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Accretion. In short, accretion
 means growth by accumulation.

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Land on Earth can form from accretion
 of soil deposited by rivers or sea.

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Planetary bodies can also grow
 from accretion as gravitational forces

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attract debris and other matter. 

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But we want to talk about
 accretion in astronomy. 

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Accretion is one way
 we can see black holes.

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Black holes have intense
 gravitational mass,

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pulling in everything in the vicinity,
 including light.

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Without that light,
 we can't see black holes, but we can see

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the disk-shaped flow of gas and dust
 that forms around some black holes.

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We call these accretion disks,
 and they can form around

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black holes of any size,
 as well as around some stars.

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Material in black hole accretion disks
 is really hot, like millions of degrees.

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This is caused by the collision 

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of particles in the accretion disk
 as they are pulled into their black hole.

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These collisions can cause
 accretion disks to release X-rays,

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which can allow us
 to locate some black holes.