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Crab Nebula

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Crab Nebula
Supernova remnant
File:Crab Nebula High Res.jpg M1, the Crab Nebula.
Observation data: J2000.0 epoch
Right ascension05h 34m 30s
Declination+22° 01m 00s
Distance6300 ly ly
Apparent magnitude (V)+8.4
Apparent dimensions (V)6 × 4 arcmins
ConstellationTaurus
Physical characteristics
Radius3 ly ly
Absolute magnitude (V)???
Notable featuresOptical pulsar
DesignationsM1, NGC 1952
See also: Lists of nebulae

The Crab Nebula (catalogue designations M1, NGC 1952 ) is a supernova remnant in the constellation of Taurus. The nebula was first observed by John Bevis in 1731, and is the remnant of a supernova that was recorded by Chinese and Arab astronomers in 1054 as being visible during daylight for 23 days. Located at a distance of about 6,300 light years (ly) from Earth, it has a diameter of 6 ly and is expanding at a rate of about 1500 kilometres per second.

A pulsar in the centre of the nebula rotates 30 times per second, emitting pulses of radiation across the electromagnetic spectrum from radio waves to gamma rays. Its discovery provided the first conclusive evidence that supernova explosions produce the extremely dense neutron stars which are observed as pulsars.

Transits of solar system objects in front of the nebula can be used to study these objects. The Sun's corona was mapped in the 1950s and 60s 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 by Charles Messier in 1758, while he was observing a bright comet visible in that year. Messier catalogued it as the first entry in his catalogue of comet-like objects. The Earl of Rosse observed the nebula with his 36- and 72-inch metal reflecting telescopes 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 is 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.[1] 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.[2] 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. It is possible that the bright new "star" was observed by Native Americans and recorded in petroglyphs.[3]

Physical conditions

File:Crab Pulsar.jpg
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). In 1953 Iosif Shklovsky proposed a theory, according to which the light from the diffuse blue region is predominantly synchrotron radiation, which is given off by electrons, ejected at speeds of up to half the speed of light, as their paths curve. [4] Three years later the theory was approved by observations. In the 1960s it was revealed, that the fast-moving electrons are deflected by a strong magnetic field of the neutron star at the centre of the nebula.[5]

The filaments are the remnants of the progenitor star's atmosphere, and consist largely of ionised helium as well as hydrogen, with carbon, oxygen, nitrogen, iron, neon and sulphur also present. The temperature of the filaments is typically between 11,000 and 18,000 K, and are of densities of about 1,300 particles/cm³.[6]

The Crab Nebula is currently expanding outwards at about 1,500 km/s.[7] Images taken several years apart reveal the slow apparent expansion of the nebula, and by comparing this angular expansion of the nebula 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,[8] 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.[9] 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.[10]

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.[11]

Central star

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 was identified as the star responsible for the nebula as long ago as 1942, when Rudolf Minkowski found that its optical spectrum was extremely unusual.[12] The region around the star was found to be a strong source of radio waves in 1949[13] and X-rays in 1963 [14], and was identified as one of the brightest objects in the sky in gamma rays in 1967 [15]. 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.[16] However, the discovery of a pulsating radio source in the centre of the Crab Nebula clearly demonstrated that pulsars were formed in supernovae. They are now understood to be rapidly rotating neutron stars, with a powerful magnetic field concentrating their radiation emissions into narrow beams.

The Crab Pulsar is believed to be about 10 km in diameter; it emits pulses of radiation every 33 milliseconds.[17] Pulses are emitted at wavelengths across the electromagnetic spectrum, from radio waves to X-rays. Like all 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.[18]

The extremely high level of energy output of the pulsar creates a 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.[19] The most dynamic feature in the inner part of the nebula is the point where one of the pulsar's polar jets slams into the surrounding material, 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.

The star that exploded as a supernova is referred to as the supernova's progenitor star. Two types of star explode as supernovae: in the so-called Type Ia supernovae, gases falling onto a white dwarf raise its mass above a critical level, the Chandrasekhar limit, causing the explosion; in Type Ib/c and Type II supernovae, the progenitor star is a supermassive star which runs out of fuel for the nuclear fusion reactions that power it, and collapses in on itself, generating 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.[20]

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 this, and the question of where the 'missing mass' is remains unresolved.[21] 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.[20] 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. [22]

Transits by solar system bodies

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

The Crab Nebula lies roughly 1½º 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 the nebula, 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.[23] When X-rays were first observed from the Crab, a lunar occultation was used to determine the exact location of their source.[14]

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.[24]

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 860 km.[25] The transit of Saturn itself could not be observed, because the satellite was passing through the Van Allen belts at the time.

References

  1. ^ Mayall N.U. (1939), The Crab Nebula, a Probable Supernova, Astronomical Society of the Pacific Leaflets, v. 3, p.145
  2. ^ 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
  3. ^ SEDS, 1054 Supernova Petrograph.
  4. ^ Shklovskii, Iosif (1953). "On the Nature of the Crab Nebula's Optical Emission". Doklady Akademii Nauk SSSR. 90: 983.
  5. ^ 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)
  6. ^ Fesen R.A., Kirshner R.P. (1982), The Crab Nebula. I - Spectrophotometry of the filaments, Astrophysical Journal, v. 258, p. 1-10
  7. ^ 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
  8. ^ Trimble, V. (1973), The Distance to the Crab Nebula and NP 0532, Publications of the Astronomical Society of the Pacific, v. 85, p. 579
  9. ^ Trimble V. (1968), Motions and Structure of the Filamentary Envelope of the Crab Nebula, Astronomical Journal, v. 73, p. 535
  10. ^ 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
  11. ^ 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
  12. ^ Minkowski R. (1942), The Crab Nebula, Astrophysical Journal, v. 96, p.199
  13. ^ Bolton J.G., Stanley G.J., Slee O.B. (1949), Positions of three discrete sources of Galactic radio frequency radiation, Nature, v. 164, p. 101
  14. ^ a b 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
  15. ^ 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
  16. ^ Del Puerto C. (2005), Pulsars In The Headlines, EAS Publications Series, v. 16, pp.115-119
  17. ^ Harnden F.R., Seward F.D. (1984), Einstein observations of the Crab nebula pulsar, Astrophysical Journal, v. 283, p. 279-285
  18. ^ Kaufmann W.J. (1996), Universe 4th edition, Freeman press, p. 428
  19. ^ 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
  20. ^ a b Davidson K., Fesen R.A. (1985), Recent developments concerning the Crab Nebula, Annual Review of Astronomy and Astrophysics, v. 23, p. 119-146
  21. ^ 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
  22. ^ 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
  23. ^ 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
  24. ^ Erickson W.C. (1964), The Radio-Wave Scattering Properties of the Solar Corona, Astrophysical Journal, v. 139, p.1290
  25. ^ Mori K., Tsunemi H., Katayama H., Burrows D.N., Garmire G.P., Metzger A.E. (2004), An X-Ray Measurement of Titan's Atmospheric Extent from Its Transit of the Crab Nebula, Astrophysical Journal, v. 607, pp. 1065-1069. Chandra images used by Mori et al can be viewed here.
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