An accretion disk is a dense, rotating disk of gas, dust, and other matter that forms around a celestial object, such as a black hole or a young star. The material in the disk is drawn towards the central object by its gravity and spirals inward, eventually accreting or merging with the central object.
The formation of an accretion disk is a result of the conservation of angular momentum. As material spirals towards the central object, it tends to retain its initial rotational motion, resulting in the formation of a disk. The material in the disk becomes increasingly dense as it approaches the central object, and friction and other processes cause it to heat up and radiate energy, often in the form of visible light or X-rays.
Accretion disks are observed in a variety of astrophysical systems, including:
Active Galactic Nuclei (AGN): Accretion disks around supermassive black holes in the centers of galaxies can emit vast amounts of energy and are believed to play a role in the evolution of galaxies.
X-ray Binaries: Accretion disks around neutron stars or black holes in binary star systems can produce intense X-ray emission, making them valuable objects for studying accretion physics.
Protostars: Young stars are surrounded by accretion disks in the early stages of their formation, as material from the surrounding molecular cloud falls onto the central object.
The study of accretion disks is important for understanding the processes that drive the evolution of celestial objects and the emission of radiation from these systems. The properties of accretion disks, such as their size, temperature, and structure, can be used to probe the physical conditions near black holes and other central objects, providing insight into the processes that govern their behavior.