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Supernovae refers to the violent explosion of massive stars in the late period of evolution or of some binary systems in the middle of its evolution process. The explosion is so powerful that the radiation it emits can illuminate the entire galaxy （at the brightest of supernovae， its luminosity is 1 billion times higher than that of the sun） and it is also the most powerful explosion second to the Big Bang from which the universe was born. Therefore， at the beginning of its appearance， it has gained lots of attention of the world. In ancient China it was called Ke Sing （guest star）. Next， more secrets of supernovae will continue to be told.
Members of the Supernova Family
According to spectroscopic analysis， astronomers have found some universalities in the absorption line in the supernova spectra， so that it’s possible to classify supernovae. Simply speaking， the first level of classification is based on the existence of a hydrogen element （H） absorption spectrum in the spectra. If there is no hydrogen， then it is classified as type I supernova， and if hydrogen exists， then it is a type II supernova. In addition， it was observed that there are hyper-bright supernovae and mutated supernovae.
Type I supernovae can also be further divided according to other spectra： A typical type I a supernova has a strong absorption line of silicon at 615 nm， and if the absorption line of the silicon element is not conspicuous or strong enough， it is then categorized as an type I b or I c supernova. Normal type II supernovae can be divided into two kinds， II-P and II-L. The curve of luminosity of II-P supernovae variating with time has an obvious "platform" period， while that of II-L supernovae seems to be linearly attenuating.
The Formation of Supernova
Studies have shown that the mechanism of I a supernova formation is： after a white dwarf mainly consisting of carbon-oxygen absorbs enough material from the companion star and reaches the upper limit of the mass （i.e.， the Chandrasekhar limit， about 1.4 times of the mass of the sun）， its electron degeneracy is not strong enough to counteract its own gravitational force， resulting in an overall collapse， and finally through a series of complex processes an uncontrolled comminuted explosions takes place， forming a type I a supernova explosion. Due to Chandrasekhar mass limits， the type I a Supernova explosion has a standard luminosity.
Type II supernova is by its nature a catastrophic explosion of massive stars caused by internal collapses. Its strong gravitational force causes the star to have an intense collapsing. A neutron star comes into being if it collapses into a dense body with a diameter of a bit more than 10 kilometers mainly composed of neutrons， of which the neutron degeneracy pressure is sufficient to counteract gravity. If the neutron degeneracy pressure is still too weak to counteract the gravity， the star will eventually turn out to be a black hole. At the moment of the formation of a neutron star or a black hole， the violently released energy and the largely ejected mass make the supernova explosion to occur. Since the massive stars are different in terms of their mass respectively， the luminosity of the supernova explosion they release is also different from each other， so that the luminosity has no standard value. When a supernova explodes， a huge amount of high-energy radiation is released， which will undoubtedly destroy any life form nearby. Fortunately， according to observation， most of the massive stars are far from the solar system， and there is no sign of explosion of nearer ones. Life on earth will not be threatened by supernovae for the time being. How to Observe Supernova
It is estimated that in galaxies of the similar size of the Milky Way， the probability of a supernova explosion is approximately one or two times every 100 years， so waiting for the next supernova explosion to happen in the Milky Way is just a waste of time. There are hundreds of millions of galaxies in the observable universe， so it’s very likely that a supernova outside the Milky Way is found. Further testification can be made through continuous searching and monitoring of the starlit sky to find "transient body" that suddenly flickers brightly. In the summer of 2015， Chinese scientists observed a supernova explosion 3.8 billion light-years away from the Indus. It is by far the brightest supernova observed， of which the highest luminosity is 570 billion times stronger than that of the sun， about 20 times of the total luminosity of hundreds of billions of stars of the entire Milky Way galaxy. It’s hundreds of times more brighter than a normal supernova， considered as superbright supernova.
What is left after the supernova explosion is known as a supernova remnant. According to records， at least 8 supernovae have appeared in the Galaxy in more than 2000 years， the most famous of which is the "Tian Guan Ke Sing" （Guest Star in the skyline）， of which a detailed script was kept in the Chinese Song Dynasty （1054）. It left a spectacular crab-like nebula （M1） after its explosion. The last recorded supernova in the Milky Way is the Kepler supernova that exploded in 1604.
