Chandrasekhar limit
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The Chandrasekhar limit is the maximum mass of a white dwarf, and is approximately 3 × 1030 kg, around 1.44 times the mass of the Sun. This number is a bit different in various articles, from 1.2 to 1.46 times the mass of the Sun and depends on the chemical composition of the star. The limit was first calculated by and thus named after the Indian-American physicist Subrahmanyan Chandrasekhar.
The heat generated by a star pushes the atmosphere of the star out. As the star runs out of fuel the atmosphere collapses back on the star's core. If the star has a mass below the Chandrasekhar limit this collapse is limited by electron degeneracy pressure, which results in a stable white dwarf. If the star has a mass above the Chandrasekhar limit it has sufficient gravity to collapse past the white dwarf stage and become a neutron star, quark star or black hole.
The Chandrasekhar limit arises from taking account of the effects of quantum mechanics in considering the behaviour of the electrons providing the degeneracy pressure supporting the white dwarf. Electrons, being fermions, cannot be on equal energy levels, so that, when an electron gas is cooling down, it is impossible to give them all minimal energy. Plenty of electrons will have to stay at higher energy levels and will thus give a certain pressure, which is purely quantum mechanical in its nature.
In the classical approximation a white dwarf may be arbitrarily massive with its volume inversely proportional to its mass. In the quantum mechanical calculation the typical energies to which degeneracy pressure forces the electrons in a massive white dwarf are non-negligible relative to their rest masses and a limiting mass emerges for a self-gravitating, spherically symmetric body supported by degeneracy pressure.
If a white dwarf in a binary system with a giant star accretes enough material to exceed the Chandrasekhar limit, the star collapses and becomes a type I supernova. When Chandrasekhar first proposed his ideas, he was vehemently opposed by the British physicist Arthur Eddington. Embittered, Chandrasekhar moved to the United States where he remained at the University of Chicago for the most part. Although Eddington was subsequently proven wrong, Chandrasekhar was magnanimous in his victory.
External links
- Chandrasekhar's Nobel Lecture, 1983 (http://www.nobel.se/physics/laureates/1983/chandrasekhar-lecture.html)de:Chandrasekhar-Grenze
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