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The Crab Nebula (catalogue designations M 1, NGC 1952, Taurus A) is a Supernova Remnant and Pulsar Wind Nebula in the Constellation of Taurus . The Nebula was first observed in 1731 by John Bevis , and corresponds to a bright supernova that was recorded by Chinese and Arab astronomers In 1054 . Located at a distance of about 6,300 Light Year s (2 Kpc ) from Earth , the Nebula has a diameter of 11 ly (3.4 pc) and is expanding at a rate of about 1,500 Kilometre s per Second . At the center of the nebula lies the Crab Pulsar , a Rotating Neutron Star , which emits pulses of Radiation from Gamma Ray s to Radio Wave s with a spin rate of 30.2 times per second. The nebula was the first astronomical object identified with a historical supernova explosion. The nebula acts as a source of radiation for studying celestial bodies that Occult it. In the 1950 s and 1960 s, the Sun 's Corona was mapped from observations of the Crab's radio waves passing through it, and more recently, the thickness of the atmosphere of Saturn 's moon Titan was measured as it blocked out X-ray s from the nebula. ORIGINS See Also: SN 1054 First observed in 1731 by John Bevis , the nebula was independently rediscovered in 1758 by Charles Messier as he was observing a bright Comet . Messier catalogued it as the first entry in his Catalogue of comet-like objects. The Earl Of Rosse observed the nebula at Birr Castle in the 1840s , and referred to the object as the Crab Nebula because a drawing he made of it looked like a Crab .Glyn Jones K. (1976), ''The Search for the Nebulae'', Journal of the History of Astronomy, v. 7, p.67 In the early 20th century, the analysis of early —a massive, exploding star, having exhausted its supply of energy from Nuclear Fusion and collapsed in on itself. Recent analyses of historical records have found that the supernova that created the Crab Nebula probably occurred in April or early May, rising to its maximum brightness of between Apparent Magnitude −7 and −4.5 (brighter than everything in the night sky except the Moon ) by July. The supernova was visible to the Naked Eye for about two years after its first observation.Collins G.W., Claspy W.P., Martin J.C. (1999), ''Reinterpretation of Historical References to the Supernova of A.D. 1054'', Publications of the Astronomical Society of the Pacific, v. 111, p. 871 Thanks to the recorded observations of Far Eastern and Middle Eastern astronomers of 1054, Crab Nebula became the first astronomical object recognized as being connected to a supernova explosion.Mayall N.U. (1939), '' The Crab Nebula, a Probable Supernova '', Astronomical Society of the Pacific Leaflets, v. 3, p.145 PHYSICAL CONDITIONS (in red) and X-ray images from Chandra X-ray Observatory (in blue).]] In Visible Light , the Crab Nebula consists of a broadly Oval -shaped mass of filaments, about 6 Arcminute s long and 4 arcminutes wide, surrounding a diffuse blue central region (by comparison, the Full Moon is 30 arcminutes across). The filaments are the remnants of the progenitor star's atmosphere, and consist largely of Ionised Helium and Hydrogen , along with Carbon , Oxygen , Nitrogen , Iron , Neon and Sulphur . The filaments' temperatures are typically between 11,000 and 18,000 K , and their densities are about 1,300 particles per Cm³ .Fesen R.A., Kirshner R.P. (1982), ''The Crab Nebula. I - Spectrophotometry of the filaments'', Astrophysical Journal, v. 258, p. 1–10 In 1953 Iosif Shklovsky proposed that the diffuse blue region is predominantly produced by Synchrotron Radiation , which is radiation given off by the curving of Electron s moving at speeds up to half the Speed Of Light .1 Three years later the theory was confirmed by observations. In the 1960s it was found that the source of the electron curved paths was the strong Magnetic Field produced by a neutron star at the center of the nebula.Burn B.J. (1973), ''A synchrotron model for the continuum spectrum of the Crab Nebula'', Monthly Notices of the Royal Astronomical Society, v. 165, p. 421 (1973) The Crab Nebula is currently expanding outwards at about 1,500 km/s.Bietenholz M.F., Kronberg P.P., Hogg D.E., Wilson A.S. (1991), ''The expansion of the Crab Nebula'', Astrophysical Journal Letters, vol. 373, p. L59-L62 Images taken several years apart reveal the slow expansion of the nebula, and by comparing this angular expansion with its Spectroscopically determined expansion velocity, the nebula's distance can be estimated. Modern observations give a distance to the nebula of about 6,300 ly,Trimble, V. (1973), ''The Distance to the Crab Nebula and NP 0532'', Publications of the Astronomical Society of the