Cosmic Lighthouses Pulsars and the Stars That Keep Perfect Time

The most consequential quarter inch in modern astronomy was easy to miss. It was a patch of scruff on a roll of chart paper, a place where the pen's line stopped being a calm wobble and turned jittery for a moment before settling down again.

The paper came out of a radio telescope in the English countryside near Cambridge. It was not a dish. It was a field of wooden posts and wire, about four and a half acres of it, strung with thousands of dipole antennas and many miles of cable. Graduate students had helped build it by hand over two years. One of them was Jocelyn Bell, a 24-year-old from Northern Ireland who had studied physics at Glasgow. Her supervisor, Antony Hewish, had designed the instrument to hunt for quasars, those distant, brilliant galactic cores. The trick was to watch for scintillation: the way a compact radio source twinkles when its signal passes through the stream of particles flowing off the Sun, much as starlight twinkles in our atmosphere.

The telescope swept across the sky as the Earth turned, and a pen recorder drew the results. It produced many yards of paper every day, and Bell's job was to read all of it, by eye, with a ruler and a lot of patience. She learned what a quasar looked like on the page. She learned which wiggles came from a passing car's ignition or a distant radio transmitter.

Then she found a bit of scruff that fit nothing she knew. It showed up at a time of night when scintillation should have been weak. When she checked back through earlier records, the same smudge appeared again, at the same position in the sky. A glitch on Earth would follow the clock. This followed the stars. Whatever was producing it was out there, among them.