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Crab Nebula

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   CAPTION: Crab Nebula

       Diffuse nebula      Lists of nebulae

   M1, the Crab Nebula.
               Observation data
   ( Epoch J2000.0)
   Type                    Supernova Remnant
   Right ascension         05^h 34^m 31.97^s
   Declination             +22° 00′ 52.1″
   Distance                6,300 ly
   Apparent magnitude (V)  +8.4
   Apparent dimensions (V) 6 × 4  arcmin
   Constellation           Taurus
           Physical characteristics
   Radius                  3 ly
   Absolute magnitude (V)  −3
   Notable features        Optical pulsar
   Other designations      M1, NGC 1952
                                        edit

   The Crab Nebula (catalogue designations M1, NGC 1952, Taurus A) is a
   supernova remnant in the constellation of Taurus. The nebula was first
   observed in 1731 by John Bevis. It is the remnant of a supernova that
   was recorded by Chinese and Arab astronomers in 1054. Located at a
   distance of about 6,300 light years (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
   kilometres per second.

   The nebula contains a pulsar in its centre which rotates thirty times
   per second, emitting pulses of radiation from gamma rays to radio
   waves. 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 1950s and 1960s, 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-rays from the nebula.

Origins

   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.

   In the early 20th century, the analysis of early photographs of the
   nebula taken several years apart revealed that it was expanding.
   Tracing the expansion back revealed that the nebula must have formed
   about 900 years ago. Historical records revealed that a new star bright
   enough to be seen in the daytime had been recorded in the same part of
   the sky by Chinese and Arab astronomers in 1054 It is possible that the
   bright new "star" was observed by Native Americans and recorded in
   petroglyphs.. Given its great distance, the daytime "guest star"
   observed by the Chinese and Arabs could only have been a supernova—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.. Thanks to the recorded observations of
   oriental astronomers of 1054, Crab Nebula became the first astronomical
   object recognized as supernova explosion connected.

Physical conditions

   The Crab Pulsar. This image combines optical data from Hubble (in red)
   and X-ray images from Chandra X-ray Observatory (in blue).
   Enlarge
   The Crab Pulsar. This image combines optical data from Hubble (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  arcminutes 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' temperature is typically between 11,000 and 18,000  K,
   and their densities are about 1,300 particles per cm³.

   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 electrons moving at speeds up to half the
   speed of light. 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 centre of the nebula.

   The Crab Nebula is currently expanding outwards at about 1,500 km/s.
   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, 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. 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.

   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 masses; although other estimates based on the information
   gleaned from the Crab Pulsar yield different numbers.

Central star

   This sequence of Hubble Space Telescope images shows features in the
   inner Crab Nebula changing over a period of four months.
   Enlarge
   This sequence of Hubble Space Telescope images shows features in the
   inner Crab Nebula changing over a period of four months.

   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. The region around the star was found to be a strong
   source of radio waves in 1949 and X-rays in 1963, and was identified as
   one of the brightest objects in the sky in gamma rays in 1967. Then, in
   1968, the star was found to be emitting its radiation in rapid pulses,
   becoming one of the first pulsars to be discovered, and the first to be
   associated with a supernova remnant.

   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. 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 stars, whose powerful magnetic field
   concentrates their radiation emissions into narrow beams.

   The Crab Pulsar is believed to be about 28-30 km in diameter; it emits
   pulses of radiation every 33  milliseconds. Pulses are emitted at
   wavelengths 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.

   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. 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

   The Crab Nebula seen in infrared by the Spitzer Space Telescope.
   Enlarge
   The Crab Nebula seen in infrared 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 dwarfs
   and massive stars. In the so-called Type Ia supernovae, 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 temperatures 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 masses. 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.

   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. 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.

Transits by solar system bodies

   Hubble Space Telescope image of a small region of the Crab Nebula,
   showing its intricate filamentary structure
   Enlarge
   Hubble Space Telescope image of a small region of the Crab Nebula,
   showing its intricate filamentary structure

   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, planets — 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. 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.

   Very rarely, Saturn transits the Crab Nebula. Its transit in 2003 was
   the first since 1296; another will not occur until 2267. Observers used
   the Chandra X-ray Observatory to observe Saturn's moon Titan as it
   crossed the nebula, and found that Titan's X-ray 'shadow' was larger
   than its solid surface, due to absorption of X-rays in its atmosphere.
   These observations showed that the thickness of Titan's atmosphere is
   880 km. The transit of Saturn itself could not be observed, because
   Chandra was passing through the Van Allen belts at the time.
   Retrieved from " http://en.wikipedia.org/wiki/Crab_Nebula"
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   with only minor checks and changes (see www.wikipedia.org for details
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