A signal that kept perfect time

Jocelyn Bell got a fast chart recording of the odd "scruff" she had been seeing for months, and it turned out to be a pulse every 1.337 seconds, steadier than a clock.

Nothing natural was supposed to be able to do that, so the source was labelled LGM-1, for Little Green Men, until a second one turned up in a different part of the sky. They are neutron stars: a sun's worth of matter in a city-sized ball, spinning fast enough to whip a beam past us.