Crab Nebula: Difference between revisions

Coordinates: Sky map 5h 34m 31.97s, +22° 00′ 52.1″
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|doi=10.1051/eas:2005070
|doi=10.1051/eas:2005070
}}</ref> 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 now are understood to be rapidly rotating [[neutron star]]s, whose powerful [[magnetic field]] concentrates their radiation emissions into narrow beams.
}}</ref> 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 now are understood to be rapidly rotating [[neutron star]]s, whose powerful [[magnetic field]] concentrates their radiation emissions into narrow beams.

The Urantia Book speaks of a small remnant star at the center of the Crab Nebula. This star, which is a pulsar, was first detected in 1967 BUT revealed in 1955 in the Urantia Book page 464:6; Paper 41:8.4...more detail in Up Close and Personal with The Urantia Book - Expanded Edition page 13


The Crab Pulsar is believed to be about 28–30&nbsp;km in diameter;<ref name="Bejgeretal2002">
The Crab Pulsar is believed to be about 28–30&nbsp;km in diameter;<ref name="Bejgeretal2002">

Revision as of 09:26, 3 June 2013

Chandra, Hubble, and Spitzer image NGC 1952

The Crab Nebula (catalogue designations M1, NGC 1952, Taurus A) is a supernova remnant and pulsar wind nebula in the constellation of Taurus.[1] Corresponding to a bright supernova recorded by Chinese astronomers in 1054, the nebula was observed later by John Bevis in 1731. At an apparent magnitude of 8.4, comparable to that of the largest moon of Saturn, it is not visible to the naked eye but can be made out using binoculars under favourable conditions.

At X-ray and gamma-ray energies above 30 keV, the Crab is generally the strongest persistent source in the sky, with measured flux extending to above 10 TeV. Located at a distance of about 6,500 light-years (2 kpc) from Earth, the nebula has a diameter of 11 light years (3.4 pc, corresponding to an apparent diameter of some 7 arc minutes) and expands at a rate of about 1,500 kilometers per second (0.5% c). It is part of the Perseus Arm of the Milky Way Galaxy.

At the center of the nebula lies the Crab Pulsar, a neutron star 28–30 km across,[2] which emits pulses of radiation from gamma rays to radio waves 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 1950s and 1960s, the Sun's corona was mapped from observations of the Crab's radio waves passing through it, and in 2003, the thickness of the atmosphere of Saturn's moon Titan was measured as it blocked out X-rays from the nebula.

Origins and history of observation

Further information: SN 1054
Reproduction of the drawing by Lord Rosse (1844), the first depiction of the nebula (color-inverted in order to appear white-on-black)

Modern understanding that the Crab nebula was created by a supernova, dates to 1921, when Carl Otto Lampland announced he had seen changes in the structure of the Crab Nebula.[3] This eventually lead to the nebula being linked to a bright supernova seen in 1054 A.D. by the 1940s.[4]

The creation of the Crab Nebula corresponds to the bright SN 1054 supernova recorded by Chinese astronomers in AD 1054.[5]

The Crab Nebula was first identified 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; in 1757, Alexis Clairaut reexamined the calculations of Edmund Halley and predicted the return of Halley's comet in late 1758 (more precisely, he predicted it would pass the perihelion in Spring 1759, with the start of its period of visibility some months earlier). The exact time of the comet's return required the consideration of perturbations to its orbit caused by planets in the Solar System such as Jupiter, which Clairaut and his two colleagues Joseph Jérôme Lefrançois de Lalande and Nicole-Reine Lepaute carried out more precisely than Halley, finding that the comet should appear in the constellation of Taurus (constellation). It is in searching in vain for the comet that Charles Messier found the Crab nebula, which he at first thought to be Halley's comet. After some observation, noticing that the object that he was observing was not moving across the sky, Messier concluded that the object was not a comet. Messier then realised the usefulness of compiling a catalogue of celestial objects of a cloudy nature, but fixed in the sky, to avoid incorrectly cataloging them as comets.

