Accretion discs are flattened, rotating formations of gas and dust found throughout the cosmos. They play a significant role in how objects in space grow and emit energy, from the birth of stars to the powering of distant galaxies.
Defining Accretion Discs
An accretion disc is a structure formed by diffuse material in orbital motion around a massive central body. This disc consists of gas, plasma, dust, or other particles pulled towards the central object by its gravitational field. As this material orbits, it slowly spirals inward, forming a flattened, disklike flow.
The material within an accretion disc does not fall directly into the central object due to its angular momentum. This rotational momentum causes the material to flatten into a disc perpendicular to the central object’s rotation axis. Within the disc, particles rub and collide, leading to friction and heating. This process converts gravitational potential energy into kinetic energy and then into heat, which is radiated away, allowing the material to gradually lose angular momentum and drift inward.
Common Locations in Space
Accretion discs are found around various celestial bodies. They surround young, newly formed stars, known as protostars, where they are called protoplanetary discs. These discs are the birthplaces of planets, with material gradually clumping together to form larger bodies.
Accretion discs also surround compact objects like black holes, both stellar-mass and supermassive, and white dwarfs. In binary star systems, a compact object can pull material from a companion star, forming an accretion disc as the gas spirals towards it. The largest accretion discs, spanning scales comparable to our solar system, are found around the supermassive black holes at the centers of active galaxies.
Formation and Dynamics
Accretion discs form when material possessing angular momentum is drawn towards a central object by gravity. As this material collapses inward, the conservation of angular momentum causes its rotation to increase, leading to the flattening of the cloud into a disc. This process is similar to how a spinning ice skater speeds up when pulling their arms inward.
For the material to move inward and accrete onto the central body, it must shed angular momentum. This occurs primarily through viscosity, which acts as a form of internal friction within the disc. Colliding particles convert their kinetic energy into heat, and this heat is then radiated away, reducing the particles’ angular momentum and allowing them to spiral closer to the central object. Magnetic fields within the disc, particularly through the magnetorotational instability, can also contribute significantly to this angular momentum transport.
As material spirals inward, its gravitational potential energy is converted into heat, causing the disc to become extremely hot and luminous. The temperature of the disc varies with distance from the central object; inner regions are hotter and emit higher-energy radiation, while outer regions are cooler. This energy release powers the emission of various forms of electromagnetic radiation, from infrared in protoplanetary discs to X-rays from discs around black holes.
Astrophysical Significance
Accretion discs are fundamental to understanding the formation and evolution of stars and planetary systems. Protoplanetary discs surrounding young stars are where planets coalesce from gas and dust. The disc’s dynamics, including angular momentum transport, directly influence how and where planets form.
These discs are also responsible for the immense luminosity observed from active galactic nuclei (AGN) and quasars. Supermassive black holes at the centers of galaxies accrete vast amounts of material through these discs, converting gravitational energy into radiation with high efficiency. This process can be significantly more efficient than nuclear fusion in generating energy.
Accretion discs around compact objects like black holes and neutron stars also produce high-energy radiation, such as X-rays and gamma rays, which astronomers use to study these otherwise difficult-to-observe entities. The study of accretion discs provides important insights into the energy output of some of the most powerful phenomena in the universe and the growth of celestial bodies across cosmic scales.