Ralph P. Eatough

Radio telescopes scattered across the globe pick up waves from distant black holes. But two telescopes placed almost side by side see only a blurry blob — fine details are beyond them. Usually, such pairs are considered useless and discarded. But that's a mistake: even a blurry blob holds clues.

Rhythmic flashes of a lighthouse in thick fog: that's what searching for pulsars looks like. The lighthouse spins, and its beam periodically sweeps across the telescope. But the fog smears the light, making the period seem slightly different. If the same lighthouse was observed twice, you can compare: the periods should nearly match, and the smearing should be similar.

In the dense family of old stars that astronomers call the cluster NGC 6791, a lonely radio beacon has been spotted — the pulsar PSR J1922+3745. Such objects are born when a massive star explodes and its core collapses into a tiny sphere the size of a city, which spins and emits radio waves like a rotating lighthouse beam.

At the center of the Milky Way lies the supermassive black hole Sagittarius A*. It is four million times more massive than the Sun, but by cosmic standards it behaves quietly: it only occasionally flares up when it swallows a clump of gas.