Nebula (catalogue designations M1, NGC 1952, Taurus A) is a
supernova remnant in the constellation of Taurus. The now-current name
is due to William Parsons, who observed the object in 1840 using a
36-inch telescope and produced a drawing that looked somewhat like a
crab. Corresponding to a bright supernova recorded by Chinese
astronomers in 1054, the nebula was observed later by English
John Bevis in 1731. The nebula was the first astronomical
object identified with a historical supernova explosion.
At an apparent magnitude of 8.4, comparable to that of Saturn's moon
Titan, it is not visible to the naked eye but can be made out using
binoculars under favourable conditions. The nebula lies in the Perseus
Arm of the
Milky Way galaxy, at a distance of about 2.0 kiloparsecs
(6,500 ly) from Earth. It has a diameter of 3.4 parsecs
(11 ly), corresponding to an apparent diameter of some
7 arcminutes, and is expanding at a rate of about 1,500
kilometres per second (930 mi/s), or 0.5% of the speed of light.
At the center of the nebula lies the Crab Pulsar, a neutron star
28–30 kilometres (17–19 mi) across with a spin rate of
30.2 times per second, which emits pulses of radiation from
gamma rays to radio waves. At
X-ray and gamma ray energies above 30
keV, the Crab
Nebula is generally the brightest persistent source in
the sky, with measured flux extending to above 10 TeV. The nebula's
radiation allows for the detailed studying of celestial bodies that
occult it. In the 1950s and 1960s, the Sun's corona was mapped from
observations of the Crab Nebula'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.
The inner part of the nebula is a much smaller pulsar wind nebula that
appears as a shell surrounding the pulsar. Some sources consider the
Nebula to be an example of both a pulsar wind nebula as well as a
supernova remnant, while others separate the two phenomena based on
the different sources of energy production and behaviour. For the
Crab Nebula, the divisions are superficial but remain meaningful to
researchers and their lines of study.
1 Observational history
1.1 First identification
1.2 Connection to SN 1054
1.3 Crab Pulsar
2 Physical conditions
2.3 Helium-rich torus
3 Central star
4 Progenitor star
5 Transits by Solar System bodies
5.3 Other objects
7 See also
10 External links
HaRGB image of the Crab
Nebula from the Liverpool Telescope, exposures
totalling 1.4 hours.
Further information: SN 1054
Modern understanding that the Crab
Nebula was created by a supernova
dates to 1921, when
Carl Otto Lampland
Carl Otto Lampland announced he had seen changes
in its structure. This eventually led to the conclusion that the
creation of the Crab
Nebula corresponds to the bright SN 1054
supernova recorded by Chinese astronomers in AD 1054. There is a
13th-century Japanese reference to this "guest star" in
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
Usaibia of a work by Ibn Butlan, a
Nestorian Christian physician
active in Baghdad at the time of the supernova.
Reproduction of the first depiction of the nebula by Lord Rosse (1844)
(colour-inverted to appear white-on-black)
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. 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
Jérôme Lalande and Nicole-Reine
Lepaute carried out more precisely than Halley, finding that the comet
should appear in the constellation of Taurus. 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
cataloguing them as comets.
William Herschel observed the Crab
Nebula numerous times between 1783
and 1809, but it is not known 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 3rd Earl of Rosse observed the nebula at
Birr Castle in
1844 using a 36-inch (0.9 m) 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-inch
(1.8 m) telescope and could not confirm the supposed resemblance,
but the name stuck nevertheless.
Connection to SN 1054
The nebula is seen in the visible spectrum at 550 nm (green
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
Changes in the cloud, suggesting its small extent, were discovered by
Carl Lampland in 1921. That same year, John Charles Duncan
demonstrated that the remnant is expanding, while Knut Lundmark
noted its proximity to the guest star of 1054.
