Perhaps the most famous image from the Hubble Space Telescope (HST) is that of the Crab Nebula. A nebula is a mixture of hot gas, dust and simple elements like hydrogen and helium.
The Crab Nebula is approximately 6,500 light-years from Earth in the constellation Taurus. The nebula itself is an astonishing 10 light-years in diameter. To put that into perspective, if the Sun were the size of a basketball, the Crab Nebula would be the size of planet Earth.
The Crab Nebula has been an important part of history, dating back to early Chinese and Arab astronomers. Historical records from 1054 described the supernova that created the Crab Nebula as a celestial event that was so bright that it was seen in the day time. It was easily the brightest object in the sky, besides the Sun and Moon, for several months.
The Nebula, as we see it today, was discovered independently by John Bevis in 1731, and again by Charles Messier in 1758. It was, in fact, the first object that Messier catalogued in his search for “comet like objects” (though most of them were not comets at all), and therefore it is also referred to as M1.
The Crab Nebula got its most familiar name in 1840 when the William Parsons, the Third Earl of Rosse, using a 36-inch telescope, created a drawing of the nebula that he thought looked like a crab. With the 36-inch telescope he was not able to fully resolve the filaments, the colored web of hot gas that permeates the nebula. But then in 1848 Parsons observed the object again, but with a larger 72-inch telescope which allowed him to see greater detail. And although he noted that his earlier drawings were not representative of the true structure of the nebula, the name Crab Nebula had already taken hold.
The Crab is part of a class of objects called Super Nova Remnants (SNRs). As the name suggests SNRs are the result of a large star going through a phase of its evolution known as a supernova. This occurs when the star no longer has enough fuel to keep from collapsing onto itself and it explodes in a violent burst of energy. The outer part of the star is driven into space, forming the “remnant” that we see today. The inner part of the star, known as the core, contracts due to gravity and forms a new type of object called a neutron star.
The Crab Pulsar
The neutron star that remains after a supernova is extremely small, usually just 10 to 15 miles across. But it is extremely dense. It is so dense, in fact, that if you had a can of soup filled with neutron star material, it would have about the same mass as the Moon. The Crab’s neutron star, roughly in the center of the nebula, spins at a very high rate, completing about 30 revolutions every second. Rotating neutron stars, like the Crab’s, are called pulsars (derived from the words PULSating stARS).
The pulsar inside the crab nebula is one of the most powerful ever observed. It injects so much energy into the nebula, we can detect light 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.