Institute of Astronomy

Black holes and companion stars

Published on 22/05/2013 
Question: 

I was wondering what it's called when there is a star next to a black hole and you can watch the black hole pulling in particles of light? I know there is a word for it i just cant find it.

Your description could match a couple of things. I think you are looking for "accretion from a binary companion". Accretion is the term we use for an object growing by accumulating matter that falls onto it. Many stars a found in multiple systems like binaries (where there are two stars). The more massive star will evolve more quickly as it burns up its fuel quicker, and so may collapse down to a black hole while its companion is still a regular star. As the companion evolves it may become a giant, puffing up in size, so the outer layers can get stripped off by the black hole. The material swirls around the black hole forming an accretion disc before eventually spiralling in. I think this is what you had in mind. The accretion disc can get very hot, hot enough to emit X-rays, in which case the system is referred to as an X-ray binary. Studying X-ray binaries has given us our best understanding of stellar mass black holes.

The other effect you could be referring to is gravitational lensing. This is when the trajectory of light appears curved because of the gravity of a massive object (often a black hole, a galaxy or a cluster of galaxies). This might match "pulling in particles of light", but you can't normally see this (since if the light is being pulled in, it can't escape for us to see): I think you're actually thinking of the stream of hot plasma being accreted from a star.