Edwin Hubble proposed associating the cloud to the star of
1054, an idea which remained controversial 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. Thanks to the recorded
observations of Far Eastern and Middle Eastern astronomers of 1054,
Nebula became the first astronomical object recognized as being
connected to a supernova explosion.
Image combining optical data from Hubble (in red) and
X-ray Observatory (in blue).
In the 1960s, because of the prediction and discovery of pulsars, the
Nebula again became a major centre of interest. It was then that
Franco Pacini predicted the existence of the
Crab Pulsar for the first
time, which would explain the brightness of the cloud. The star was
observed shortly afterwards in 1968. 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 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,
3C 58, is home to a pulsar, but its identification using Chinese
observations from 1181 is sometimes contested.
Hubble image of a small region of the Crab Nebula, showing
Rayleigh–Taylor instabilities in its intricate filamentary
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. 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.
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. 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 centre of the nebula.
Even though the Crab
Nebula is the focus of much attention among
astronomers, its distance remains an open question, owing 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 (6,500 ± 1,600 ly).
Along its longest visible dimension, it thus measures about
4.1 ± 1 pc (13 ± 3 ly) across.[d]
Nebula currently is expanding outward at about
1,500 km/s (930 mi/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. In 1973, an analysis
of many methods used to compute the distance to the nebula had reached
a conclusion of about 1.9 kpc (6,300 ly), consistent with
the currently cited value.
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. 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.
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) is estimated to be
One of the many nebular components (or anomalies) of the Crab Nebula
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.
Main article: Crab Pulsar
Slow-motion movie of the Crab Pulsar, taken with OES
Data from orbiting observatories show unexpected variations in the
X-ray output, likely tied to the environment around its
central neutron star.
NASA's Fermi spots 'superflares' in the Crab Nebula.
At the center 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. 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.
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. 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
Crab Pulsar is believed to be about 28–30 km
(17–19 mi) 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 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
Nebula 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
This sequence of Hubble images shows features in the inner Crab Nebula
changing over a period of four months.
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 'dead' white dwarf raise its mass until it nears
a critical level, the Chandrasekhar limit, resulting in a runaway
nuclear fusion explosion that obliterates the star; in Type Ib/c and
Type II supernovae, the progenitor star is a massive star whose core
runs out of fuel to power its nuclear fusion reactions and collapses
in on itself, releasing gravitational potential energy in a form that
blows away the star's outer layers. The presence of a pulsar in the
Nebula 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☉. Stars with masses lower than 8 M☉ 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 M☉ would have produced a nebula with a different
chemical composition from that observed in the Crab Nebula. Recent
studies, however, suggest the progenitor could have been a
super-asymptotic giant branch star in the 8 to 10 M☉ range that
would have exploded in an electron-capture supernova.
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 M☉, with 2–3 M☉ being the generally accepted
value. The neutron star mass is estimated to be between 1.4 and
The predominant theory to account for the missing mass of the Crab
Nebula 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
Transits by Solar System bodies
Chandra image showing Saturn's moon Titan transiting the nebula.
Nebula lies roughly 1.5 degrees 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 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
When X-rays were first observed from the Crab Nebula, a lunar
occultation was used to determine the exact location of their
The Sun's corona passes in front of the Crab
Nebula every June.
Variations in the radio waves received from the Crab
Nebula 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
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). The transit of Saturn
itself could not be observed, because Chandra was passing through the
Van Allen belts
Van Allen belts at the time.
Nebula seen in radio, infrared, visible light, ultraviolet,
X-rays, and gamma-rays (8 March 2015)
Nebula – five observatories (10 May 2017)
Nebula – five observatories (animation; 10 May 2017)
Nebula in fiction
Lists of nebulae
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^ Apparent Magnitude of 8.4 – distance modulus of
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^ distance × tan( diameter_angle = 420″ ) =
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7001130000000000000♠13±3 light year diameter
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Coordinates: 5h 34m 31.97s, +22° 00′ 52.1″