William Herschel observed the Crab nebula numerous times between 1783 and 1809, but we do not know whether he was aware of its existence in 1783, or if he discovered it independently of Messier and Bevis. After several observations, he concluded that it was composed of a group of stars.

The Earl of Rosse observed the nebula at Birr Castle in 1844 using a 36-inch telescope, and referred to the object as the "Crab Nebula" because a drawing he made of it looked like a crab (he observed it again later, in 1848, using a 72-telescope and could not confirm the supposed resemblance, but the name stuck nevertheless).[6][7]

In 1913, when Vesto Slipher registered his spectroscopy study of the sky, the Crab nebula was again one of the first objects to be studied.

In the early twentieth 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 become visible on Earth 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 astronomers in 1054.[8][9]

The Crab Nebula seen in infrared by the Spitzer Space Telescope.

Changes in the cloud, suggesting its small extent, were discovered by Carl Lampland in 1921.[3] That same year, John Charles Duncan demonstrated that the remnant is expanding,[10] while Knut Lundmark noted its proximity to the guest star of 1054, but did not mention the comments of his two colleagues. In 1928, Edwin Hubble proposed associating the cloud to the star of 1054, an idea which remained confidential until the nature of supernovae was understood, and it was Nicholas Mayall who indicated that the star of 1054 was undoubtedly the supernova whose explosion produced the Crab Nebula. The search for historical supernovae started at that moment: seven other historical sightings have been found by comparing modern observations of supernova remnants with astronomical documents of past centuries. Given its great distance, the daytime "guest star" observed by the Chinese 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 analysis of historical records have found that the supernova that created the Crab Nebula probably appeared 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.[11] 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.[9]

In the 1960s, due to the prediction and discovery of pulsars, the Crab nebula again became a major centre of interest. It was then that Franco Pacini predicted the existence of a neutron star for the first time, which would explain the brightness of the cloud. This neutron star was observed shortly afterwards in 1968, a shining confirmation of the theory of the formation of these objects at the time of certain supernovae. The discovery of the Crab pulsar, and the knowledge of its exact age (almost to the day) allows for the verification of basic physical properties of these objects, such as characteristic age and spin-down luminosity, the orders of magnitude involved (notably the strength of the magnetic field), along with various aspects related to the dynamics of the remnant. The particular role of this supernova to the scientific understanding of supernova remnants was crucial, as no other historical supernova created a pulsar whose precise age we can know for certain. The only possible exception to this rule would be SN 1181 whose supposed remnant, 3C58, is home to a pulsar, but its identification using Chinese observations from 1181 is sometimes contested.

Physical conditions

Crab Nebula
Nebula
Hubble Space Telescope photograph (2005)
Observation data: J2000.0 epoch
Right ascension05h 34m 31.94s[12]
Declination+22° 00′ 52.2″[12]
Distance6.5 ± 1.6 kly (2.0 ± 0.5 kpc)[13] ly
Apparent magnitude (V)+8.4
Apparent dimensions (V)420″ × 290″[14][a]
ConstellationTaurus
Physical characteristics
Radius5.5 light year (1.7 pc) [15] ly
Absolute magnitude (V)−3.1 ± 0.5[b]
Notable featuresOptical pulsar
DesignationsMessier 1,[12] NGC 1952,[12] Sharpless 244
See also: Lists of nebulae
The Crab Nebula video by NASA

In visible light, the Crab Nebula consists of a broadly oval-shaped mass of filaments, about 6 arcminutes long and 4 arcminutes wide (by comparison, the full moon is 30 arcminutes across) surrounding a diffuse blue central region. In three dimensions, the nebula is thought to be shaped like a prolate spheroid.[14] 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 sulfur. The filaments' temperatures are typically between 11,000 and 18,000 K, and their densities are about 1,300 particles per cm3.[16]

