How massive is that black hole?

A black hole is a point in space that is a cosmic sink—the gravitational attraction is so strong that not even light can escape. A supermassive black hole (SMBH) is enormous, with a mass on the order of millions to billions of times that of the Sun (1). It's not clear how such an entity arises. It could be the result of the merger of smaller black holes or the collapse of either a stellar cluster or large gas clouds. Every large galaxy is thought to contain a SMBH. To understand how they arise, we need to know how massive they are. On page 789 of this issue, Burke et al. (2) describe a method to make this determination on the basis of radiation emissions from the accretion disks of SMBHs. The approach also shows a connection between SMBHs and much less massive objects, such as white dwarf stars.