How fast does the Crab Pulsar spin?

It rotates about 30 times per second, once every 33.7 milliseconds. Its spin is slowly decreasing as it loses energy to the surrounding nebula.

What is the difference between a neutron star and a pulsar?

A neutron star is the ultra-dense collapsed core of a massive star. A pulsar is a neutron star whose rotating beams of radiation happen to sweep across Earth, so we see regular pulses.

Who discovered pulsars?

Jocelyn Bell Burnell discovered the first pulsar in 1967 while a graduate student at the University of Cambridge. The 1974 Nobel Prize in Physics for the discovery went to her supervisor Antony Hewish and to Martin Ryle.

How dense is a neutron star?

A neutron star packs more than the Sun's mass into a sphere about 20 km wide. A sugar-cube-sized piece would weigh roughly a billion tonnes on Earth.

Can you see the Crab Pulsar with a telescope?

The Crab Nebula is visible in small telescopes as a faint smudge, but the pulsar itself is a 16th-magnitude star. Seeing it flash requires large telescopes and fast cameras.

A spinning city-sized star left by a supernova

Crab Pulsar

Neutron star · pulsar · ≈ 6,500 ly

The Crab Pulsar, known to astronomers as PSR B0531+21, is a neutron star at the centre of the Crab Nebula, about 6,500 light-years away in the constellation Taurus. It is the collapsed core of a massive star whose explosion was recorded by Chinese astronomers in 1054. Roughly 20 kilometres across yet heavier than the Sun, it spins about 30 times every second.

As it turns, beams of radiation sweep past Earth like a lighthouse, producing pulses seen from radio waves to gamma rays. The pulsar's spin is gradually slowing, and the energy it loses powers the glowing nebula around it. It is one of the most studied objects in high-energy astronomy, and a cornerstone of our understanding of neutron stars.

Key facts

Distance: ≈ 6,500 light-yearsEstimates range roughly 6,000–7,000
Diameter: ≈ 20 km
Mass: ≈ 1.4 solar massesTypical neutron star value; not measured directly
Rotation period: ≈ 33.7 millisecondsAbout 30 turns per second
Spin-down: ≈ 38 nanoseconds per day
Magnetic field: ≈ 10⁸ teslaAround a trillion times Earth's
Supernova: SN 1054Seen from Earth in July 1054
Discovered: 1968Radio pulses detected at Green Bank
Constellation: TaurusIn Messier 1, the Crab Nebula

Born in the supernova of 1054

In July 1054 astronomers at the Song dynasty court in China recorded a 'guest star' near the horns of Taurus. It was bright enough to be seen in daylight for about three weeks and stayed visible at night for nearly two years. Today we know they were watching a massive star explode as a supernova.

The debris from that explosion forms the Crab Nebula, catalogued by Charles Messier as the first entry in his list, M1. It is about 11 light-years across and still expanding at around 1,500 kilometres per second. Its tangled filaments and blue-white glow are energised by the pulsar at its centre, the collapsed core that survived the blast.

What neutron stars are

When a star of more than about eight solar masses runs out of fuel, its iron core collapses under its own gravity in a fraction of a second. Electrons are forced into protons, turning them into neutrons, and the core shrinks from thousands of kilometres across to about 20. If the remaining core is less than roughly two to three solar masses, the pressure of tightly packed neutrons halts the collapse, and a neutron star is born.

Neutron stars are the densest objects that we can directly observe. A sugar-cube-sized piece of neutron star matter would weigh roughly a billion tonnes on Earth. Collapse also spins them up and concentrates their magnetic fields, in the same way a skater spins faster when pulling their arms in, so newborn neutron stars rotate rapidly and are intensely magnetic.

Pulsars: cosmic lighthouses

A pulsar is a spinning neutron star that sends beams of radiation out from its magnetic poles. Because the magnetic axis is tilted from the spin axis, the beams sweep around the sky. If one crosses Earth we see a brief pulse once per rotation, with clock-like regularity. Pulsars have been found spinning anywhere from once every few seconds to more than 700 times a second.

