Implementation and Quality/Performance Settings
Previous versions of SpaceEngine (starting from 0.980) had flat, two-dimensional accretion disks: this was a “fast and cheap” solution, and worked even on lower-end PCs. Volumetric disks require more powerful hardware, since the shaders have to integrate the emission and opacity of multiple points of a volume along a ray. This volume contains procedurally-generated and animated noise, which are used to create the shape of the hot plasma “clouds” rotating around the central black hole. But things are even more complicated, since we also have to integrate light rays in a warped space-time! This is the only way to correctly render objects (like the accretion disk) near the event horizon.



Reproducing Famous Black Holes in SpaceEngine
We have also been trying to faithfully reproduce the accretion disks around some famous objects, such as the supermassive black holes at the center of M 87 and the Milky Way, and some well-known stellar-mass black holes like SS 433 and Cygnus X-1. For example, based on known data, the accretion disk around the Milky Way’s black hole (known as Sagittarius A*) is almost transparent and is hot and geometrically thick (i.e. “puffy”), with a very low radiative efficiency – almost none of the energy in the disk, perhaps just one part per million, is emitted as light. This hot, advection-dominated disk produces strong outflows, similar to stellar winds, that fuel a broad jet, which accelerates in the directions perpendicular to the disk, carrying much (perhaps most) of the matter away before it can reach the black hole – maybe as little as 1% of it reaches the event horizon, with the rest being flung out into the galaxy or shot into distant orbit around the black hole to return again in the future. Matter is accreted onto this disk at a very slow rate, less than one millionth of a solar mass per year – not much more than the famous black hole Cygnus X-1, which is smaller by a factor of 50,000! We are living in an inactive galaxy, with a starving central black hole, but it may be that this is why we are alive in the first place…

