Stellar evolution

In astronomy, stellar evolution is the sequence of changes that a star undergoes during its lifetime, the millions or billions of years during which it emits light and heat. Over the course of that time, the star will change radically.

Birth

Stellar evolution begins with a giant molecular cloud (GMC), also known as a stellar nursery. Most of the 'empty' space inside a galaxy actually contains around 0.1 to 1 particle per cm³, but inside a GMC, the typical density is a few million particles per cm³. A GMC contains 100,000 to 10,000,000 times as much mass as our Sun by virtue of its size: 50 to 300 light years across.

Related Topics:
Giant molecular cloud - Stellar nursery - Galaxy - Sun - Light year

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As a GMC orbits the galaxy, one of several events might occur to cause its gravitational collapse. GMCs may collide with each other, or pass through dense regions of spiral arms. A nearby supernova explosion can also be a trigger, sending shocked matter into the GMC at very high speeds. Finally, galactic collisions can trigger massive bursts of star formation as the gas clouds in each galaxy are compressed and agitated by the collision.

Related Topics:
Gravitational collapse - Spiral arm - Supernova

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A collapsing GMC fragments as it collapses, breaking into smaller and smaller chunks. Fragments with masses of less than about 50 solar masses are able to form into stars. In these fragments, the gas is heated as it collapses due to the release of gravitational potential energy, and the cloud becomes a protostar as it forms into a spherical rotating object.

Related Topics:
Solar mass - Potential energy - Protostar

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This initial stage of stellar existence is almost invariably hidden away deep inside dense clouds of gas and dust. Often, these star-forming cocoons can be seen in silhouette against bright emission from surrounding gas, and are then known as Bok globules.

Related Topics:
Silhouette - Bok globule

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Very small protostars never reach temperatures high enough for nuclear fusion to begin; these are brown dwarfs of less than 0.1 solar mass. They die away slowly, cooling gradually over hundreds of millions of years. The central temperature in more massive protostars, however, will eventually reach 10 megakelvins, at which point hydrogen begins to fuse into helium. The star begins to shine. The onset of nuclear fusion sets up a hydrostatic equilibrium in which energy released by the core prevents further gravitational collapse. The star then exists in a stable state.

Related Topics:
Nuclear fusion - Brown dwarf - Megakelvin - Hydrogen - Helium - Hydrostatic equilibrium

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