The Crab Pulsar is young and energetic. It loses rotational energy at a rate of about 100,000 times the Sun's total power output, and that energy flows into a wind of particles that lights up the nebula. Its pulses have been detected from radio to very high-energy gamma rays, and it occasionally emits unusually powerful 'giant pulses' of radio waves.

Jocelyn Bell Burnell's discovery

In November 1967 Jocelyn Bell Burnell, a graduate student at the University of Cambridge, noticed a strange, regular signal repeating every 1.34 seconds in data from a radio telescope she had helped to build. Half-jokingly nicknamed LGM-1, for 'little green men', it turned out to be the first known pulsar. Her supervisor Antony Hewish shared the 1974 Nobel Prize in Physics for the work.

Magnetars

Magnetars are neutron stars with magnetic fields up to a thousand times stronger than a typical pulsar's, the strongest known in the universe. Starquakes in their crusts can unleash bursts of X-rays and gamma rays. In 2004 a giant flare from the magnetar SGR 1806−20 was detected across the galaxy and briefly disturbed Earth's upper atmosphere.

Discovering the Crab Pulsar

After pulsars were discovered in 1967, astronomers searched for one in the Crab Nebula, which had long been suspected of hiding an energy source. In 1968 David Staelin and Edward Reifenstein, using the radio telescope at Green Bank in West Virginia, detected pulses coming from the nebula, and the period was soon measured at about 33 milliseconds.

In January 1969 astronomers at Steward Observatory in Arizona saw the pulsar flashing in visible light, making it the first pulsar seen optically. The discovery linked pulsars firmly to neutron stars and supernovae, because only a tiny, dense object could spin so fast without flying apart. It also confirmed that the nebula's glow is powered by the pulsar's slowing spin.

When neutron stars collide

Some neutron stars orbit in pairs. Over millions of years they lose energy as gravitational waves and spiral together until they merge. On 17 August 2017 the LIGO and Virgo detectors recorded such a merger, called GW170817, in the galaxy NGC 4993 about 130 million light-years away. Two seconds later, gamma-ray telescopes caught a short burst from the same direction.

Telescopes around the world then saw a kilonova, the glow of radioactive debris thrown out by the collision. Its light showed that neutron star mergers forge large amounts of heavy elements, likely including gold and platinum. GW170817 was the first cosmic event observed in both gravitational waves and light.

Surprising facts about Crab Pulsar

The Crab Pulsar's surface spins at nearly 2,000 kilometres per second at its equator.
Its spin slows by only about 38 billionths of a second each day.
Chinese astronomers recorded the supernova that made it nearly a thousand years ago.
The first pulsar signal was briefly nicknamed 'little green men' because it was so regular.
The Crab Pulsar was the first pulsar ever seen flashing in visible light.

Frequently asked questions

How fast does the Crab Pulsar spin?: It rotates about 30 times per second, once every 33.7 milliseconds. Its spin is slowly decreasing as it loses energy to the surrounding nebula.
What is the difference between a neutron star and a pulsar?: A neutron star is the ultra-dense collapsed core of a massive star. A pulsar is a neutron star whose rotating beams of radiation happen to sweep across Earth, so we see regular pulses.
Who discovered pulsars?: Jocelyn Bell Burnell discovered the first pulsar in 1967 while a graduate student at the University of Cambridge. The 1974 Nobel Prize in Physics for the discovery went to her supervisor Antony Hewish and to Martin Ryle.
How dense is a neutron star?: A neutron star packs more than the Sun's mass into a sphere about 20 km wide. A sugar-cube-sized piece would weigh roughly a billion tonnes on Earth.
Can you see the Crab Pulsar with a telescope?: The Crab Nebula is visible in small telescopes as a faint smudge, but the pulsar itself is a 16th-magnitude star. Seeing it flash requires large telescopes and fast cameras.

Keep exploring

The 3D views in Ember’s Universe are scientific visualisations: physically based where possible, compressed or exaggerated where the real scales would hide everything.

Sources