Pulsars
Stars that pulse like clocks, beginning with the 1967 chart on which a Cambridge graduate student found the first one, and the weeks her group spent asking whether it was artificial.
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Little green men: Jocelyn Bell and the first pulsar, 1967
In 1967 Jocelyn Bell, a graduate student at Cambridge, found a radio source that pulsed every 1.337 seconds. Her group labelled it LGM, for Little Green Men, and by their later accounts its senior astronomers weighed whether it might be artificial. This article sets out the chart, the tests that showed it was a pulsar, and the question about artificial signals it left open.