Astellar collision is the coming together of twostars^[1] caused bystellar dynamics within astar cluster, or by theorbital decay of abinary star due tostellar mass loss orgravitational radiation, or by other mechanisms not yet well understood.

Any stars in theuniverse 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, andsupergiants are very different in type, mass, temperature, and radius, and accordingly produce different types of collisions and remnants.^[2]

About half of all the stars in the sky are part of binary systems, with two stars orbiting each other. Some binary stars orbit each other so closely that they share the same atmosphere, giving the system a peanut shape. While most suchcontact binary systems are stable, some do become unstable and either eject one partner or eventually merge.

Astronomers predict that events of this type occur in theglobular clusters of theMilky Way galaxy about once every 10,000 years.^[2] On 2 September 2008 scientists first observed a stellar merger in Scorpius (namedV1309 Scorpii), though it was not known to be the result of a stellar merger at the time.^[3]

White dwarfs are the remnants of low-mass stars which, if they form a binary system with a star on a sufficiently close orbit, can cause large stellar explosions known as type Ia supernovae. The normal route by which this happens involves a white dwarf drawing material off amain sequence orred giant star to form anaccretion disc.

Much more rarely, a type Ia supernova occurs when two white dwarfs orbit each other closely.^[4] Emission ofgravitational waves causes the pair to spiral inward. When they finally merge, if their combined mass approaches or exceeds theChandrasekhar limit,carbon fusion is ignited, raising the temperature. Since a white dwarf consists ofdegenerate matter, there is no safe equilibrium betweenthermal 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 asupernova explosion.^[4] In a matter of seconds, all of the white dwarf's mass is thrown into space.^[5]

Neutron star mergers occur in a fashion similar to the raretype Ia supernovae resulting from merging white dwarfs. When twoneutron 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 theTolman–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 createsshort gamma-ray bursts^[6] andkilonovae.^[7]

Agravitational 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 distantgalaxy, the first such merger to be observed via gravitational radiation.^{[8][9][10][11]}

If a neutron star collides withred giant of sufficiently low mass and density, the merger is conjectured to produce aThorne–Żytkow object, a hypothetical type ofcompact star containing a neutron star enveloped by a red giant.

When two low-mass stars in a binary system merge,mass may be thrown off in the orbital plane of the merging stars,creating anexcretion disk from which new planets can form.^[12]

While the concept of stellar collision has been around for several generations of astronomers, only the development of new technology has made it possible for it to be more objectively studied. For example, in 1764, a cluster of stars known asMessier 30 was discovered by astronomerCharles Messier. In the twentieth century, astronomers concluded that the cluster was approximately 13 billion years old.^[13] TheHubble Space Telescope resolved the individual stars of Messier 30. With this new technology, astronomers discovered that some stars, known asblue stragglers, appeared younger than other stars in the cluster.^[13] Astronomers then hypothesized that stars may have "collided", or "merged", giving them more fuel so they continued fusion while fellow stars around them started going out.^[13]

While stellar collisions may occur very frequently in certain parts of the galaxy, the likelihood of a collision involving theSun is very small. A probability calculation predicts the rate of stellar collisions involving the Sun is 1 in 10²⁸ years.^[14]For comparison, the age of the universe is of the order 10¹⁰ years. The likelihood of close encounters with the Sun is also small. The rate is estimated by the formula:

N ≈ 4.2 ·D² Myr⁻¹

whereN is the number of encounters per million years that come within a radiusD of the Sun inparsecs.^[15] For comparison, the mean radius of the Earth's orbit, 1AU, is4.82 × 10⁻⁶ parsecs.

Our star will likely not be directly affected by such an event because there are no stellar clusters close enough to cause such interactions.^[14]

The explosion that created theKleinmann–Low Nebula, located in theOrion Nebula, is suspected to have involved a dynamical interaction between multiple stars and might have involved aprotostellar merger. The radio source I is suspected to be either a close binary or the result of such a merger.^[16][17]

An analysis of the eclipses of KIC 9832227 initially suggested that its orbital period was indeed shortening,and that the cores of the two stars would merge in 2022.^{[18][19][20][21]}However subsequent reanalysis found that one of the datasets used in the initial prediction contained a 12-hour timing error, leading to a spurious apparent shortening of the stars' orbital period.^{[22][23][24][25]}

The mechanism behind binary star mergers is not yet fully understood, and remains one of the main focuses of those researching KIC 9832227 and other contact binaries.

• "Pau Amaro Seoane MODEST working group 4 "Stellar Collisions"". Retrieved20 May 2013.

• Stellar dynamics
• Stellar astronomy
• Impact events
• Concepts in stellar astronomy

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