Phys.org
Accretion disc
The flat, ferociously hot spiral of matter falling onto a compact object.
Gas does not fall straight into a black hole or a young star, because it arrives with sideways motion it cannot shed. Instead it settles into a disc and spirals inward slowly, rubbing against itself. That friction is the point: it heats the inner disc to millions of degrees, which is why some of the brightest objects in the universe are powered by material that has not fallen in yet. A quasar is an accretion disc around a supermassive black hole, outshining every star in its galaxy combined. The same physics, at gentler temperatures, is how planets form around a new star.
Where it has come up lately
Related terms
Event horizon
The boundary around a black hole past which nothing gets out — not a surface, but a point of no return.
Singularity
The point where our description of gravity stops producing meaningful answers.
Schwarzschild radius
How small you would have to squeeze something to make it a black hole.
Supermassive black hole
Millions to billions of solar masses, one at the centre of essentially every large galaxy.
Questions
What is Accretion disc?
The flat, ferociously hot spiral of matter falling onto a compact object.
Why does it matter?
A quasar is an accretion disc around a supermassive black hole, outshining every star in its galaxy combined. The same physics, at gentler temperatures, is how planets form around a new star.