Pacific, v. 85, p. 579 meaning that it is about 11 ly in length. Tracing back its expansion consistently yields a date for the creation of the nebula several decades after 1054, implying that its outward velocity has accelerated since the supernova explosion.Trimble V. (1968), ''Motions and Structure of the Filamentary Envelope of the Crab Nebula'', Astronomical Journal, v. 73, p. 535 This acceleration is believed to be caused by energy from the pulsar that feeds into the nebula's magnetic field, which expands and forces the nebula's filaments outwards.Bejger M., Haensel P. (2003), ''Accelerated expansion of the Crab Nebula and evaluation of its neutron-star parameters'', Astronomy and Astrophysics, v.405, p.747–751 Estimates of the total mass of the nebula are important for estimating the mass of the supernova's progenitor star. Estimates of the amount of matter contained in the filaments of the Crab Nebula range from about 1–5 Solar Mass es;Fesen R.A., Shull J.M., Hurford A.P. (1997), ''An Optical Study of the Circumstellar Environment Around the Crab Nebula'', Astronomical Journal v.113, p. 354-363 although other estimates based on the investigation of the Crab Pulsar yield different numbers. CENTRAL STAR See Also: Crab Pulsar images shows features in the inner Crab Nebula changing over a period of four months. Credit: NASA / ESA .]] At the centre of the Crab Nebula are two faint stars, one of which is the star responsible for existence of the nebula. It was identified as such in 1942 , when Rudolf Minkowski found that its optical spectrum was extremely unusual.Minkowski R. (1942), ''The Crab Nebula'', Astrophysical Journal, v. 96, p.199 The region around the star was found to be a strong source of radio waves in 1949Bolton J.G., Stanley G.J., Slee O.B. (1949), ''Positions of three discrete sources of Galactic radio frequency radiation'', Nature, v. 164, p. 101 and X-rays in 1963,Bowyer S., Byram E.T., Chubb T.A., Friedman H. (1964), ''Lunar Occulation of X-ray Emission from the Crab Nebula'', Science, v. 146, pp. 912–917 and was identified as one of the brightest objects in the sky in Gamma Ray s in 1967 .Haymes R.C., Ellis D.V., Fishman G.J., Kurfess J.D., Tucker, W.H. (1968), ''Observation of Gamma Radiation from the Crab Nebula'', Astrophysical Journal, v. 151, p.L9 Then, in 1968, the star was found to be emitting its radiation in rapid pulses, becoming one of the first Pulsar s to be discovered. Pulsars are sources of powerful Electromagnetic Radiation , emitted in short and extremely regular pulses many times a second. They were a great mystery when discovered in 1967, and the team which identified the first one considered the possibility that it could be a signal from an advanced civilization.Del Puerto C. (2005), ''Pulsars In The Headlines'', EAS Publications Series, v. 16, pp.115–119 However, the discovery of a pulsating radio source in the centre of the Crab Nebula was strong evidence that pulsars were formed by supernova explosions. They are now understood to be rapidly rotating Neutron Star s, whose powerful Magnetic Field concentrates their radiation emissions into narrow beams. The Crab Pulsar is believed to be about 28–30 km in diameter;M. Bejger and P. Haensel (2002), ''Moments of inertia for neutron and strange stars: Limits derived for the Crab pulsar'', Astronomy and Astrophysics , v. 396, p. 917–921 it emits pulses of radiation every 33 Millisecond s.Harnden F.R., Seward F.D. (1984), ''Einstein observations of the Crab nebula pulsar'', Astrophysical Journal, v. 283, p. 279–285 Pulses are emitted at Wavelength s across the Electromagnetic Spectrum , from radio waves to X-rays. Like all isolated pulsars, its period is slowing very gradually. Occasionally, its rotational period shows sharp changes, known as 'glitches', which are believed to be caused by a sudden realignment inside the neutron star. The Energy released as the pulsar slows down is enormous, and it powers the emission of the synchrotron radiation of the Crab Nebula, which has a total Luminosity about 75,000 times greater than that of the Sun.Kaufmann W.J. (1996), ''Universe'' 4th edition, Freeman press, p. 428 The pulsar's extreme energy output creates a unusually dynamic region at the centre of the Crab Nebula. While most astronomical objects evolve so slowly that changes are visible only over timescales of many years, the inner parts of the Crab show changes over timescales of only a few days.Hester J.J., Scowen P.A., Sankrit R., Michel F.C., Graham J.R., Watson A., Gallagher J.S. (1996), ''The Extremely Dynamic Structure of the Inner Crab Nebula'', Bulletin of the American Astronomical Society, Vol. 28, p.950 The most dynamic feature in the