The Crab Nebula is one of the most intensely studied and most beautiful of objects to be found in the night sky. This beautifully coloured swirling cloud of interstellar gas can easily be seen with binoculars and low grade telescopes. It is so beautiful that it has inspired many amateur astronomers to pursue their hobby.
The Crab Nebula extends over an area of sky approximately one fifth of that of a full moon. Within this area of sky observers can see an extraordinary complex web of red, brown and white filaments. A turquoise blue light radiates from the gasps between the filaments. Although the nebula was first observed by John Bevis in 1731 it was not named as the Crab Nebula until 1848, In 1848 William Parsons, the third Earl of Rosse, made a sketch of the nebula. He thought that it looked like a crab.
Although beautiful, and often photographed as a feature for glossy, coffee table books about astronomy it has taken astronomers a long time to understand the processes at work in the Crab Nebula. Their understanding has developed from a series of scientific clues.
In the early twentieth century scientists identified that the nebula was evolving. Two photos taken of the nebula at an interval of two years were not identical. The nebula is thought to be some 6 ,300 light years from Earth with a diameter of about 1.3 light years. Changes that are discernable on this scale from Earth must be the product of very high energy events.
The exact location of the Crab Nebula is intriguing. It is thought to lie within one of outer spirals of the galaxy and is thought to be in the form of a squat sphere of gas expanding outwards from a central source. The central point is thought to be a the same location as an event witnessed by Chinese and Arab astronomers in 1056 when the astronomers saw a very bright object in the sky. Moreover, changes seen in the twentieth century photographic plates imply that the nebula has expanded from a central point dating from the middle of the 11th century.
Spectroscopic analysis has identified that the colourful arms of the nebula are composed of ionized helium, hydrogen along with molecules of carbon, oxygen, nitrogen, iron, neon and sulphur.
In 1953 Josif Shklovsky demonstrated that the mysterious blue glow found within the nebula was the product of electrons decelerating in an intense magnetic field.
More detailed spectroscopic analysis has shown that the Crab Nebula is a rich source of radio and gamma waves. The radio waves appear to be pulsed and are emitted at regular intervals of 33 milliseconds. The discovery of the regular radio emissions in 1967 was so enigmatic that some thought that an extra-terrestrial life form was signalling from within the nebula
Unravelling these clues has led scientists to the forefront of theories of solar evolution and decay. The chemical composition of the gas clouds can be explained by nucleo-synthesis. Elements up to the atomic weight of iron are produced by the nuclear fusion reactions which power stars. During the course of the reaction the energy released by nuclear fusion is able to offset the gravitational forces which encourage a star to collapse in on itself. However, when the fuel is extinguished, gravitational forces dominate and a star can implode. This is thought to have happened to a central star on the site of the Crab Nebula in 1056. Depending upon the mass of the star the collapse can be so rapid that the imploding gas builds up a tremendous temperature and pressure causing the collapse to stall and the gas to explode at high velocity back into space. Sometimes a similar effect is seen if too much water is forced =down a bathroom plughole. The explosion can be very rapid and is almost certainly the cause of the 1056 observation.
The puzzles of the strong magnetic field, gamma ray and regular radio wave emissions can be solved by proposing that the star continues to collapse. Having shed some of the incoming gas, pressures and densities continue to increase. Conditions become so intense that protons decay by nuclear reaction into neutron and neutrinos. Ultimately a very compact core of neutrons is formed containing the mass of the star in a sphere of just 20 kilometres in radius. A neutron star of this type is thought to lie at the heart of the Crab Nebula.
The regular radio signal that emanates from the Crab Nebula is an example of a pulsar. The exact reasons why neutron stars emit radio and gamma rays is unclear, hover the radio waves are believed to be omitted along the magnetic access of the neutron star. Neutron stars are known to spin vigorously, The reason for this is similar to that of a pirouetting dancer who pulls in her arms. To conserve angular momentum an object of smaller radius has to spin faster. When the spinning neutron star points in the direction of Earth a radio burst is transmitted in our direction. This is the origin of the 33 milli-second signal.
Although not the first pulsar to be discovered the discovery of the 33 millisecond radio signal from the Crab Nebula in 1968 was a milestone event in the unravelling of the Crab Nebula and super nova science.
