Beginner · 1:22

Pulsar

How a dead star becomes a cosmic lighthouse

Each is simulated from real physics and measurements; anything no eye could see, like magnetic fields and radio waves, is drawn and labelled as drawn.

Below are the 7 chapters of this simulation in Cosmic Scale: each one's caption, then the fuller explanation the app shows under “Read more”. Colours, arrows and lines mentioned in the text belong to the 3D scene in the app.

Open Cosmic Scale then the Phenomena tab

1. A city-sized star

When a star of more than eight Suns dies, its core collapses into a neutron star: about 1.4 times the Sun's mass in a ball some 22 km across.

A big star shines by fusing light atoms into heavier ones in its core. Once the core has turned to iron, that fire goes out, because fusing iron gives no energy. With nothing left to hold it up, the core, about as big as Earth, falls in on itself in less than a second.

The crush is so great that electrons are pressed into protons, turning them into neutrons. The fall stops only when the neutrons are packed as tightly as in the nucleus of an atom. The rest of the star is blasted into space in a supernova, and the neutron star is left behind.

Apart from black holes, nothing known is denser: a sugar cube of neutron star would weigh about as much as all the people on Earth together. Its gravity bends light. The ring of stretched stars around it is starlight bent by its pull, and you can see about four fifths of its surface at once.

2. A spinning magnet

The collapse spun it up and squeezed its magnetic field to trillions of times the strength of Earth's. Its magnetic poles are tilted away from the axis it spins on.

Think of a skater who pulls in their arms and spins faster. The dying star's core was turning slowly, but when it shrank from the size of Earth to the size of a city, it spun hundreds of thousands of times faster. Newborn neutron stars can turn many times a second.

The squeeze also packed the star's magnetic field into a far smaller space, which made it enormously stronger. A few neutron stars, called magnetars, are a thousand times stronger still: the strongest magnets known.

As on Earth, the magnetic poles are not where the spin axis comes out. The white dashed line is the axis the star spins on, the violet one its magnetic axis, tilted away from it. That tilt is what will turn the star into a lighthouse.

3. Two beams

Above the magnetic poles, particles torn from the surface race out along the field and shine two narrow beams of radio waves. Radio is invisible, so here it is drawn green.

A spinning magnet makes electricity: that is how the generators in power stations work. A neutron star is a gigantic one. Its spin drives voltages of trillions of volts, strong enough to tear electrons and other charged particles out of its surface.

Charged particles can only move along the magnetic field lines, like beads on a wire. Above each magnetic pole the lines lead out into space, and there the particles are flung outwards at nearly the speed of light. On the way they give off radio waves in a narrow cone: the beam. Exactly how they do it is a puzzle scientists are still working on.

The beams' energy comes from the star's spin, so every pulsar slowly winds down over millions of years. Some, like the Crab and Vela pulsars, also flash in visible light, X-rays and gamma rays.

4. A cosmic lighthouse

As the star turns, its beams sweep round the sky like a lighthouse's. Each time one crosses Earth, radio telescopes catch a pulse.

The beams are fixed to the magnetic poles, and those are tilted, so as the star spins each beam sweeps round in a circle across the sky. The star does not switch on and off: like a lighthouse's lamp it shines all the time, and we see a flash only while a beam points our way.

The gold arrow points the way to Earth. The green line above the caption is the radio signal as a telescope on Earth records it: flat while the beams point elsewhere, a spike each time one sweeps past us.

Most pulsars' beams never cross Earth, so we cannot see them at all. Even so, more than 4,000 pulsars have been found, nearly all of them in our own galaxy.

5. Its real speed

Slowed down, you can follow each turn. The Vela pulsar really turns 11 times a second, and the fastest known 716 times. Pick a pulsar to see and hear it.

Until now the spin was slowed down so you could follow it. The button at the top switches between slow motion and real speed, and the list beside it holds four real pulsars, from the first one ever found, which turns once every 1.34 seconds, to the fastest known.

The fastest, PSR J1748−2446ad, turns 716 times a second: its equator whirls round at about a fifth of the speed of light. Pulsars this fast are old ones spun up again. A companion star poured gas onto them, pushing them round faster and faster, like a hand pushing a merry-go-round.

With the sound on you can hear the difference. Slow pulsars beat like a drum, fast ones buzz, and at hundreds of pulses a second the beats blur into a musical note.

6. The light cylinder

At 4,262 km from the axis, the field would have to move at the speed of light to keep up. It can't, so from there the field lines open and stream away in a wind.

The magnetic field is anchored in the star and turns with it, like the spokes of a wheel. The farther out along a spoke, the faster it moves, just as the edge of a merry-go-round moves faster than its middle.

At some distance from the axis, the field would have to move at the speed of light to keep up, and nothing can go that fast. That distance marks out an imaginary cylinder round the star: the light cylinder. For this pulsar it lies 4,262 km from the axis. The faster a pulsar spins, the smaller its light cylinder.

Inside it, the field lines loop back to the star, and the particles on them turn with it. Lines that would reach beyond cannot close: they open up and trail out in a spiral, carrying a wind of particles away at nearly the speed of light. Around the Crab pulsar, this wind lights up the whole Crab Nebula, about 11 light-years across.

7. Clocks in the sky

Jocelyn Bell Burnell found the first pulsar in 1967. Pulsars keep time so steadily that a map of 14 of them, on the Pioneer and Voyager probes, shows where our Sun is.

In 1967 Jocelyn Bell Burnell, a student at Cambridge, spotted a bit of “scruff” on the paper charts of a radio telescope she had helped to build: pulses 1.34 seconds apart, as regular as a clock. The team half-jokingly called the source LGM-1, for “little green men”, until more were found and they turned out to be spinning neutron stars.

The 1974 Nobel Prize for the discovery went to her supervisor, Antony Hewish, but not to her. In 2018 she won a $3 million Breakthrough Prize and gave all of it to help students from groups under-represented in physics.

The fastest pulsars keep time about as well as atomic clocks. Timing one revealed the first planets ever found around another star, in 1992. By timing 68 of them for 15 years, astronomers found signs in 2023 of gravitational waves, ripples in space itself, washing through our galaxy.

The map on the Pioneer plaques and Voyager records shows the direction, distance and beat of 14 pulsars as seen from the Sun. Pulsars slow down steadily, so anyone who found it could work out not only where the probes came from but roughly when they set off.

Objects in this story

Real photos