Stars are sometimes born very close together and when one of them swells up into a red giant, the outermost layers of the red giant can be transferred to its companion star.

But if one of those stars died first and became a black hole, then there can be fireworks, as shown in this painting. The very strong gravity near the event horizon causes the gas from the companion star to be accelerated to enormous speeds as it approaches. Friction causes the gas to heat up so that the gas emits very high energy light--the light of x-rays. So, a cool star emitting x-rays and in a fast orbit with an unseen companion suggests the presence of a black hole. The binary system Cygnus X-1 was our first candidate for black hole-dom.