Dwarf white
A white dwarf is the residue of a extinct star. It is the penultimate phase of the evolution of the stars whose mass lies between 0,8 and 1,4 times that of the Sun, before its black transformation into dwarf.
Formation of dwarf white
The dwarf white ones at the end of the lifetime form the residue of the stars which did not explode in Supernova. At the end of their life, these stars have amalgamated most of their Hydrogène in Hélium. Deprived of fuel, they crumble on itself under the effect of the gravitation. The pressure and the temperature of the increasing heart, the fusion of helium in element heavier, in particular in Carbon, start. This new energy makes inflate the star, which becomes a giant red. However helium is consumed very quickly; when the fusion of helium stops, the contraction of star begins again. Its low mass not making it possible to reach sufficient temperatures and pressures to start the fusion of carbon, the heart crumbles dwarf white, while the external layers of star come to rebound on this solid surface and are projected in space in the form of nebulous planet gear. The result of this process is thus dwarf white surrounded by a primarily made up gas cloud hydrogen and helium not consumed in fusion (and of a little carbon). This dwarf white then cools very slowly in dwarf black.
Physical characteristic of dwarf white
Pressure of degeneration of the electrons
The dwarf white ones are degenerated stars, i.e. which they can be maintained only thanks to quantum effects. In the absence of these effects, nothing would come to stop the collapse of dwarf white, and it would become a black hole. But when this collapse takes place, the electrons carbon atoms constituting stars are increasingly close from/to each other (with the quantum direction of the term). When the diameter of dwarf white is sufficiently small, the electrons then occupy the whole of the quantum energy levels at their disposal: two opposite electrons of Spin are then in each energy level. As from this moment, the dwarf white one cannot crumble any more, except if its mass is higher than 1,4 solar mass. In case contrary, Principle of exclusion of Pauli, which stipulates that two Fermions (thus consequently 2 electrons) cannot coexist in two identical states quantum, prevents the electrons of star from forming a more compact building (there cannot be more than 2 electrons per atomic level). Thus, by the quantum effects, a force being opposed to the gravitation, called " pressure of degeneration of the fermions" , appears.If the dwarf white one has a mass higher than 1,4 solar mass, the pressure of degeneration of the electrons is not sufficient to thwart the gravitation: the electrons combine then with the protons to form neutrons (the principle of exclusion of Pauli is thus not violated), which cause them-also to it appearance of a pressure of degeneration, which can maintain the star in the form of a neutron star. Lastly, if the star is too massive, nothing prevents its collapse, and one obtains a Black hole.
Density and composition of dwarf white
The density of dwarf white is very high: dwarf white of a solar Masse has a ray of about size of that of the Ground. Because of the quantum phenomena, the diameter of dwarf white then does not depend almost more its Température, contrary to what occurs in stars in activity; it depends mainly on its Masse: the higher the mass of dwarf white is, the more its diameter is low because of the Gravitation. The dwarf white ones are made up in major part of carbon (and a little hydrogen), in an extremely condensed state.
Temperature
The temperature of dwarf white is extremely high, this heat having been stored during the very important gravitational collapse of star. The radiative surface of dwarf white being extremely weak, those take very a long time to cool. They thus populate the corner lower right of the diagram of Hertzsprung-Russell, that of not very luminous stars but nevertheless hot. Once its completely exhausted energy, dwarf white cannot even any more radiate and thus becomes a black Naine in good logic. Fact even of this absence of radiation, it is naturally not possible to observe them - and less still to estimate the proportion of it - by the current means (2007).
Supernovas of the 1a type
The supernovas of the type 1a (SN1a) are examples particularly interesting of the use of dwarf white for the determination of distances. In a double system, one of two stars, being transformed into dwarf white before his/her companion, accrète of the matter of this one by gravitational effects. When the mass of dwarf white exceeds 1,4 solar masses, it cannot be maintained and explodes in SN1a, visible with very high distances, and whose curve of luminosity is always the same one, which makes it possible to know their distance.
Simple: White dwarf
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