Knowledge Link： The Significance of Supernova Explosion
Any of the heavier elements in nature， such as calcium in bone， iron in blood， gold in jewelry， and uranium for nuclear power plants， are all produced and released into space by internal nuclear fusion of stars or supernova explosions， which means supernovae can produce abundant heavy elements for our universe. Meanwhile， by compressing nearby nebulae， the shock waves caused by supernova explosions can enable the formation of new stars （and planets）. It is never too much to say that human wouldn’t exist without supernova.
Members of the Supernova Family
According to spectroscopic analysis， astronomers have found some universalities in the absorption line in the supernova spectra， so that it’s possible to classify supernovae. Simply speaking， the first level of classification is based on the existence of a hydrogen element （H） absorption spectrum in the spectra. If there is no hydrogen， then it is classified as type I supernova， and if hydrogen exists， then it is a type II supernova. In addition， it was observed that there are hyper-bright supernovae and mutated supernovae.
Type I supernovae can also be further divided according to other spectra： A typical type I a supernova has a strong absorption line of silicon at 615 nm， and if the absorption line of the silicon element is not conspicuous or strong enough， it is then categorized as an type I b or I c supernova. Normal type II supernovae can be divided into two kinds， II-P and II-L. The curve of luminosity of II-P supernovae variating with time has an obvious "platform" period， while that of II-L supernovae seems to be linearly attenuating.
The Formation of Supernova
Studies have shown that the mechanism of I a supernova formation is： after a white dwarf mainly consisting of carbon-oxygen absorbs enough material from the companion star and reaches the upper limit of the mass （i.e.， the Chandrasekhar limit， about 1.4 times of the mass of the sun）， its electron degeneracy is not strong enough to counteract its own gravitational force， resulting in an overall collapse， and finally through a series of complex processes an uncontrolled comminuted explosions takes place， forming a type I a supernova explosion. Due to Chandrasekhar mass limits， the type I a Supernova explosion has a standard luminosity.
Type II supernova is by its nature a catastrophic explosion of massive stars caused by internal collapses. Its strong gravitational force causes the star to have an intense collapsing. A neutron star comes into being if it collapses into a dense body with a diameter of a bit more than 10 kilometers mainly composed of neutrons， of which the neutron degeneracy pressure is sufficient to counteract gravity. If the neutron degeneracy pressure is still too weak to counteract the gravity， the star will eventually turn out to be a black hole. At the moment of the formation of a neutron star or a black hole， the violently released energy and the largely ejected mass make the supernova explosion to occur. Since the massive stars are different in terms of their mass respectively， the luminosity of the supernova explosion they release is also different from each other， so that the luminosity has no standard value. When a supernova explodes， a huge amount of high-energy radiation is released， which will undoubtedly destroy any life form nearby. Fortunately， according to observation， most of the massive stars are far from the solar system， and there is no sign of explosion of nearer ones. Life on earth will not be threatened by supernovae for the time being. How to Observe Supernova
It is estimated that in galaxies of the similar size of the Milky Way， the probability of a supernova explosion is approximately one or two times every 100 years， so waiting for the next supernova explosion to happen in the Milky Way is just a waste of time. There are hundreds of millions of galaxies in the observable universe， so it’s very likely that a supernova outside the Milky Way is found. Further testification can be made through continuous searching and monitoring of the starlit sky to find "transient body" that suddenly flickers brightly. In the summer of 2015， Chinese scientists observed a supernova explosion 3.8 billion light-years away from the Indus. It is by far the brightest supernova observed， of which the highest luminosity is 570 billion times stronger than that of the sun， about 20 times of the total luminosity of hundreds of billions of stars of the entire Milky Way galaxy. It’s hundreds of times more brighter than a normal supernova， considered as superbright supernova.
What is left after the supernova explosion is known as a supernova remnant. According to records， at least 8 supernovae have appeared in the Galaxy in more than 2000 years， the most famous of which is the "Tian Guan Ke Sing" （Guest Star in the skyline）， of which a detailed script was kept in the Chinese Song Dynasty （1054）. It left a spectacular crab-like nebula （M1） after its explosion. The last recorded supernova in the Milky Way is the Kepler supernova that exploded in 1604.
Knowledge Link： The Significance of Supernova Explosion
Any of the heavier elements in nature， such as calcium in bone， iron in blood， gold in jewelry， and uranium for nuclear power plants， are all produced and released into space by internal nuclear fusion of stars or supernova explosions， which means supernovae can produce abundant heavy elements for our universe. Meanwhile， by compressing nearby nebulae， the shock waves caused by supernova explosions can enable the formation of new stars （and planets）. It is never too much to say that human wouldn’t exist without supernova.