In 1967, Jocelyn Bell Burnell and Antony Hewish observed, in the night sky, strong radio pulses separated by 1.33 seconds, like some sort of cosmic alarm clock. While these radio bursts were all but certain to be natural, the source was named Little Green Men-1, or LGM-1. What Burnell and Hewish had, in fact, observed was what has come to be called a pulsar.

But what, exactly, is a pulsar? ...