What Is an Accretion Disc? Matter Spiralling In and Heating Up

Material falling towards a massive object almost never falls directly in, because it arrives with some sideways motion and that rotation is conserved as it approaches, causing it to swing around rather than plunge. Collisions between particles moving on different paths cancel out motion perpendicular to the average plane of rotation while preserving the rotation itself, so the cloud flattens into a disc. Within the disc, friction between adjacent rings moving at different speeds transfers rotation outward and allows material to drift inward gradually, which is why the structure is described as accreting. That same friction converts gravitational energy into heat, and the temperature rises steeply towards the centre, reaching millions of degrees near a compact object and producing radiation across the spectrum up to X-rays.

The same structure appears at enormously different scales:

• •Around forming stars, where the disc is the material from which planets subsequently assemble
• •In close binary systems, where one star draws material from a companion
• •Around neutron stars and stellar-mass black holes, where the inner disc is hot enough to emit X-rays
• •Around supermassive black holes at the centres of galaxies, powering quasars and active galactic nuclei
• •Around planets during formation, producing the regular satellite systems of the giant planets
• •Planetary rings, which are a cold and stable relative rather than an accreting structure

Accretion onto a compact object is the most efficient process for converting mass into energy known in nature, which is the reason these structures dominate the list of luminous objects. Nuclear fusion in stars converts under one percent of the mass involved into energy. Accretion onto a black hole can convert around ten percent, and onto a rapidly rotating one considerably more, because the material can fall far deeper into the gravitational well before it is lost, releasing energy the whole way. That efficiency is why quasars, powered by accretion onto supermassive black holes, outshine entire galaxies containing hundreds of billions of stars, and why they were detectable across the observable universe before anyone understood what they were. It also sets a limit, since radiation from the disc pushes back on infalling material and caps how fast a given object can accrete.

These structures are too small and too distant to resolve directly in almost every case, so their existence is inferred from what they emit and how it varies. The spectrum is one route, since a disc has a temperature that rises towards the centre and therefore emits a characteristic blend of wavelengths that a uniform source would not. Variability is another, since material falling in produces flickering on timescales set by the size of the emitting region, and a rapid flicker implies a small one. Spectral lines emitted by the disc are broadened and shifted by the rotation, and modelling the shape of a line recovers the velocity and therefore the geometry. Direct imaging became possible for the region around two nearby supermassive black holes using a network of radio telescopes operating as a single instrument, which produced the first pictures of the shadow cast by the object on the light around it.

The basic picture is secure and important details are not. The friction that allows material to drift inward is far stronger than ordinary molecular viscosity can explain, and the accepted mechanism involves magnetic fields in the ionised gas generating turbulence, which was proposed decades after the discs were first modelled and is still being worked out in detail. Jets of material shot outward along the rotation axis at near light speed are observed in many systems and the process launching them is not fully understood, though magnetic fields are clearly involved. The behaviour of the innermost region, where general relativity dominates, is directly relevant to testing that theory and has become observationally accessible through imaging of the region around nearby supermassive black holes and through gravitational wave observations of merging compact objects.

Infalling material arrives with sideways motion that is conserved, so it swings around rather than dropping in, and collisions flatten it into a disc while friction lets it drift inward and heats it enormously. Accretion onto a black hole converts around ten percent of mass into energy against under one percent for fusion, which is why quasars outshine whole galaxies.