Supernova explosion is one of the violent explosions of the universe which outshines the entire galaxy. The supernova explosion releases tremendous amount of energy and light into the interstellar medium. The brightness of supernova is so large that it can be seen from the billions of light years away. The brightness of supernova is sometimes even larger than the brightness of galaxy in which it occurred. The supernova also releases radiations like gamma rays which can be harmful for any life supporting environment present around the supernova. The effect of supernova on the environment can be seen from many light years away.
History of supernova Explosion
The supernovae in any galaxy are not too frequent and there are normally two to three supernovae observed per century. The supernovae occurred in our galaxy were observed in the past by Chinese, European and Islamic astronomers. There are records that Chinese astronomers have observed a supernova in 185 AD which is today known as SN 185 (SN means supernova and 185 is the year of explosion). In 1006, the Chinese and Islamic astronomers have observed the brightest supernova called SN 1006. In 1054, another supernova was observed which was so bright that it could also be viewed in daytime for many months. The remnant of SN 1054 is known as CrabNebula. European astronomers had observed two latest supernovae in our galaxy known as SN 1572 and SN 1604, these supernovae could be observed by the naked eyes. SN 1572 was observed by the Tycho Brahe in Cassiopeia and SN 1604 was observed by Johannes Kepler.
In 1885, a supernova was observed in the Andromeda Galaxy. After the development of modern telescope and radio astronomy, the supernova in faraway galaxies could also be observed. These days, amateur astronomers also help in finding supernovae in other galaxies by comparing the view of closer galaxies with the earlier photographs.
Types of Supernova Explosion
The nature of all the supernovae are not same, they have different light spectrum which indicate the presence of different elements. Light spectrum is obtained by passing the light of supernova from prism which splits the light into its component just like rainbow. The pattern obtained after passing through prism is called light spectrum.
The supernovae are broadly classified into two categories based on presence of hydrogen line in the light spectrum of supernova. The supernova which does not show hydrogen line in the spectrum is called type I supernova and the supernova which shows the presence of hydrogen line in the spectrum is called type II supernova. There are further classifications of these two categories but we will not go in detail. We will just classify type I supernova, the type I supernova which shows presence of silicon line is called type Ia supernovaand type Ib/c does not show presence of silicon absorption line.
There are broadly two mechanism of supernova explosion, thermonuclear runaway and gravitational core collapse.
Type Ia Supernova due to Thermonuclear runaway
Thermonuclear runaway supernova occurs in binary stars in which one of them has become white dwarf. As we have learned in earlier post “stellar evolution” that the core of stars which have mass less than 1.4 times the mass of sun end up their life into white dwarf. This limit of 1.4 times of solar mass is also known as Chandrasekhar Limit. The white dwarf is supported by the electron degeneracy pressure.
Binary stars are the system of two stars which revolves around the center of mass of both stars. Usually one of the star is massive than the other star. Since the massive star burns their nuclear fuel faster than smaller stars, the massive star becomes red giant much earlier than smaller star. The red giant sheds its mass to form accretion disk around binary star system. After some time, the nuclear reactions of massive star stops and if the mass of the core is less than 1.4 times the solar mass then the core collapses and converts into white dwarf. Now we have binary star system with one normal star and other white dwarf.
As the hydrogen in the core of smaller star exhausts, it becomes red giant. The white dwarf starts pulling matter from the outer layer of red giant. The matter flows from the red giant to the white dwarf forming accretion disk around the white dwarf. As this matter deposits on the surface of white dwarf, the mass of white dwarf increases. This also increases the temperature of white dwarf. If the increasing mass becomes more than 1.4 times the solar mass then the white dwarf may collapse but just before the beginning of collapse, the thermonuclear reaction ignites. The thermonuclear runaway reaction makes the white dwarf unstable and the whole white dwarf blows away with tremendous explosion called type Ia supernova.
