With binoculars or a small telescope, just one degree northwest of the star zeta (ζ) Tauri in the horns of the constellation Taurus, the Bull, you can see the famous Crab Nebula, the remnant of a massive star that exploded nearly 1,000 years ago. The Crab Nebula takes its name from a drawing made by the Irish amateur astronomer Lord Rosse in 1844 using his 36-inch telescope at Birr Castle in Ireland. The drawing resembles a horseshoe crab.
The Crab was first noted by physician Jon Bevis in 1731 and independently recorded by Charles Messier in 1758. This is the first object in Messier’s famous catalog, so it’s commonly known as M1. It’s the only supernova remnant in the catalog. Messier noted the resemblance of this object to a comet. Indeed, the Crab Nebula inspired him to record the position of more nebulous objects to prevent confusion during his searches for new comets. Said Messier of the Crab, “This nebula had such a resemblance to a comet, in its form and brightness, that I endeavored to find others, so that astronomers would not confuse these same nebulae with comets just beginning to shine”.
M1 was also confused for a comet by many amateur stargazers in late 1985 who were searching nearby for their first glimpses of Halley’s Comet.
The Crab Nebula lies in the horns of the constellation Taurus, the small V-shaped constellation that lies high in the sky in late winter in the northern hemisphere and lower over the northern horizon in the southern hemisphere. In a small telescope, the nebula appears as an oval splotch about 6′x4′ about one degree northwest of 3rd magnitude zeta Tauri. This makes it easy to find. At magnitude 8.4, it’s bright enough to be visible in 7×50 binoculars in dark sky. At 30x in a small telescope, the nebula fits in the same field of view as zeta Tauri. At higher-power, the nebula reveals a pinched off region near its centre. The brightness of the nebula is fairly uniform, though the pinched-off region appears slightly fainter. The delicate tendrils seen in images of M1 are visible only with difficulty in larger telescopes.
M1 expanding (1054 – 2010) from emivanov on Vimeo.
Astronomers now know the Crab was created by a core-collapse supernova because the event left a pulsar, a dense city-sized remnant of the stellar core that spins around once every 33 milliseconds. The 16th-magnitude pulsar can be seen visually with a 20-inch or larger telescope. But astronomers learn much more about this object through observations at radio and X-ray wavelengths.
The supernova itself was observed by Chinese and Arabic astronomers in the year 1054. When it exploded, the star appeared in the daytime sky and shone as much as six times brighter than Venus, then slowly faded from view. The nebula lies some 6,300 light years from Earth and its visible portion spans 11 light years.
The video above shows an animation of the evolution of the Crab Nebula since its birth in 1054.
Since M1 lies about 1.5 degrees off the ecliptic, it’s sometime occulted by the Moon, and this helps astronomers map the X-ray emission from the central region of the nebula. The Sun’s corona also passes in front of M1 every June, and the X-rays from the nebula help astronomers infer the physical nature of the corona. Astronomers also used the space-based Chandra X-ray observatory to observe Saturn’s moon Titan passing in front of the Crab. This helped determine the thickness of Titan’s atmosphere.
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