Stellar merger

A stellar collision is the coming together of two stars. The mechanisms that drives such collisions include gravity, gravitational radiation, and other mechanisms not well understood. Astronomers predict that events of this type occur in the globular clusters of our galaxy about once every 10,000 years. Scientists have only recently been able to observe a stellar merger in Scorpius in 2008, though it was not known to be the result of a stellar merger at the time. A series of stellar collisions in a dense cluster over a short period of time can lead to an intermediate-mass black hole via "runaway stellar collisions".

Any star in the universe can be involved in a stellar collision; whether it is 'alive' (meaning fusion is still active in the star) or dead (with fusion no longer taking place). White dwarf stars, neutron stars, black holes, main sequence stars, giant stars, and supergiants, for example, are very different in type, mass, temperature, and radius, and will react differently.

On 25 August 2017, a possible gravitational wave event, GW170817, that may be associated with the collision of two neutron stars in a distant galaxy, was reported.

White dwarfs are the remnants of low-mass stars and, if they form a binary system with another star, they can cause large stellar explosions known as type Ia supernova. The normal route by which this happens involves a white dwarf drawing material off a main sequence or red giant star to form an accretion disc. However, a much rarer brand of type Ia supernova occurs when two white dwarfs orbit each other closely. Emission of gravitational waves causes the pair to spiral inward. When they finally merge, if their combined mass approaches or exceeds the Chandrasekhar limit, carbon fusion is ignited, raising the temperature. Since a white dwarf consists of degenerate matter, there is no safe equilibrium between thermal pressure and the weight of overlying layers of the star. Because of this, runaway fusion reactions rapidly heat up the interior of the combined star and spread, causing a supernova explosion. In a matter of seconds, all of the white dwarf's mass is thrown into space.

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