In 1953 Iosif Shklovsky proposed that the diffuse blue region is predominantly produced by synchrotron radiation, which is radiation given off by the curving motion of electrons in a magnetic field. The radiation corresponded to electrons moving at speeds up to half the speed of light.[17] Three years later the theory was confirmed by observations. In the 1960s it was found that the source of the curved paths of the electrons was the strong magnetic field produced by a neutron star at the center of the nebula.[18]

Distance

Even though the Crab Nebula is the focus of much attention among astronomers, its distance remains an open question due to uncertainties in every method used to estimate its distance. In 2008, the consensus was that its distance from Earth is 2.0 ± 0.5 kpc (6.5 ± 1.6 kly). Along its longest visible dimension, it thus measures about 13 ± 3 light year across.[c] The Crab Nebula currently is expanding outward at about 1,500 km/s.[19] Images taken several years apart reveal the slow expansion of the nebula,[20] and by comparing this angular expansion with its spectroscopically determined expansion velocity, the nebula's distance can be estimated. In 1973, an analysis of many methods used to compute the distance to the nebula had reached a conclusion of about 6.3 kly, consistent with the currently cited value.[14]

The Crab Pulsar itself was discovered in 1968. Tracing back its expansion (assuming a constant decrease of expansion speed due to the nebula's mass) yielded a date for the creation of the nebula several decades after 1054, implying that its outward velocity has decelerated less than assumed since the supernova explosion.[21] This reduced deceleration 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 outward.[22]

Mass

Estimates of the total mass of the nebula are important for estimating the mass of the supernova's progenitor star. The amount of matter contained in the Crab Nebula's filaments (ejecta mass of ionized and neutral gas; mostly helium[23]) is estimated to be 4.6 ± 1.8 M☉.[24]

Helium-rich torus

One of the many nebular components (or anomalies) of the Crab is a helium-rich torus which is visible as an east-west band crossing the pulsar region. The torus composes about 25% of the visible ejecta. However, it is suggested by calculation that about 95% of the torus is helium. As yet, there has been no plausible explanation put forth for the structure of the torus.[25]

Central star

Main article: Crab Pulsar
Hubble Space Telescope image of a small region of the Crab Nebula, showing Rayleigh–Taylor instabilities in its intricate filamentary structure. Credit: NASA/ESA.

At the centre of the Crab Nebula are two faint stars, one of which is the star responsible for the existence of the nebula. It was identified as such in 1942, when Rudolf Minkowski found that its optical spectrum was extremely unusual.[26] The region around the star was found to be a strong source of radio waves in 1949[27] and X-rays in 1963,[28] and was identified as one of the brightest objects in the sky in gamma rays in 1967.[29] Then, in 1968, the star was found to be emitting its radiation in rapid pulses, becoming one of the first pulsars 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 who identified the first one considered the possibility that it could be a signal from an advanced civilization.[30] 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 now are understood to be rapidly rotating neutron stars, whose powerful magnetic field concentrates their radiation emissions into narrow beams.

The Urantia Book speaks of a small remnant star at the center of the Crab Nebula. This star, which is a pulsar, was first detected in 1967 BUT revealed in 1955 in the Urantia Book page 464:6; Paper 41:8.4...more detail in Up Close and Personal with The Urantia Book - Expanded Edition page 13

The Crab Pulsar is believed to be about 28–30 km in diameter;[31] it emits pulses of radiation every 33 milliseconds.[32] 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.[33]

The pulsar's extreme energy output creates an 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.[34] 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.

  • The Crab Pulsar. This image combines optical data from Hubble (in red) and X-ray images from Chandra X-ray Observatory (in blue).
    The Crab Pulsar. This image combines optical data from Hubble (in red) and X-ray images from Chandra X-ray Observatory (in blue).
  • Data from orbiting observatories show unexpected variations in the Crab Nebula's X-ray output, likely tied to the environment around its central neutron star.
  • NASA's Fermi spots 'superflares' in the Crab Nebula.

