Three Size Classes

Black holes are not all alike. Astronomers sort them by mass into three broad groups:

• Stellar-mass black holes, a few to a few dozen solar masses, born when massive stars die
• Supermassive black holes, millions to billions of solar masses, anchoring the centers of galaxies
• Intermediate-mass black holes, the elusive middle ground that telescopes are only now starting to pin down

The gap between stellar and supermassive is one of the field's open puzzles: it is still unclear how the giants grew so large so early.