ACCRETION DISCS

Many powerful phenomena come not from stars, but from the dusty discs around them

There are many kinds of astronomical objects that grow or evolve by gravitationally attracting and harvesting nearby material, a process termed ‘accretion’. The accretion disc is simply what we call the gathered material. It is the machine that surrounds an object and allows it to grow larger and more massive. For example, stars are born at the centre of protoplanetary discs, illustrated above. As dust and gas orbit in the disc, it loses momentum and spirals into the central regions, where it accumulates.

In binary stars, accretion discs are created when a secondary star fills its Roche lobe and spills material into the lobe surrounding an adjacent primary star. The gas leaves the inner Lagrangian point and runs in a narrow stream towards and around the primary star, creating a ring-like flow. Friction in this ring causes the gas to heat up, converting potential energy into kinetic energy. The gas also loses angular momentum, so it drops down to lower orbits and spreads slowly inwards, forming a fully fledged disc. The inner regions of an accretion disc around a stellar mass black hole can have temperatures measuring millions of Kelvin – hot enough that they emit most of their energy as X-rays. Astronomers find these emissions useful because they can detect the disc easily and thus infer the presence of a black hole, even if the latter emits no radiation. Supermassive black holes also exhibit accretion discs, although they are orders of magnitude larger and cooler.