You can't see a black hole until a star wanders too close. Then tidal forces stretch the star like a cook rolling out dough, and some of the material falls into the hole, flaring up as a final goodbye. Such events, called tidal disruptions, are a rare chance to weigh a black hole that would otherwise remain invisible.

Astronomers studied 42 such flares and compared them with ordinary galaxies. It turned out that in large galaxies where stars no longer form, black holes are heavier than in galaxies with fresh stars. This has been known for a while: the idea is that the black hole 'blows away' gas and stops star formation.

But among the galaxies with tidal disruptions, there was a surprise. In small galaxies, about a billion times the Sun's mass, it's the opposite: 'sleeping' galaxies host lighter black holes than their star-forming neighbors of the same size. Such a black hole weighs only a few million Suns, while in the neighbors it's tens of times more massive.

Perhaps these galaxies lost their gas due to external influences — for example, passing through a cluster where hot gas 'blows away' cold gas, like wind blowing leaves. Then stars stopped forming, and the black hole was left on short rations. If such cases aren't rare, we may need to rethink the role of black holes in galaxy evolution: sometimes they're not conductors but spectators.