M1 - The Crab Nebula
Credits: Keith Turnecliff, Long Itchington
In 1054, Chinese astronomers took notice of a “guest star” that was, for nearly a month, visible in the daytime sky. The “guest star” they observed was actually a supernova explosion, which gave rise to the Crab Nebula, a six-light-year-wide remnant of the violent event. With an apparent magnitude of 8.4 and located 6,500 light-years from Earth in the constellation Taurus, the Crab Nebula can be spotted with a small telescope and is best observed in January. The nebula was discovered by English astronomer John Bevis in 1731, and later observed by Charles Messier who mistook it for Halley’s Comet. Messier’s observation of the nebula inspired him to create a catalog of celestial objects that might be mistaken for comets. The blue in the filaments in the outer part of the nebula represents neutral oxygen. Green is singly ionized sulfur, and red indicates doubly ionized oxygen. These elements were expelled during the supernova explosion. A rapidly spinning neutron star (the ultra-dense core of the exploded star) is embedded in the center of the Crab Nebula. Electrons whirling at nearly the speed of light around the star’s magnetic field lines produce the eerie blue light in the interior of the nebula. The neutron star, like a lighthouse, ejects twin beams of radiation that make it appear to pulse 30 times per second as it rotates.
Facts about M1 by Keith Turnecliff
The Crab Nebula (catalogue designations M1, NGC 1952, Taurus A) is a supernova remnant in the constellation of Taurus.
The current name was caused by William Parsons, 3rd Earl of Rosse, observing the object in 1840 using a 36-inch telescope and produced a drawing that looked somewhat like a crab.
Corresponding to a bright supernova recorded by Chinese astronomers in 1054, the nebula was observed later by English astronomer John Bevis in 1731. The nebula was the first astronomical object identified with a historical supernova explosion.
At an apparent magnitude of 8.4, comparable to that of Saturn's moon Titan, it is not visible to the naked eye but can be made out using binoculars under favourable conditions. The nebula lies in the Perseus Arm of the Milky Way galaxy, at a distance of about 2.0 kiloparsecs (6,500 ly) from Earth. It has a diameter of 3.4 parsecs (11 ly), corresponding to an apparent diameter of some 7 arcminutes, and is expanding at a rate of about 1,500 kilometres per second (930 mi/s), or 0.5% of the speed of light.
At the center of the nebula lies the Crab Pulsar, a neutron star 28–30 kilometres (17–19 mi) across with a spin rate of 30.2 times per second, which emits pulses of radiation from gamma rays to radio waves. At X-ray and gamma ray energies above 30 keV, the Crab Nebula is generally the brightest persistent source in the sky, with measured flux extending to above 10 TeV. The nebula's radiation allows detailed study of celestial bodies that occult it. In the 1950s and 1960s, the Sun's corona was mapped from observations of the Crab Nebula's radio waves passing through it, and in 2003, the thickness of the atmosphere of Saturn's moon Titan was measured as it blocked out X-rays from the nebula.