. The star again will live a period of stability which in the case of the sun will last 1 billion years. To start we will describe the evolution of a star which is familiar for us: sun. We will describe his evolution from its current form to its death.
The sun is an average star, mainly made up of hydrogen, which provide him its energy, and of helium.
It is in hydrostatic balance since nearly 4 billion year and will still remain it during 4 to 5 billion years. The life of a star is primarily determined by the fight between the gravity which tends to make break down star on itself and the pressure of gas heated by the photons produced during the fusion of hydrogen in the heart of star.
The ruptures of balance intervene when the internal fuel becomes exhausted (hydrogen initially then helium). During the exhaustion of hydrogen in the heart of star (when helium accounts for 12% of the mass of star), there is displacement of fusions on the peripheral layers of the star, which involves a dilation of this one and an increase in its luminosity. The star of the principal sequence of diagram HR (Figure 5.6 ) moves towards the branch of the red giants. The heart of star becomes inert and starts to break down under the weight of its own gravity. The pressure in the center is now primarily supported by gas of Fermi composed by the electrons. A packing and temperature do not result any more in one increase of the pressure. The temperature thus increases until reaching the value allowing the lighting of helium. As the medium is degenerated, fusion is explosive and one sees appearing the helium flash. It will intervene in 4 billion years for the sun.
When the temperature exceeds the temperature of Fermi of the electrons, the pressure is again supported by . The star again will live a period of stability which in the case of the sun will last 1 billion years.
When in its turn helium comes to be exhausted, the same phenomenon
appears, the star dilates again. However, this time, the temperature of
the heart does not reach that of lighting of carbon, balance is done
then between pressure of Fermi of the electrons and gravitation. The
star, after having exhausted all its fuel, contracts for the last time
to reach the size of a planet. This object, which remains very hot,
behaves from now on like a black body at a temperature of 
and appears white to us. It is thus quite naturally that it is called dwarf white.
This star is from now on sterile and starts to cool for gently dying out.