inner part of the nebula is the point where the pulsar's equatorial wind slams into the bulk of the nebula, 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. PROGENITOR STAR by the Spitzer Space Telescope .]] The star that exploded as a supernova is referred to as the supernova's ''progenitor star''. Two types of star explode as supernovae: White Dwarf s and massive stars. In the so-called Type Ia Supernova e, gases falling onto a white dwarf raise its mass until it nears a critical level, the Chandrasekhar Limit , resulting in an explosion; in Type Ib/c and Type II supernovae, the progenitor star is a massive star which runs out of fuel to power its Nuclear Fusion reactions and collapses in on itself, reaching such phenomenal Temperature s that it explodes. The presence of a pulsar in the Crab means it must have formed in a core-collapse supernova; Type Ia supernovae do not produce pulsars. Theoretical models of supernova explosions suggest that the star that exploded to produce the Crab Nebula must have had a Mass of between 8 and 12 Solar Mass es. Stars with masses lower than 8 solar masses are thought to be too small to produce supernova explosions, and end their lives by producing a Planetary Nebula instead, while a star heavier than 12 solar masses would have produced a nebula with a different chemical composition to that observed in the Crab.Davidson K., Fesen R.A. (1985), ''Recent developments concerning the Crab Nebula'', Annual Review of Astronomy and Astrophysics, v. 23, p. 119–146 A significant problem in studies of the Crab Nebula is that the combined mass of the nebula and the pulsar add up to considerably less than the predicted mass of the progenitor star, and the question of where the 'missing mass' is remains unresolved.Fesen R.A., Shull J.M., Hurford A.P. (1997), ''An Optical Study of the Circumstellar Environment Around the Crab Nebula'', Astronomical Journal v.113, p. 354–363 Estimates of the mass of the nebula are made by measuring the total amount of light emitted, and calculating the mass required, given the measured temperature and density of the nebula. Estimates range from about 1–5 solar masses, with 2–3 solar masses being the generally accepted value. The neutron star mass is estimated to be between 1.4 and 2 solar masses. The predominant theory to account for the missing mass of the Crab is that a substantial proportion of the mass of the progenitor was carried away before the supernova explosion in a fast Stellar Wind . However, this would have created a shell around the nebula. Although attempts have been made at several different wavelengths to observe a shell, none has yet been found.Frail D.A., Kassim N.E., Cornwell T.J., Goss W.M. (1995), ''Does the Crab Have a Shell?'', Astrophysical Journal, v. 454, p. L129–L132 TRANSITS BY SOLAR SYSTEM BODIES image of a small region of the Crab Nebula, showing / ESA .]] The Crab Nebula lies roughly 1½ ° away from the Ecliptic —the plane of Earth's orbit around the Sun. This means that the Moon — and occasionally, Planet s — can Transit or Occult the nebula. Although the Sun does not transit the nebula, its Corona passes in front of it. These transits and occultations can be used to analyse both the nebula and the object passing in front of it, by observing how radiation from the nebula is altered by the transiting body. Lunar transits have been used to map X-ray emissions from the nebula. Before the launch of X-ray-observing satellites, such as the Chandra X-ray Observatory , X-ray observations generally had quite low Angular Resolution , but when the Moon passes in front of the nebula, its position is very accurately known, and so the variations in the nebula's brightness can be used to create maps of X-ray emission.Palmieri T.M., Seward F.D., Toor A., van Flandern T.C. (1975), ''Spatial distribution of X-rays in the Crab Nebula'', Astrophysical Journal, v. 202, p. 494–497 When X-rays were first observed from the Crab, a lunar occultation was used to determine the exact location of their source. The Sun's corona passes in front of the Crab every June. Variations in the radio waves received from the Crab at this time can be used to infer details about the corona's density and structure. Early observations established that the corona extended out to much greater distances than had previously been thought; later observations found that the corona contained substantial density variations.Erickson W.C. (1964), ''The Radio-Wave Scattering Properties of the Solar Corona'', Astrophysical Journal, v. 139, p.1290 Very rarely, at the time. IN FICTION : ''See Crab Nebula In Fiction .'' REFERENCES EXTERNAL LINKS
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