The Crab Nebula (catalogue designations M1, NGC 1952, Taurus A) is a supernova remnant and pulsar wind nebula in the constellation of Taurus and possibly the most intensely studied bright nebula in the Universe.
The Crab Nebula is approximately 6,500 light years from Earth, has an absolute magnitude (visible brightness) of 8.4 and is approximately 10 light years in diameter. To put its diameter into perspective, if the Sun was the size of a basketball, the Crab Nebula would be the size of Earth.
At the centre of the nebula is the Crab Pulsar, a neutron star approximately 30 km across and spinning at about 30 revolutions every second.
According to historical records from 1054, the supernova explosion that resulted in the formation of the Crab Nebula was seen on Earth beginning on July 4th, by Chinese and Arab astronomers. Recorded as a “guest star” by Chinese astronomers, it was visible in daylight for 23 days and at night for almost 2 years. The explosion was described by Yang Wei-te, the court astronomer to the Sung emperor, as yellow in colour. The event was probably also recorded by Anasazi Indian artists, in present day Arizona and New Mexico, as findings in Navaho Canyon and White Mesa indicate. In addition, Mimbres Indian art from New Mexico has also been found which possibly depicts the supernova. There are no records of observation at the time by Europeans.
The discovery of the object as a nebula is attributed to John Bevis, an English physician and amateur astronomer, who made the discovery in 1731 and added it to his sky atlas, Uranographia Britannica. Some 27 years later, on August 28, 1758, Charles Messier independently found a “nebulosity above the southern horn of Taurus…”, whilst he was looking for the first predicted return of Halley’s Comet. Messier at first thought it was a comet but realised that it had no apparent motion. It was this discovery that led to Messier compiling his famous catalogue of nebulae and star clusters so that others would not confuse them with comets “just beginning to shine”. This is the origin of the designation of M1, number 1 in Messier’s catalogue. Messier later acknowledged the original discovery made by John Bevis after learning of it in a letter of June 10, 1771.
The Crab Nebula got its most familiar name in 1840 when William Parsons, the Third Earl of Rosse, observed the nebula using a 36 inch telescope and created a drawing that he thought looked like a crab. The 36 inch telescope however did not allow Parsons to fully resolve the filaments, that coloured web of hot gas that permeates the nebula. When in 1848 Parsons again observed the object with a 72 inch telescope, it allowed him to see the nebula in greater detail. Although he noted that his earlier drawings did not represent the true structure of the nebular, the name Crab Nebular had already become familiar.
The Crab Nebular is part of a class of objects known as Supernova Remnants (SNRs), that are the result of a large star going through a phase of its evolution known as a supernova. This phase occurs when the star no longer has enough fuel to keep from collapsing onto itself and it explodes in a violent surge of energy. The outer part of the star is driven into space, forming the remnant that can be observed today. It has been suggested that had the Crab Supernova occurred within 26 light years of the Sun, most living organisms on Earth’s surface would have been destroyed, as the resultant radiation would have caused the breakdown of Earth’s ozone layer.
The filaments of the Crab consist of the material ejected in the supernova explosion, spread over a distance approximately 10 light years in diameter, which is still rapidly expanding at the incredible velocity of 1,800 km/sec. These filaments are composed mainly of ionized helium and hydrogen together with oxygen, carbon, iron, nitrogen, neon and sulphur. These filaments are the remnants from the outer layers of the former star, the progenitor, whilst the inner blue coloured nebula emits light consisting of highly polarized synchrotron radiation which is emitted by high-energy electrons in a strong magnetic field. This explanation was first proposed by the Soviet astronomer J. Shklovsky in 1953.
In 1968, the Crab Pulsar was detected as a pulsating radio source at the centre of the nebula by astronomers of the Arecibo Observatory 300 meter radio telescope in Puerto Rico. It has been established that this pulsar is a rapidly rotating neutron star which rotates at around 30 times per second. This neutron star is an extremely dense object with more density than an atomic nucleus, concentrating more than one solar mass in a volume of 30 kilometres across. Its rotation is slowly decelerating by magnetic interaction with the nebula, providing the energy source which produces the shining effect of the nebula. It has been estimated that this energy source is 100,000 times more powerful that the Sun.
Astronomers are aware of around 1000 pulsars, this number growing almost daily due to radio telescope discoveries. However, the Crab pulsar is one of the youngest and most energetic pulsars known. While some pulsars have been observed to pulse in X-rays and gamma-rays, the Crab pulsar seems to pulse in almost every wavelength, pulsing in radio, optical, X-rays and gamma-rays and its nebula is also visible over that very broad range of wavelengths.
On January 8, 2007, a team of astronomers announced evidence that the Crab Pulsar may have four magnetic poles, rather than the usual two. It has been suggested that the two additional poles may have been “frozen” into the pulsar during its formation in a supernova explosion. Normally a pulsar beam from only one pole would be observed, although a weaker second signal can sometimes be detected if the beam from the other pole is pointing in Earth’s direction when it comes into view. The Crab pulsar is known to emit weaker secondary pulses, which indicate that the primary and secondary pulses are different, something that would be difficult to explain as coming from opposite magnetic poles. This has led to some astronomers suggesting that the secondary pulses are related to an additional pair of magnetic poles.
On April 12, 2011, a blast of gamma rays, the highest-energy light in the Universe, was detected being emitted by the Crab nebula. The cause of the gamma ray flare, described at the Third Fermi Symposium in Rome, mystified astronomers.
From 1054 AD to the present day, the Crab Nebula continues to fascinate.
