy and contain considerably more stars and are much older than the less dense open clusters found in the disk. These clusters are fairly common. There are about 155 of these currently known in the Milky Way galaxy alone. The larger the galaxy, the more globular clusters there are.
Globular clusters are generally composed of hundreds of thousands of low metal old stars. They are free of gas and dust. Many think that all of the gas and dust was turned into stars. This type of cluster can contain a high density of stars, perhaps as many as 100 to 1000 stars per cubic parsec in the core of the cluster; however, they are not thought to be favorable locations for planetary systems. Planetary orbits are dynamically unstable within the cores of dense clusters because of the perturbations of passing stars. A planet orbiting at a single astronomical unit around a star within the core of a dense cluster would only survive about 108 years. No known cluster of this types is currently displaying active star formation, which makes scientist think that they are among the oldest objects in the galaxy.
The very high star density causes close interactions and near-collisions of stars relatively often. Because of these chance encounters, some exotic classes of stars, such as blue stragglers are much more common in globular clusters. A blue straggler is formed from the collision and accretion of two stars, possibly as a result of an encounter with a binary system. The resulting star has a higher temperature than comparable stars in the cluster with the same luminosity, so it differs from main sequence stars.
Globular clusters are still little understood. This lack of understanding leads to constant debate about their components and formation in the scientific community. Perhaps, if we understood them better, we would understand out galaxy better.
We have written many articles about globular clusters for Universe Today. Here’s an article about the Globular Cluster 47 Tucanae, and here’s an article about the new globular cluster discovered.
If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.
We’ve done many episodes of Astronomy Cast about stars. Listen here, Episode 12: Where Do Baby Stars Come From?
References:
http://en.wikipedia.org/wiki/Globular_cluster
http://csep10.phys.utk.edu/astr162/lect/clusters/globular.html
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