Open cluster

An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud, and are still gravitationally bound to each other. Open clusters are found only in spiral and irregular galaxies, in which active star formation is occurring. They are usually less than a few hundred million years old: they become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic centre, as well as losing cluster members through internal close encounters.

Formation

All stars are originally formed in multiple systems, because only a cloud of gas containing many times the mass of the Sun will be heavy enough to collapse under its own gravity, but such a heavy cloud cannot collapse into a single star.

Related Topics:
Multiple systems - Mass - Sun - Gravity

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The formation of an open cluster begins with the collapse of part of a giant molecular cloud, a cold dense cloud of gas containing up to many thousands of times the mass of the Sun. Many factors may trigger the collapse of a giant molecular cloud (or part of it) and a burst of star formation which will result in an open cluster, including shock waves from a nearby supernova and gravitational interactions. Once a giant molecular cloud begins to collapse, star formation proceeds via successive fragmentations of the cloud into smaller and smaller clumps, resulting eventually in the formation of up to several thousand stars. In our own galaxy, the formation rate of open clusters is estimated to be one every few thousand years {{ref|Battinelli}}.

Related Topics:
Giant molecular cloud - Mass of the Sun - Supernova

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Once star formation has begun, the hottest and most massive stars (known as OB stars) will emit copious amounts of ultraviolet radiation. This radiation rapidly ionizes the surrounding gas of the giant molecular cloud, forming an H II region. Stellar winds from the massive stars and radiation pressure begin to drive away the gases; after a few million years the cluster will experience its first supernovae, which will also expel gas from the system. After a few tens of millions of years, the cluster will be stripped of gas and no further star formation will take place. Typically, less than 10% of the gas originally in the cluster will form into stars before it is dissipated {{ref|Battinelli2}}.

Related Topics:
OB star - Ultraviolet - H II region - Stellar wind - Radiation pressure - Supernova

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It is common for two or more separate open clusters to form out of the same molecular cloud. In the Large Magellanic Cloud, both Hodge 301 and R136 are forming from the gases of the Tarantula Nebula, while in our own galaxy, tracing back the motion through space of the Hyades and Praesepe, two prominent nearby open clusters, suggests that they formed in the same cloud about 600 million years ago {{ref|Eggen}}.

Related Topics:
Large Magellanic Cloud - Hodge 301 - R136 - Tarantula Nebula - Hyades - Praesepe

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Sometimes, two clusters born at the same time will form a binary cluster. The best known example in the Milky Way is the Double Cluster of h Persei and χ Persei, but at least 10 more double clusters are known to exist {{ref|Subramaniam}}. Many more are known in the Small and Large Magellanic Clouds — they are easier to detect in external systems than in our own galaxy because projection effects can cause unrelated clusters within the Milky Way to appear close to each other.

Related Topics:
Double Cluster - Small - Large Magellanic Cloud - Projection effect

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