Accretion disks are common in astronomy and are found around:

* Stars: Young stars form from accretion disks of gas and dust.

* Black holes: Black holes can also have accretion disks formed from material pulled in by their immense gravity.

* Neutron stars: These dense, collapsed stars can also have accretion disks.

The particles in an accretion disk orbit the central object and collide with each other. These collisions cause the particles to lose energy and spiral inwards towards the central object. This process can generate immense heat and light, making accretion disks visible in many astronomical objects.

Here are some additional points about accretion disks:

* They are often very thin: This is because the particles are orbiting in the same plane and are constantly colliding, smoothing out the disk.

* They can be very hot: The friction from the particles colliding generates a lot of heat.

* They are important for the formation of stars and planets: Accretion disks provide the material and energy needed for these objects to form.

Let me know if you'd like to know more about accretion disks or any specific type of accretion disk!