Observatory · the lighthouse
Neutron stars and pulsars
When a big star dies, its core falls in on itself in less than a second. What is left is a ball about 24 kilometres across, the size of a city, with more mass than the Sun: a neutron star. It spins, sometimes hundreds of times a second, and its magnetic field throws two beams of radio waves out of its poles. If the field is tilted, the beams sweep round like a lighthouse, and every time one passes over the Earth a radio telescope picks up a tick.
The lighthouse
Fig. I. A neutron star and its beams, and the pulses received, one line a turn.
- Pulsar
- Turns a second
- Its equator moves at
- Shown
- Seen from the Earth
The star spins round the vertical axis; the beams come out along the magnetic axis, tilted away from it. Each time a beam crosses the dashed line to the Earth, a pulse arrives, and on the right every turn adds one more line, the newest at the bottom. The pulses are never quite the same, but the average shape is as steady as a fingerprint. Turn the line of sight away and the beams miss us completely: most pulsars in the galaxy are invisible to us for that reason. Set the tilt to 90° and look from the side, and both beams sweep past: a pulse and, half a turn later, an interpulse.
Press listen to hear each pulsar at its true speed, worked out from the same beams. The first ever found ticks a little faster than once a second. The Crab, the remains of a supernova seen in 1054, turns 30 times a second and buzzes. The fastest known spins 716 times a second and sings: a star more massive than the Sun, turning faster than a kitchen blender.
Why it spins so fast
Fig. II. A dying star's core falling in, spinning up as it shrinks.
- Radius
- One turn in
- Magnetic field
A skater who pulls in her arms spins faster: the spin is kept, so a smaller body has to turn quicker. A dying star's core, about the size of the Earth, falls in to a ball of 12 kilometres: 500 times smaller, so it turns 500 × 500 = 250,000 times faster. A core that turned once an hour comes out turning about 70 times a second. The magnetic field is squeezed into the same small surface and gets 250,000 times stronger too; that field, spun round at that speed, is what powers the beams. The ball is drawn on a squashed scale: at true scale the last moments would be a dot.
The beams cost energy, so every pulsar slowly runs down; the Crab loses about 36 billionths of a second a day. A few are spun up again, by gas falling onto them from a companion star, and end up as the millisecond pulsars, the steadiest clocks in nature, hundreds of turns a second for billions of years.
Who worked it out
Where it already matters
Little green men. In 1967 Jocelyn Bell, a PhD student in Cambridge, noticed a bit of "scruff" in the paper charts of a new radio telescope: a pulse every 1.337 seconds, too regular for anything natural they knew. They half-jokingly named it LGM-1, for little green men, until she found a second one elsewhere in the sky. The Nobel prize of 1974 went to her supervisor, Antony Hewish, and Martin Ryle, not to her; she later gave her own Breakthrough Prize money of 2018 to students who are underrepresented in physics.
An album cover. In 1970 Harold Craft plotted 80 pulses of that first pulsar one under the other, as on the right. Nine years later the picture became the cover of Joy Division's Unknown Pleasures, and it has been on T-shirts ever since.
Clocks in space. In 1974 Hulse and Taylor found a pulsar orbiting another neutron star. Its ticks showed the two spiralling slowly together, losing exactly the energy Einstein's gravitational waves should carry off, the Nobel prize of 1993 and the prelude to the chirp. In 1992 the first planets ever found outside the solar system turned up around a pulsar, from tiny delays in its ticks. And in 2023 timing networks of dozens of millisecond pulsars, among them European telescopes in Westerbork and Effelsberg, found the first evidence of a slow background of gravitational waves washing through the galaxy.
An address. The plaques on Pioneer 10 and 11 and the Voyager golden records carry a map of 14 pulsars, each line marked with its period, to show where the Sun is to anyone who finds them.