Messier 1, also known as the “Crab Nebula” and NGC 1952, is a supernova remnant and pulsar
wind nebula in the constellation of Taurus. 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. Corresponding to a bright supernova recorded by
Chinese astronomers in 1054, the nebula was observed later by English astronomer John Bevis
in 1731. The nebula was the first astronomical object identified with a historical supernova
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% c.
At the center of the nebula lies the Crab Pulsar, a neutron star 28–30 kilometres 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 is generally the strongest
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'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 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 solar masses (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 to that observed in the Crab. Recent studies,
however, suggest the progenitor could have been a super-Asymptotic giant branch star 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 2 M.
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.
The first and second images comprise 10 x Luminance (90 seconds each) and 5 x Red, Green and
Blue (90, 90 and 72 seconds each), 0.5m f/2.9 ASA Astrograph with FLI ML3200 camera at an
altitude of 65 degrees on 5 November 2015. The fields of view are 34’ x 22’ and 18’ x 12’
The third and fourth images comprise 6 x Luminance (300 seconds each), 0.101m f/5.4 Tele Vue
refractor with FLI ML16200 camera at an altitude of 55 degrees on 19 September 2017. The fields
of view of the images are 132’ x 132’ (2.2 degrees square) and 42' x 42', respectively.
Hills Observatory: 1 January 2013 to 8 October 2018