Crab Nebula

> Crab Nebula

The Crab Nebula (also known as Messier Object 1, M1 or NGC 1952) is a gaseous diffuse nebula in the constellation Taurus. It is the remnant of a supernova that was recorded by Chinese and Arab astronomers in 1054 as being visible during daylight for 23 days. Located at a distance of about 6500 ly from Earth, it has a diameter of 6 ly and is expanding at a rate of 1000 km per second. A neutron star in the center of the nebula rotates 30 times per second.

Crab Nebula

Messier Object 1, the Crab Nebula. In the nebula's very center lies a pulsar: a neutron star rotating 30 times a second.

At the center of the nebula is the Crab Pulsar, a neutron star remnant of the supernova which is roughly 10 km in diameter. It was discovered in 1969. The Crab Pulsar rotates once every 33 milliseconds, or 30 times each second, and the beams of radiation it emits interact with the nebular gases to produce complex patterns of wind and fluorescence. The most dynamic feature in the inner part of the nebula is the point where one of the pulsar's polar jets slams into the surrounding material forming a shock front. The shape and position of this feature shifts rapidly, with the equatorial wind appearing as a series of wisp-like features that steepen, brighten, then fade as they move away from the pulsar to well out into the main body of the nebula.

The Crab Pulsar

This image combines optical data from Hubble (in red) and X-ray images from Chandra (in blue).

The Crab Nebula is often used as a calibration source in X-ray astronomy. It is very bright in X-rays and the flux density and spectrum are known to be constant, with the exception of the pulsar. The pulsar provides a strong periodic signal that is used to check the timing of the X-ray detectors. In X-ray astronomy, 'Crab' and 'milliCrab' are sometimes used as units of flux density. Very few X-ray sources ever exceed one Crab in brightness.

The Crab Nebula derived its name from its appearance in a drawing made by Lord Rosse in 1844 using his 36-inch telescope. In this sketch it does indeed resemble a crab, but upon reobserving the nebula in 1848 with his newly-built 72-inch telescope he produced a much more accurate drawing which bore little resemblance to the original. However, the name "Crab Nebula" stuck.

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