The stars are shined by nuclear fusion of hydrogen from birth to about 90% of the time of its life. As the hydrogen was ran out, nuclear fusion of helium generated in nuclear fusion of hydrogen is beginning, and the center temperature of the star will reach 100 million degrees or more. The stars are swelling with a temperature rise. Then it becomes a red giant with a temperature of surface low. A star with a weight of 8 times or less of the sun, the outer layer was blown off leaving a core, white dwarf.
White dwarfs is an extremely dense bodies, but with a weight of the same extent as the sun, a size just about Earth, and a weight 1000kg per 1cm3. The key here is that there is a limit to the weight of the white dwarf. This is called the "Chandrasekhar limit" with a weight of 1.4 times of the sun.
It can be described as follows in a little detail. Underpinning the structure of the white dwarfs is "degeneracy pressure" of electrons. There is no possibility that any two Fermi particles such as electrons take the same state. In other words, as the states of lower energy than an electron are occupied by other electrons, that electron can not become the same state with the other electrons, and it cannot be an energy below that. At this moment, the energy is called Fermi energyE and the corresponding pressure is called degeneracy pressureE Here, even if the speed of the electrons corresponding to the Fermi energy reached the speed of light, the weight can't support the gravity, this is the Chandrasekhar limit.
A white dwarf and a star join together with gravity, in the system which gas flows into the white dwarfs from star, the gas lies thick and the white dwarf will gain weight. Then, as soon as the weight of the star reaches the Chandrasekhar limit, the stars gravity is not supported by the electron degeneracy pressure, a large explosion is termed a Type Ia supernova explosion will cause.
When the Ia type supernova explosion, fusion is proceeded rapidly, heavy elements such as iron are being generated and scattered a shower in space, this promotes the chemical evolution of the universe. In addition, if it is assumed that the weight of white dwarfs raising a Ia supernova explosion is to be constant at 1.4 times of the sun in the condition of the explosion, and that the brightness is also a constant, it is possible to measure the distance of distant galaxies from the apparent brightness. By comparing the rate of expansion of the universe which can be determined in the redding of light and the distance obtained, it can be shown that the space is accelerated expanding. As a result, it can be evidenced that the main component of the universe is dark energy.
In this way, the white dwarfs will play a major role in the study of cosmology and chemical evolution of the universe. In particular, it is important to know that the weight of the white dwarfs in a close binary there is a possibility that the Ia type supernovae explosions in the future. It may affect the frequency of type Ia supernovae explosions, and it holds the key of chemical evolution of space. Also, weight value of Chandrasekhar limit of 1.4 times the weight of the sun was a theoretical calculation result, so the true value need to be explored.