A Stellar collision is the coming together of two astronomical bodies, which through the force of gravity, merge into one larger unit. In a more basic sense, a stellar collision is a collision of stars.[1] Astronomers predict that over several hundred such collisions can take place somewhere in our galaxy, where events of this type occur about once every 10,000 years.[2]. It is also predicted, that under the correct conditions, even three or four stars can collide and merge into a single star. While much is unknown about this theory, there are new strides being made every day in order to try to prove that this phenomenon occurs in the universe. Scientist have not yet been able to physically see a collision due to technological constraints, but they are able to detect the gravitational waves created by these collisions, allowing them to make predictions about what really occurs.[3]
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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.) Examples would include White Dwarf stars, Neutron Stars, Black holes, Main Sequence Stars, Giant stars, and Supergiants. Differerent types, masses, temperatures, and radiuses will result in different collisions or mergers. All astronomical bodies have different reactions in the way they collide due to forces such as gravity, and due to the physical parameters of the star.[2]
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 1a supernovae. When a white dwarf comes near a "live star", they get caught in each other's gravity and begin a slow revolution around each other. While this happens, the white dwarf is rotating and begins sucking gas from its companion star until it reaches the Chandrasekhar limit, in which carbon fusion begins in the core increasing the interior temperature. Since the white dwarf consists of degenerate matter, there is no safe relation between pressure and temperature. Because of this, the reactions spreads outward at runaway speed with no barrier to stop it. This causes a collapse of outside layers causing a supernova shockwave, and in a matter of seconds, all of the white dwarfs mass is thrown into space.[4]
Neutron star collisions occur in a similar fashion to type 1a supernovae. When two neutron stars travelling through the galaxy get caught inside each other's gravity, they begin to revolve around each other, moving in closer as time passes. Once the two neutron stars meet in the middle, a huge collision occurs. This creates a magnetic field that is trillions times stronger than that of Earth, in a matter of one or two milliseconds. Astronomers believe that this event is what creates certain kinds of gamma-ray bursts.[5]
While the theory 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 proven. The theory of stellar collisions started in 1764 when a cluster of stars known as Messier 30 was discovered by astronomer Charles Messier. After years of observation, fellow astronomers concluded that the cluster was approximately 13 billion years old.[6]. Around 250 years later, the Hubble Telescope resolved the individual stars of Messier 30. With this new technology, astronomers discovered that some stars, known as “blue stragglers”, were younger than other stars in the cluster.[6] 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.[6]
While stellar collisions may occur very frequently in certain parts of the galaxy, the likelihood of a collision involving the Sun is very small. A probability calculated predicts the odds of a stellar collision involving the Sun is 1 in 10 trillion, trillion years.[3] The likelihood of close encounters with the Sun is also small. The rate is estimated as follows:
where N is the number of encounters per million years that come within a radius D of the Sun in parsecs.[7] For comparison, the mean radius of the Earth's orbit, 1 AU, is 4.82 × 10−6 parsecs.
Even though our star will likely not be directly affected by such an event, the Earth may very easily be impacted by a nearby collision. Astronomers say that if a stellar collision happens within 100 light years of the Earth, it could possibly result in the planet's destruction.[3] This is still very unlikely though because there are no stellar clusters this close to the Solar system.