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There's a ghostly remnant of star death out there in the night-time sky. It can't be seen with the naked eye. However, stargazers can glimpse it through a telescope. It looks like a faint wisp of light, and astronomers have long called it the Crab Nebula.
The Ghostly Remains of a Dead Star
This faint, fuzzy-looking object is all that remains of a massive star that died in a supernova explosion thousands of years ago. The most famous recent image of this cloud of hot gas and dust was taken by the Hubble Space Telescope and shows amazing detail of the expanding cloud. That's not quite how it looks from a backyard-type telescope, but it's still worth searching out 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 debris cloud 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. Our star will end its days as a planetary nebula.
The Crab Through History
For anyone alive in the year 1054, the Crab would have been so bright they 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 people who lived in the U.S. desert southwest also noted its presence. Weirdly enough, there are NO mentions of it in European histories of the time, which is somewhat odd, since there WERE people observing the sky. Some historians have suggested that perhaps wars and famines kept people from paying much attention to celestial sights. Whatever, the reasons, the historical mentions of this awesome sight were pretty limited.
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.
What Made the Crab What It Is Today?
The Crab belongs to a class of objects called supernova remnants (which astronomers shorten down to "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. That expansion of the stellar material is called "mass loss", and it actually begins long before the star dies. It gets more intense as the star ages, and so astronomers recognize mass loss as a hallmark of a star that is aging and dying, particularly if there's a LOT of it happening.
At some point, the outward pressure from the core can't hold back the massive weight of the outer layers, They collapse in and then everything blasts back out in a violent burst of energy. That sends 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 someone had a can of soup filled with neutron star material, it would have about the same mass as Earth's Moon!
The pulsar itself is 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 astronomers 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 it appears as a sort of cloudy area in the middle of the HST image.
Astronomers continue to study the Crab and chart the outward motion of its remnant clouds. The pulsar remains an object of great interest, as well as the material it's "lighting up" as it swings its searchlight-like beam around during its rapid spin.
Edited by Carolyn Collins Petersen.