An Accretion Disk is a disc-shaped cloud of gases, debris, etc that is spiraling in towards a central stellar body. Typically this central unit is a young star, a protostar, a white dwarf, a neutron star, or a black hole.
This is how young stars accumulate mass, by slowly drawing it in via an accretion disk. It takes several million years for a newborn star to finish absorbing everything it's going to suck up out of the system.
In some binary systems that feature a main sequence star and a black hole, the star will regularly jettison plasma (see Coronal Mass Ejection) that immediately spins into the accretion disk of the paired black hole.
The same gravitational forces that cause the matter to circle around and flow into the central body also cause the matter to emit electromagnetic radiation. Accretion discs of young stars and protostars radiate in the infrared, those around neutron stars and black holes in the X-ray part of the spectrum.
Game and Story Use
- Artifacts of a lost Precursor civilization might be found amidst the accretion disc of a neutron star. Prior to the star going supernova and collapsing into the neutron stage, there was a flourishing Type II civilization here. Now all that's left is the irradiated remains of their strongest stellar megastructures and a few pockmarked planets, all slated for destruction at the hands of the star that birthed them.
- If you're playing fast and loose with gravity (and really, who doesn't?), you could have the PCs spacecraft get caught in the disk, with their only escape being to fly out the winding length of the spiral, passing all your plot points and encounters in the process.