Progenitor star

This sequence of Hubble Space Telescope images shows features in the inner Crab Nebula changing over a period of four months, Credit: NASA/ESA

The star that exploded as a supernova is referred to as the supernova's progenitor star. Two types of stars 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 that 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 9 and 11 M☉.[25][35] 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.[36]

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.[24] 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.[36] 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, a phenomenon commonly seen in Wolf-Rayet stars. However, this would have created a shell around the nebula. Although attempts have been made at several wavelengths to observe a shell, none has yet been found.[37]

Transits by solar system bodies

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.[38] When X-rays were first observed from the Crab, a lunar occultation was used to determine the exact location of their source.[28]

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.[39]

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 (550 mi).[40] The transit of Saturn itself could not be observed, because Chandra was passing through the Van Allen belts at the time.

See also

Notes

  1. ^ Size as measured on a very deep plate taken by Sidney van den Bergh in late 1969.[14][41]
  2. ^ Apparent Magnitude of 8.4 - distance modulus of 11.5 ± 0.5 = −3.1 ± 0.5
  3. ^ distance × tan( diameter_angle = 420″ ) = 4.1 ± 1.0 pc diameter = 13 ± 3 light year diameter

References

  1. ^ and not, as its name might suggest, in Cancer. The now-current name is due to William Parsons, 3rd Earl of Rosse who observed the object in 1840 using a 36-inch telescope and produced a drawing that looked somewhat like a crab. The drawing was published in Observations on Some of the Nebulae, Philosophical Transactions of the Royal Society of London vol. 134 (1844), fig. 81 (plate xviii, p. 321)
  2. ^ "Crab Nebula: The Spirit of Halloween Lives on as a Dead Star Creates Celestial Havoc". Chandra X-ray observatory. October 24, 2006. Retrieved February 11, 2012. 12 miles across
  3. ^ a b C. O. Lampland, Observed Changes in the Structure of the "Crab" Nebula (N. G. C. 1952), Publications of the Astronomical Society of the Pacific, 33, 79–84 (1921) Bibcode:1921PASP...33...79L
  4. ^ A short biography of Jan Hendrik Oort, Section 7
  5. ^ "the spectacular supernova (stellar explosion) of 1054, which produced the Crab Nebula, went virtually unrecorded in Islamic texts even though it was widely noted in China." Owen Gingerich, Islamic astronomy, Scientific American, April 1986 v254 p74(10). There is also a 13th-century Japanese reference to this "guest star" in Meigetsuki. The event was long considered unrecorded in Islamic astronomy, but in 1978 a reference was found in a 13th-century copy made by Ibn Abi Usaybi'a (1194–1270) of a work by Ibn Butlan, a Nestorian Christian physician active in Baghdad.
  6. ^ Glyn Jones, K. (1976). "The Search for the Nebulae". Journal for the History of Astronomy. 7: 67. Bibcode:1976JHA.....7...67B.
  7. ^ Rossi, B.B. "The Crab Nebula ancient history and recent discoveries". NASA. NTRS. Retrieved October 1, 1969. {{cite web}}: Check date values in: |accessdate= (help)
