Crab Nebula

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Crab Nebula,

diffuse gaseous nebulanebula
[Lat.,=mist], in astronomy, observed manifestation of a collection of highly rarefied gas and dust in interstellar space. Prior to the 1960s this term was also applied to bodies later discovered to be galaxies, e.g.
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 in the constellation Taurus; cataloged as NGC 1952 and M1, the first object recorded in Charles Messier's catalog of nonstellar objects (see Messier catalogMessier catalog
, systematic list of nebulae and star clusters. A first list, compiled and published in 1771 by Charles Messier, contained 45 objects. The final list, published in 1784, contained 103 objects; some of these were later removed from the list.
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). It is the remnant of a supernovasupernova,
a massive star in the latter stages of stellar evolution that suddenly contracts and then explodes, increasing its energy output as much as a billionfold. Supernovas are the principal distributors of heavy elements throughout the universe; all elements heavier than
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 that was observed in 1054 by Chinese and Arab astronomers to be as bright as Venus; markings in northern New Mexico depict a star near a crescent moon that might be a record of this supernova. Only three other supernovas have been observed in our galaxy since then. The explosion of the Crab Nebula produced a large expanding shell of delicate filaments. The filaments contain ionized gas in which unusually energetic electrons twist through magnetic fields at speeds close to that of light, emitting synchrotron radiation. Although this radiation is what makes supernova remnants visible in radio wavelengths, in this nebula it is so strong that observers can see the filaments through moderate-sized optical telescopes under good conditions. The nebula is also a strong emitter of X rays. At its center is a pulsarpulsar,
in astronomy, a neutron star that emits brief, sharp pulses of energy instead of the steady radiation associated with other natural sources. The study of pulsars began when Antony Hewish and his students at Cambridge built a primitive radio telescope to study a
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, PSR 0531+21, that spins 30 times per second. The youngest pulsar observed, it gives off radiation at radio, optical, ultraviolet, X-ray, and gamma ray wavelengths, as well as electrons that power the synchrotron radiation in the surrounding nebula.

Crab nebula

(Ml; NGC 1952) A turbulent expanding mass of gas and dust with luminous twisting filaments of ionized gas, lying about 2000 parsecs away and located in the sky near the star Zeta (ζ) Tauri in the constellation Taurus. It is a supernova remnant, the result of a supernova (probably of type II) that was almost certainly observed by Chinese and Japanese astronomers in 1054 and was sufficiently bright (magnitude about –5) to be visible in daylight for more than three weeks. At present it is about four parsecs in diameter. The Crab nebula emits synchrotron radiation of all wavelengths and is a particularly strong source of X-rays (Taurus X-1) and radio waves (Taurus A). It was named the Crab nebula in 1848 by its first modern observer, the Irish astronomer William Parsons, 3rd Earl of Rosse (1800–67).

The star whose explosion produced the Crab nebula is now a young optical pulsar (the Crab pulsar NP 0532), identified as such in 1967. Its pulsations are also observed at radio, infrared, X-ray, and gamma-ray wavelengths. These pulsations have a period of only 0.0331 seconds. The pulsar is the power house for the Crab nebula: energy is being lost by the pulsar in the form of highly energetic electrons, causing the pulsar to slow down very gradually; the electrons interact with the intense magnetic field extending into the nebula and radiate synchrotron emission. The energy loss of the pulsar equals the total energy radiated by the nebula (1030 to 1031 J s–1). The ultraviolet component of this radiation ionizes the gas in the filaments, causing the atoms to fluoresce. The Crab nebula is the defining member of the class of filled supernova remnants, or plerions.

Crab Nebula

 

a galactic nebula, the result of a supernova explosion in 1054 in Taurus. The distance to the nebula is 1,700 parsecs, and its radius is about 1 parsec. It is expanding at a speed of 1,000–1,500 km/sec. The nebula has the form of an elongated ellipsoid and a filamentary structure. The total mass of gas in the Crab Nebula is about 0.1 solar mass. About 80 percent of the nebula's luminosity (it has a visual stellar magnitude of 8.5) is concentrated not in the filaments but in the amorphous mass occupying the interior of the ellipsoid.

The Crab Nebula is a source of radio emission (Taurus A). In the interval between the radio and the optical regions of the spectrum there is a noticeable maximum in the radiation, the nature of which is not yet known. In the direction of shorter wavelengths, the spectrum extends to the X-ray region; only the central part of the nebula and the star itself radiate in this region. The radiation of the amorphous mass at all wavelengths is produced by fast (relativistic) electrons (with energies of 108–1012 eV) moving in a magnetic field (of intensity 10-3 oersted, or 8 X 10-2 A/m); this is what is known as synchrotron radiation. The generation of particles and of the magnetic field is associated with the remnant of the supernova, which is a pulsar with a radius of about 10 km, rotating with a period of 0.033 sec and producing bursts of optical, X-ray, and radio radiation. The star has a magnetic field of large intensity. The rapid rotation of this field produces electromagnetic effects, resulting in the acceleration of the particles and bursts of radiation. The field itself “twists” and then expands into the nebula. The pressure of the field and the particles accelerates the expansion of the nebula.

The study of the Crab Nebula, a unique celestial object, has made it possible to solve a number of problems of stellar evolution connected with the origin of pulsars.

S. B. PIKEL'NER

Crab Nebula

[′krab ′neb·yə·lə]
(astronomy)
A gaseous nebula in the constellation Taurus; an amorphous mass which radiates a continuous spectrum involved in a mesh of filaments that radiate a bright-line spectrum.
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