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Accretion is the adding of material to an existing mass. In astrophysics it has two similar meanings. One relates to the gravitational pulling of other objects into a larger object, which then adds mass to itself, usually referred to as an accretion disc. The other, in nebular theory, addresses the sticking together of microscopic dust and ice to create a larger object. Both will be discussed in this article.
The most common definition of accretion is the growth of a massive object by gravity, slowly adding more matter. This matter is usually typically gaseous and creates in an accretion disc. Accretion discs are common around small stars or remnants of stars in a binary system, or near black holes at the center of spiral galaxies.
The second definition refers to the slow accumulation of minute particles of dust and ice to increase the size of an object. On occasion, a very large object may attract a smaller object and the resulting collision can lead to the growth of the larger object. This is thought to have happened with several of the largest asteroids. Asteroids are covered in a layer of dust that has accreted on them during their travels through the universe.
Here on Earth, parts of our planet go through a geological accretion process. Plate accretion explains the addition of material to a tectonic plate. As tectonic plates collide, one of the plates may subduct, or slide under the other. The plate which is being subducted floats on the asthenosphere and is pushed against the bottom of other plate. This scraping causes the sediment to come off the subducted plate and form a mass of material called the accretionary wedge, which attaches itself to the subducted plate. Also, there is landmass accretion. As water flows it allows the addition of sediment to a coastline or riverbank, increasing land area. The most noteworthy landmass accretion is the deposition of alluvium, containing precious metals, onto riverbanks and into river deltas.
The process of accretion occurs every where in the universe. The process may explain the initial development of all large masses in the universe.
We’ve also recorded an episode of Astronomy Cast just about Black Holes. Listen here, Episode 18: Black Holes, Big and Small.