Stellar collision

A stellar collision is the coming together of two stars caused by stellar dynamics within a star cluster, or by the orbital decay of a binary star due to stellar mass loss or gravitational radiation, or by other mechanisms not yet well understood. A stellar collision is the coming together of two stars caused by stellar dynamics within a star cluster, or by the orbital decay of a binary star due to stellar mass loss or gravitational radiation, or by other mechanisms not yet well understood. Astronomers predict that events of this type occur in the globular clusters of our galaxy about once every 10,000 years. On 2 September 2008 scientists first observed a stellar merger in Scorpius (named V1309 Scorpii), 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 stars in the universe can collide, whether they are '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 are very different in type, mass, temperature, and radius, and so react differently. A gravitational wave event that occurred on 25 August 2017, GW170817, was reported on 16 October 2017 to be associated with the merger of two neutron stars in a distant galaxy, the first such merger to be observed via gravitational radiation. 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 supernovae. 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. Much more rarely, a 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. Neutron star mergers occur in a fashion similar to the rare type Ia supernovae resulting from merging white dwarfs. When two neutron stars orbit each other closely, they spiral inward as time passes due to gravitational radiation. When they meet, their merger leads to the formation of either a heavier neutron star or a black hole, depending on whether the mass of the remnant exceeds the Tolman–Oppenheimer–Volkoff limit. This creates a magnetic field that is trillions of times stronger than that of Earth, in a matter of one or two milliseconds. Astronomers believe that this type of event is what creates short gamma-ray bursts and kilonovae. If a neutron star collides with red giant of sufficiently low mass and density, both can survive in the form of a peculiar hybrid known as Thorne–Żytkow object, with a neutron star surrounded by a red giant.