Da-Bin Lin

In April 2021, in a galaxy 35 megaparsecs away, supernova SN 2021foa erupted. It immediately puzzled astronomers: its spectrum feverishly switched between hydrogen and helium lines — as if the star, in agony, was frantically shedding its outer layers.

At the heart of nearly every large galaxy shines an active nucleus — a supermassive black hole surrounded by a flat, rotating disk of incandescent gas. The disk itself flickers, but astrophysicists suspect that within it lurks a whole swarm of small, stellar-mass black holes.