Nebulae · supernova remnant

The Crab Nebula: what is left of a star that exploded in 1054

The Crab Nebula, an orange-pink shell of gas round a bright blue-white centre, drawn in Exo-View

On 4 July 1054 Chinese astronomers recorded a new star in the constellation Taurus, bright enough to see by day for three weeks and by night for nearly two years. It was a star blowing itself apart, 6,500 light-years away, and what is left of it, 970 years on, is the Crab Nebula: a cloud of gas 11 light-years across, still expanding, with a pulsar at its heart.

Fly to the Crab Nebula Watch Betelgeuse explode

Distance: About 6,500 light-years
Width: About 11 light-years, growing by about 1,500 km every second
Kind: Supernova remnant
Seen in: The constellation Taurus, near the star Zeta Tauri
Centre: A pulsar, a neutron star about 20 km across, spinning about 30 times a second
Catalogue number: M1, the first object in Messier's list

The guest star of 1054

Chinese court astronomers wrote of a 'guest star' that appeared in the summer of 1054 and was brighter than Venus. It was visible in daylight for 23 days and in the night sky for 653 days. It may also have been noted in Japan and the Arab world; European records of it are missing. Nobody knew what it was until the 20th century, when the nebula, which had been found in 1731, was seen to be growing, and traced back to a start about 900 years earlier.

The lighthouse at its heart

At the centre is what the star's core became: a neutron star, about one and a half times the Sun's mass and about 20 km across, spinning about 30 times a second. It sweeps beams of radio waves, light and X-rays round like a lighthouse, so it flashes at us 30 times a second: a pulsar. Its energy lights the whole nebula and keeps its glow bright long after the explosion.

Why 'Crab'?

The Irish astronomer Lord Rosse drew it in the 1840s with arms like claws and called it the Crab; the name stuck, though modern photographs look more like a ball of threads. Charles Messier saw it in 1758 while hunting for a comet, noted it as a fixed fuzzy patch that was not one, and made it number 1 in his list of things not to be mistaken for comets.

In Exo-View

The key below flies from the Sun to the Crab Nebula, a shell of orange and pink gas round a blue-white glow. The shape is drawn from the nebula's photographs, an artist's impression, and the plate says so. You can fly on to Betelgeuse, another star expected to end this way.