The main light source from a black hole is a structure called an accretion disk. Black holes grow by consuming matter, a process scientists call accretion, and by merging with other black holes. A stellar-mass black hole paired with a star may pull gas from it, and a supermassive black hole does the same from stars that stray too close. The gas settles into a hot, bright, rapidly spinning disk. Matter gradually works its way from the outer part of the disk to its inner edge, where it falls into the event horizon. Isolated black holes that have consumed the matter surrounding them do not possess an accretion disk and can be very difficult to find and study.