  8. ^ Lundmark, K. (1921). "Suspected New Stars Recorded in Old Chronicles and Among Recent Meridian Observations". Publications of the Astronomical Society of the Pacific. 33: 225. Bibcode:1921PASP...33..225L. doi:10.1086/123101.
  9. ^ a b Mayall, N.U. (1939). "The Crab Nebula, a Probable Supernova". Astronomical Society of the Pacific Leaflets. 3: 145. Bibcode:1939ASPL....3..145M.
  10. ^ John C. Duncan, Changes Observed in the Crab Nebula in Taurus, Proceedings of the National Academy of Sciences, 7, 179–181 (1921) Bibcode:1921PNAS....7..179D
  11. ^ Collins, George W., II; Claspy, William P.; Martin, John C. (1999). "A Reinterpretation of Historical References to the Supernova of A.D. 1054". Publications of the Astronomical Society of the Pacific. 111 (761): 871–880. arXiv:astro-ph/9904285. Bibcode:1999PASP..111..871C. doi:10.1086/316401.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  12. ^ a b c d "SIMBAD Astronomical Database". Results for NGC 1952. Retrieved February 12, 2012.
  13. ^ Kaplan, D. L.; Chatterjee, S.; Gaensler, B. M.; Anderson, J. (2008). "A Precise Proper Motion for the Crab Pulsar, and the Difficulty of Testing Spin-Kick Alignment for Young Neutron Stars". Astrophysical Journal. 677 (2): 1201. arXiv:0801.1142. Bibcode:2008ApJ...677.1201K. doi:10.1086/529026.
  14. ^ a b c d Trimble, Virginia Louise (1973). "The Distance to the Crab Nebula and NP 0532". Publications of the Astronomical Society of the Pacific. 85 (507): 579. Bibcode:1973PASP...85..579T. doi:10.1086/129507.
  15. ^ Carroll, Bradley W.; Ostlie, Dale A. (2007). An Introduction to Modern Astrophysics, Second Edition. Pearson Addison-Wesley. ISBN 0-8053-0402-9.[verification needed]
  16. ^ Fesen, R. A.; Kirshner, R. P. (1982). "The Crab Nebula. I – Spectrophotometry of the filaments". Astrophysical Journal. 258 (1): 1–10. Bibcode:1982ApJ...258....1F. doi:10.1086/160043.
  17. ^ Shklovskii, Iosif (1953). "On the Nature of the Crab Nebula's Optical Emission". Doklady Akademii Nauk SSSR. 90: 983. Bibcode:1957SvA.....1..690S.
  18. ^ Burn B.J. (1973). "A synchrotron model for the continuum spectrum of the Crab Nebula". Monthly Notices of the Royal Astronomical Society. 165: 421. Bibcode:1973MNRAS.165..421B.
  19. ^ Bietenholz, M. F.; Kronberg, P. P.; Hogg, D. E.; Wilson, A. S. (1991). "The expansion of the Crab Nebula". Astrophysical Journal Letters. 373: L59–L62. Bibcode:1991ApJ...373L..59B. doi:10.1086/186051.
  20. ^ "Animation showing expansion from 1973 to 2001". Astronomy Picture of the Day. NASA. December 27, 2001. Retrieved March 20, 2010.
  21. ^ Trimble, Virginia Louise (1968). "Motions and Structure of the Filamentary Envelope of the Crab Nebula". Astronomical Journal. 73: 535. Bibcode:1968AJ.....73..535T. doi:10.1086/110658.
  22. ^ Bejger, M.; Haensel, P. (2003). "Accelerated expansion of the Crab Nebula and evaluation of its neutron-star parameters". Astronomy and Astrophysics. 405 (2): 747–751. arXiv:astro-ph/0301071. Bibcode:2003A&A...405..747B. doi:10.1051/0004-6361:20030642.
  23. ^ Green, D. A.; Tuffs, R. J.; Popescu, C. C. (2004). "Far-infrared and submillimetre observations of the Crab nebula". Monthly Notices of the Royal Astronomical Society. 355 (4): 1315–1326. arXiv:astro-ph/0409469. Bibcode:2004MNRAS.355.1315G. doi:10.1111/j.1365-2966.2004.08414.x.
  24. ^ a b Fesen, Robert A.; Shull, J. Michael; Hurford, Alan P. (1997). "An Optical Study of the Circumstellar Environment Around the Crab Nebula". Astronomical Journal. 113: 354–363. Bibcode:1997AJ....113..354F. doi:10.1086/118258.
