In 1935 Fritz Zwicky and has colleague Walter Baade coined the phrase supernovae. Supernovae are visually stunning, complicated, stellar explosions. They are a solar requiem. These extremely energetic explosions are the final actions in the lifetime of a star. A supernova quickly liberates more energy than that of a star in its entire lifetime. Such events are rarely seen. Their aftermaths known as supernova remnants, are spectacular clouds of expanding gas and dust. These can be seen by amateur astronomers using good telescopes.
Supernovae are named after the years in which they occur. The most famous are SN 185, SN 1006, SN 1054, SN 1572 (Tycho’s Nova) and SN 1604 (Kepler’s Star). Chinese astronomers made the first observation of a supernova in 185 AD. The supernovae of 1006 is believed to be the brightest on record. The supernova of 1054 produced the famous Crab Nebula. The supernovae of 1604 was the last recorded in the Milky Way. SN 1885 in Andromeda was the first seen outside of the Milky Way.
A systematic search for supernovae began in the 1960s when astronomers realised that they could be used to measure galactic distances. They found that the peak intensity of a supernovae is related to its distance from Earth. The search is painstaking. Astronomers must compare before and after photographs to identify new light sources and it is difficult to locate a supernovae before the maximum intensity has peaked. The Katzman Automatic Imaging Telescope plus the introduction of charged coupled devices have greatly simplified the work. Astronomers can now electronically check the images. In the last few years scientists have typically found several hundred supernovae each year. Rather surprisingly, the most remote supernovae have recently been found to be dimmer than expected. This implies that the university is expanding faster than expected and has deep implications for our understanding of cosmology.
Scientists can use computer models to determine the date of supernovae explosions by studying the remnants. They estimate that on average a supernova explodes every fifty years or so in the Milky Way. They are not being recorded because they are fleeting. The Supernovae Early Warning System (SNEWS) intends to make their discovery much easier. SNEWS was set up in 2005 to detect neutrinos. According to theory, a supernova releases these sub-atomic particles several hours before emitting electromagnetic energy. The neutrino detector will alert electromagnetic astronomers.
Astronomers classify supernovae according to the chemical composition of the remnant. The key distinction is the presence of hydrogen. A Type II supernova contains hydrogen: a Type I supernovae does not. Some supernovae do not fit neatly into these classifications. Others such as SN 1987K appear to change classification, changing from a Type I to Type II in a matter of weeks or months.
The physics behind Type I and Type II supernovae are somewhat different. Both address the question of what happens to a star when it runs out of fuel. Without fuel the gas cools and the star contracts.
The formation of a Type II supernova is easiest to understand. The process applies to stars of at least nine solar masses. The collapsing star generates sufficient temperature and pressure to ignite the helium core and prevent further collapse. Fusion takes place forming carbon. When this fuel is burnt out the star collapses once more, higher temperatures and pressures are generated and the star burns through fusion a heavier set of nuclei. After several cycles, each burning heavier nuclei, the star makes a final burn of silicon to nickel 56. The final burn takes place over a period of several days at most. Thereafter, for most stars gravitational forces dominate and the star implodes. The core collapses in on itself at speeds approaching one fifth of the speed of light. Under intense heat and pressure the core absorbs energy and degenerates into helium and free neutrons. If the star has a mass of less than twenty times that of the sun it collapses into a neutron star in which the gravitational force is eventually balanced by the nuclear forces between neutrons. Neutron stars are extraordinary objects some thirty miles in diameter with the density of an atomic nucleus. During the collapse about 99% of the gravitational energy is converted into neutrinos – only 1% of the energy is converted into a supernova explosion. Scientists are still grappling with the precise mechanism that causes the explosion, If the star has a mass of between twenty and fifty solar masses the nuclear force is insufficient to half the gravitational collapse and the star degenerates into a black hole without the formation of a supernova. If the star has a mass greater than 140 solar masses it forms a supernova and does not leave a black hole remnant.
Most Type I supernovae are generated by burnt out stars that suffer from core collapse and undergo a fate similar to that described for Type II supernovae. The only difference is that Type I stars are stripped of their hydrogen, possible by solar wind, possibly because of the influence of a companion star. Some are though to be caused by the collapse of massive stars of over 20 solar masses known charmingly as Wolf-Rayet stars.
Stars have insufficient mass to degenerate into a neutron star may generate a Type 1a supernovae. Some 97% of all the stars in the galaxy are of this size. The last stage of their stellar evolution they become white dwarfs. By this stage nuclear fusion has ceased and the star has contracted into a very dense object, typically with the mass of the Sun and the radius of the Earth. The gravitational collapse is balanced by quantum forces but the condition is unstable. If extra mass is added to the system, from the collision of two stars or from a binary companion, the core temperate can rise sufficiently to allow carbon fusion. Once started a runaway reaction can take place in which all the heavy elements in a matter of seconds. The resulting energy is released as a supernova.
The supernovae produced form very large stars are known as pair-instability supernovae. They are responsible for most of the heavy metals that have been found in the universe.
Supernovae play an important role in the distribution of heavy elements throughout the universe. The Big Bang produced the hydrogen, helium and lithium in the universe. Stars and supernovae manufactured the elements of higher mass. Type II supernovae are responsible for all the elements heavier than iron. The higher elements were distributed into interstellar space through the explosion of super novae.
Supernovae are also important in the process of star formation. The expanding shock wave from a supernova can sufficiently compress local gas clouds to ignite the fusion reactions. Stars that form in this way are called second or third generation stars because of their use of recycled material. Dust and chemicals thrown out by the supernovae may be important for the formation of solar systems and in the development of life respectively.
This review shows that supernovae are spectacular astronomical events which hold clues to many of the fundamental problems of astronomy. They are useful for determining intergalactic distances, are important in the formation of heavy metals contribute to our understanding of black holes and may even be important to the origin of solar system and life itself.
