The Crab Nebula

Hubble Space Telescope image of the Crab Nebula. NASA

There's a ghostly remnant of star death out there in the night-time sky. You can't see it with the naked eye. However, you can glimpse it through a telescope. It looks like a faint wisp of light, and astronomers have long called it the Crab Nebula.

This ghostly apparition is all that remains of a massive star that died in a supernova explosion thousands of years ago. Perhaps the most famous image (seen here) of this cloud of hot gas and dust has been taken by the Hubble Space Telescope and shows amazing detail of the expanding cloud.

If YOU want to take a look, you'll need a telescope and a place away from bright lights to spot it. The best times to look at night are from November through March each year.

The Crab Nebula lies about 6,500 light-years from Earth in the direction of the constellation Taurus. The cloud we see has been expanding ever since the original explosion, and now it covers an area of space about 10 light-years across. People often ask if the Sun will explode like this. Thankfully, the answer is "no".  It's not massive enough to create such a sight. It will end its days as a planetary nebula. 

What Made the Crab What It Is Today?

The Crab belongs to a class of objects called supernova remnants (SNR). They are created when a star many times the mass of the Sun collapses in on itself and then rebounds out in a catastrophic explosion. This is called a supernova. Why does the star do this?  Massive stars eventually run out of fuel in their cores at the same time they are losing their outer layers to space.

At some point, the outward pressure of the core can't hold back the massive weight of the outer layers, They collapse in on the core. Everything blasts back out in a violent burst of energy, sending huge amounts of stellar material out to space. This forms the “remnant” that we see today. The leftover core of the star keeps contracting under its own gravity.

Eventually, it forms a new type of object called a neutron star.

The Crab Pulsar

The neutron star at the heart of the Crab is very small, probably just a few miles across. But it is extremely dense. If you had a can of soup filled with neutron star material, it would have about the same mass as Earth's Moon. It's roughly in the center of the nebula and spins very fast, about 30 times a second. Rotating neutron stars like this are called pulsars (derived from the words PULSating stARS).

The pulsar inside the Crab is one of the most powerful ever observed. It injects so much energy into the nebula that we can detect light streaming away from the cloud in virtually every wavelength, from low-energy radio photons to the highest energy gamma-rays.

The Pulsar Wind Nebula

The Crab Nebula is also referred to as a pulsar wind nebula, or PWN. A PWN is a nebula that is created by the material that is ejected by a pulsar interacting with random interstellar gas and the pulsar’s own magnetic field. PWNs are often difficult to distinguish from SNRs, since they often look very similar. In some cases, objects will appear with a PWN but no SNR. The Crab Nebula contains a PWN inside the SNR, and if you look closely it appears as the sort of cloudy area in the middle of the HST image.

The Crab Through History

If you had lived in the year 1054, the Crab would have been so bright you could see it in the daytime. It was easily the brightest object in the sky, besides the Sun and Moon, for several months. Then, as all supernova explosions do, it began to fade. Chinese astronomers noted its presence in the sky as a "guest star", and it's thought that the Anasazi peopel who lived in the U.S. desert southwest also noted its presence. 

The Crab Nebula got its name in 1840 when William Parsons, the Third Earl of Rosse, using a 36-inch telescope, created a drawing of a nebula he spotted that he thought looked like a crab. With the 36-inch telescope he was not able to fully resolve the colored web of hot gas around the pulsar. But, he tried again a few years later with a bigger telescope and then he could see greater detail.

He noted that his earlier drawings were not representative of the true structure of the nebula, but the name Crab Nebula was already popular. 

Edited by Carolyn Collins Petersen.