  25. ^ a b MacAlpine, Gordon M.; Ecklund, Tait C.; Lester, William R.; Vanderveer, Steven J.; Strolger, Louis-Gregory (2007). "A Spectroscopic Study of Nuclear Processing and the Production of Anomalously Strong Lines in the Crab Nebula". Astronomical Journal. 133 (1): 81–88. arXiv:astro-ph/0609803. Bibcode:2007AJ....133...81M. doi:10.1086/509504.
  26. ^ Minkowski, R. (1942). "The Crab Nebula". Astrophysical Journal. 96: 199. Bibcode:1942ApJ....96..199M. doi:10.1086/144447.
  27. ^ Bolton, J. G.; Stanley, G. J.; Slee, O. B. (1949). "Positions of three discrete sources of Galactic radio frequency radiation". Nature. 164 (4159): 101–102. Bibcode:1949Natur.164..101B. doi:10.1038/164101b0.
  28. ^ a b Bowyer, S.; Byram, E. T.; Chubb, T. A.; Friedman, H. (1964). "Lunar Occultation of X-ray Emission from the Crab Nebula". Science. 146 (3646): 912–917. Bibcode:1964Sci...146..912B. doi:10.1126/science.146.3646.912. PMID 17777056.
  29. ^ 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 Letters. 151: L9. Bibcode:1968ApJ...151L...9H. doi:10.1086/180129.
  30. ^ Del Puerto, C. (2005). "Pulsars In The Headlines". EAS Publications Series. 16: 115–119. doi:10.1051/eas:2005070.
  31. ^ Bejger, M.; Haensel, P. (2002). "Moments of inertia for neutron and strange stars: Limits derived for the Crab pulsar". Astronomy and Astrophysics. 396 (3): 917–921. arXiv:astro-ph/0209151. Bibcode:2002A&A...396..917B. doi:10.1051/0004-6361:20021241.
  32. ^ Harnden, F. R.; Seward, F. D. (1984). "Einstein observations of the Crab nebula pulsar". Astrophysical Journal. 283: 279–285. Bibcode:1984ApJ...283..279H. doi:10.1086/162304.
  33. ^ Kaufmann, W. J. (1996). Universe (4th ed.). W. H. Freeman. p. 428. ISBN 0-7167-2379-4.
  34. ^ 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. 28 (2): 950. Bibcode:1996BAAS...28..950H.
  35. ^ Nomoto, K. (1985). "Evolutionary models of the Crab Nebula's progenitor". The Crab Nebula and related supernova remnants; Proceedings of the Workshop. Cambridge University Press. pp. 97–113. Bibcode:1985cnrs.work...97N.
  36. ^ a b Davidson, K.; Fesen, R. A. (1985). "Recent developments concerning the Crab Nebula". Annual Review of Astronomy and Astrophysics. 23 (507): 119–146. Bibcode:1985ARA&A..23..119D. doi:10.1146/annurev.aa.23.090185.001003.
  37. ^ Frail, D. A.; Kassim, N. E.; Cornwell, T. J.; Goss, W. M. (1995). "Does the Crab Have a Shell?". Astrophysical Journal Letters. 454 (2): L129–L132. arXiv:astro-ph/9509135. Bibcode:1995ApJ...454L.129F. doi:10.1086/309794.
  38. ^ Palmieri, T. M.; Seward, F. D.; Toor, A.; van Flandern, T. C. (1975). "Spatial distribution of X-rays in the Crab Nebula". Astrophysical Journal. 202: 494–497. Bibcode:1975ApJ...202..494P. doi:10.1086/153998.
  39. ^ Erickson, W. C. (1964). "The Radio-Wave Scattering Properties of the Solar Corona". Astrophysical Journal. 139: 1290. Bibcode:1964ApJ...139.1290E. doi:10.1086/147865.
  40. ^ 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. 607 (2): 1065–1069. arXiv:astro-ph/0403283. Bibcode:2004ApJ...607.1065M. doi:10.1086/383521. Chandra images used by Mori et al. can be viewed here [1].
  41. ^ van den Bergh, Sidney (1970). "A Jetlike Structure Associated with the Crab Nebula". Astrophysical Journal Letters. 160: L27. Bibcode:1970ApJ...160L..27V. doi:10.1086/180516.

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