Image white dwarf

The white dwarfs

The white dwarfs represent the final stage of evolution for stars with a small mass: they are very small compact stars that rotate rapidly. To get an idea of their size they have been compared to the Sun, on the right of the figure is a dot that represents a white dwarf and on the left is the Sun. It is evident that the sizes of the white dwarfs are inferior to those of a normal star. Its size is comparable to that of a planet like Earth.

How does a white dwarf form? After the red giant stage, when almost all the helium in the nucleus has been transformed into carbon, the star no longer produces enough energy to stop the gravitational contractions and it is destined to collapse rapidly; as a consequence of this, the outer layers will once more start to push in the inner ones, and as a result the pressure and the temperature of the nucleus will increase. If this happens the temperature could reach such high levels that the transformation of carbon particles is triggered off. However, if the mass of the star is less than a certain critical level, that is one and a half times the mass of the Sun, the outer layers won’t be heavy enough to allow such an internal temperature to be reached; and the carbon particles will not undergo transformation.

On the other hand, if the nuclear transformation of the carbon particles is not set off, then new energy is not produced and the pressure on the outer layers does not occur. As a consequence, the star continues to collapse until the particles are so close to each other that the star can contract no more. The star is now in a stable equilibrium but without the production of anymore energy in the nucleus: the star has now become a white dwarf which will continue to undergo a progressive process of cooling.

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