Glossary term: accretion

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Description: Accretion is the process of an astrophysical object attracting additional matter, typically gas or dust, by its gravitational pull. Unless that matter is already moving straight towards the attracting object, the basic laws of physics ("conservation of angular momentum") do not allow the falling matter to reach the attracting object directly. That is why, typically, infalling matter will collect in what is called an accretion disk: a swirling disk of gas and dust surrounding the attracting object. From the inner rim of the disk, matter can fall onto the central object. For a compact central object, matter falling onto the accretion disk will have gained an enormous amount of energy while falling. As this energy is deposited in the disk, the disk can heat up to temperatures of hundreds of thousands or even millions of Kelvin. Accretion onto a compact object is the most efficient physical way of heating up matter. In this way, accretion disks around black holes or neutron stars are some of the brightest objects in the universe. Accretion also plays an important role in the formation of young stars.

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