As this type Ia supernova occurs near the Chandrasekhar Limit, every type Ia supernova has the nearly same brightness. That’s why these are also known as standard candles. We know that light intensity follows inverse square law means light intensity decreases as the square of the distance from the source. The distance of supernova can be estimated by comparing the observed intensity with the standard intensity of supernova.
Using these standard candles it is proved that the rate of expansion of universe is accelerating. As these supernovae occurred in the past millions years ago, the light spectrum has red shift corresponding to the rate of expansion of universe occurring at that time. The time of these supernovae explosions can be found using the distance measured by standard candle and speed of light. Thus the rate of expansion of universe could be compared for different times in past. It was found that the rate of expansion is accelerating with time.
Type II Supernova due to Core Collapse
Core collapse supernova occurs when the mass of core of star is more than 1.4 times the solar mass. As we have learned that in the massive star the chain of nuclear reaction stops at iron. As the iron has highest stability and the conversion of iron into higher element do not produce energy instead they requires input of energy. Thus the nuclear reaction stops at iron and the pressure produced due to nuclear reaction stops. The core starts collapsing due to gravity and density of core increases. For the core of star massive than 1.4 times the solar mass, the gravitational pressure is much larger than the electron degeneracy pressure. Thus the electron degeneracy breaks and electron fuses into the proton forming neutron. The core collapse stops whens neutron degeneracy pressure develops against the gravitational pressure. The core is now called neutron star.
The gravitational potential energy decreases due to collapse of core, which is released as outburst and blows away the outer layers. This tremendous amount of energy is released in the form of explosion called type II supernova. The energy released in the supernova is equivalent to the energy released by sun in entire life. This explosion expels the matter of outer layer at velocity of 30000 km/s and creates a shock wave into the surrounding interstellar medium.
When the core of star is heavier than about 3 times the solar mass then the gravitation pressure overcome the neutleron degeneracy pressure and the collapse does not stop. The core and many layers above it collapses to the point of singularity and results in the black hole.
Impact of Supernova Explosion
Supernovae play an important role in the evolution of universe. The supernova distributes the heavy elements in the interstellar medium. The shock waves originated by supernova also helps in the formation of new stars. Supernova can also affect the biosphere of any planet having life if the distance of supernova from planet is not large.
Formation of heavy elements
The energy released in the supernova is so large that the hydrogen in the upper layer undergo nuclear chain reaction and give rise to element heavier than iron like copper, gold, silver and even uranium etc. The supernova throws these newly formed heavier elements into the interstellar medium. Later these heavy elements participate in the formation of new stars and planetary systems. We know that the planets of our solar system are made up of different type of heavy elements. These various types of elements are remnant of supernovae that occurred in past much before the formation of our sun. The life on Earth is also the result of these supernova remnants.
Formation of new stars
The shock wave originated from supernova propagates through the interstellar medium and applies pressure to the gas clouds present in the space. The pressure applied through shock wave creates region of high density which triggers the process of formation of new stars. Thus supernova plays an important role in the formation of new stars.
Impact on Earth
Depending upon the type and energy of supernova, the biosphere of Earth may get affect from as far as 3000 light years. The cosmic rays originated from supernova when encounters the atmosphere of Earth, various type of chemical reaction takes place in the biosphere. For e.g. the nitrogen gas converts into nitrogen oxides and also the ozone layer gets depleted. In 1996, it was theorized that the traces of past supernovae might be detectable on the Earth in the form of metal isotopes on the rocks. Iron-60 was found to be deposited on the deep-sea rock in the Pacific Ocean. In 2009, the nitrate ion was found to be deposited in the Arctic ice which was supposed to be the traces of SN 1006 and SN 1054.
Recent estimates predict that the type II supernovashould be closer than 26 light years to destroy the half of the ozone layer of Earth. The depletion of ozone layer will lead to exposure to harmful Ultra Violet radiation of Sun that will cause skin cancer. Thus supernovae have potential to destroy life on Earth but for that it has be very close to our solar system.
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