How big is Crab Nebula? | Scale of the Universe

The Crab Nebula is the cloud of gas left behind by a star that exploded as a supernova — the violent death of a star — and was seen from Earth in the year 1054. It lies about 6,500 light-years away in the constellation Taurus. A light-year is the distance light travels in one year, about 9.46 trillion kilometers. The nebula is oval, about 13 light-years long and 9 light-years wide, so it averages about 11 light-years, or 110 trillion kilometers, across.

Size

On the sky, the Crab Nebula covers about 7 by 5 arcminutes, where an arcminute is 1/60 of a degree. An angle becomes a length only when the distance is known. At NASA's distance of 6,500 light-years, the nebula is about 13 light-years long and 9 light-years wide. The team behind NASA's Chandra X-ray Observatory describes it as about 10 light-years across.

The distance is the main uncertainty. A catalog of supernova remnants kept at the University of Cambridge gives about 6,200 light-years, from the neutron star's parallax, the small shift in its position as Earth orbits the Sun. Older estimates range from about 5,500 to 7,800 light-years. Across that range, the nebula's length would be anywhere from about 11 to 16 light-years.

The nebula is not a neat oval in three dimensions either. A 2021 study measured how fast its glowing gas moves toward and away from Earth and used this to rebuild its shape in 3D. It found a shape like a heart, with no glowing gas more than about 6.5 light-years from the center.

Size comparisons

The Black Pillar, a 9.5-light-year tower of gas in the Eagle Nebula, is a little shorter than the Crab Nebula's average width.
The Orion Nebula, about 24 light-years across, is about twice as wide as the Crab Nebula.
The Crab Nebula's length is about three times the distance from the Sun to Proxima Centauri, the nearest star, 4.2 light-years away.
If the nebula were shrunk to the size of Earth, the neutron star at its center would be a speck about 2 micrometers across, about one-fortieth the width of a human hair. A micrometer is a millionth of a meter.

The supernova of 1054

In 1054, astronomers in China recorded a "guest star" that could be seen in the daytime sky for nearly a month. Astronomers in Japan saw it too. The Crab Nebula is the expanding wreckage of that explosion.

The English astronomer John Bevis discovered the nebula in 1731. Charles Messier later mistook it for Halley's Comet, and that mistake led him to start a catalog of objects that could be confused with comets. The Crab Nebula is its first entry, Messier 1. The name comes from a drawing made in 1844 by the Irish astronomer Lord Rosse.

Tracing the motion of the nebula's filaments backward points to an explosion around the year 1130, about three-quarters of a century later than the recorded date. Astronomers explain the gap by the neutron star's wind of particles, which has sped the gas up since the explosion.

The pulsar at the center

At the center of the nebula is a neutron star, the crushed core of the star that exploded. It packs about the mass of the Sun into a ball about 25 kilometers (15 miles) across. That is the width NASA's NICER telescope has measured for other neutron stars; the Crab's own has never been measured. It spins about 30 times a second and sweeps two beams of radiation across space like a lighthouse, so it seems to pulse. A spinning neutron star seen this way is called a pulsar.

The pulsar powers the nebula's glow. It pours high-energy particles into the nebula, and electrons moving at nearly the speed of light spiral around magnetic field lines, giving off the bluish light inside the nebula. In Hubble's pictures, the orange filaments around the bluish light are the tattered remains of the star, mostly hydrogen.