Crab Nebula (M1, NGC 1952)
The Crab Nebula lies about 6,300 light-years away in the constellation Taurus, measures roughly 10 light-years across, and is expanding at an average speed of 1,800 km/s (some 0.2 arcsecond per year). Surprisingly, its expansion rate seems to be accelerating, driven by radiation from the central pulsar. Its luminosity at visible wavelengths exceeds 1,000 times that of the Sun and comes from two major contributions, revealed spectroscopically by Roscoe Frank Sanford in 1919 and confirmed photographically by Walter Baade and Rudolph Minkowski in 1930. First, a reddish component, with an emission line spectrum like that of emission nebula, forms a far-flung chaotic web of bright filaments. Second, a bluish component, with a continuous spectrum due to highly polarized synchrotron radiation, supplies a diffuse background in the central region of the Crab. This synchrotron component is emitted by high-speed electrons spiraling in a strong magnetic field.
The Crab in radio waves and X-raysIn 1948, the Crab was identified as a strong source of radio waves, and catalogued as Taurus A and, later, as 3C 144. In 1964, it was also found to be a bright X-ray source (Taurus X-1), emitting 100 times more energy in X-rays than it does an optical wavelengths. The collapsed remnant of the star that produced the nebula, the Crab pulsar, was first detected in 1968.
History of observationsThe Crab Nebula was discovered in 1731 by the British physician and amateur astronomer John Bevis (1695-1771) according to Charles Messier, who independently found it on Aug. 28, 1758, and first thought it was a comet, when looking for Halley's Comet on its first predicted return. It inspired Messier to begin his famous catalogue and formed its first entry, M1. It was christened the "Crab" on the basis of a drawing made by the Earl of Rosse (William Parsons) in about 1844. The first photo of M1 was obtained in 1892 and the first serious investigation of its spectrum was carried out in 1913-15 by Vesto Slipher.
See it for yourselfThe Crab can be found starting from Zeta Tauri, a third-magnitude star ENE of Aldebaran. Lying about 1° N and 1° W of Zeta, the Crab is visible as a dim patch in 7 × 50 or 10 × 50 binoculars. With more magnification, it is seen as a nebulous oval patch, surrounded by haze. In telescopes larger than 10 cm in aperture, some detail in its shape becomes apparent, with the suggestion of mottled or streaky structure in the inner part of the nebula.
Crab pulsar (PSR 0531+21, NP0532)
The Crab Nebula in science fictionThe Crab Nebula is the location for the first meeting between humans and intelligent extraterrestrials in Murray Leinster's story "First Contact" (1945).
Related categories• NEBULAE AND STAR CLUSTERS
• MESSIER CATALOGUE
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