The main question of stellar physics is why the stars shine? How long have they been shone? What are the processes capable to generate the huge amount of energy the stars emit?

Sources of energy: Nuclear Reactions

What are the processes capable to generate the huge amount of energy the stars emit?

The basic calculations show that such an usual source of energy like the chemical oxidation or burning is not enough as a stellar source of energy. Even if the whole Sun was made of carbon and also oxygen to support the burning, the Sun would be burned totally in a few thousand years. Another source of energy could be the gravitational collapse. When the massive body is collapsing because of the gravity the outer layers literally falling down to the center and the energy of this matter is turning into a heat. The calculations demonstrate that the Sun collapsing from the almost infinite big body to the present size could emit the present power less than 50 million years.

A powerful source of energy could be the transformation of the matter to energy. Even Einstein has shown that the matter and the energy are connected by a simple formula: 

E = mc2,

Where E is the energy, produced by the matter with a mass m and c is the speed of light.

What are the processes during them the matter is transforming to energy? These are the nuclear fusion. Elements into the cores of the stars interact and transform to other elements emitting the colossal amount of energy. In 1920 Arthur Eddington proposed that stars obtained their energy from nuclear fusion of hydrogen to form helium. In 1939 Hans Bethe analyzed the possibilities for nuclear fusion which hydrogen was fused into helium. He proposed two processes according him could produce the energy in the stellar cores: proton-proton cycle and carbon-nitrogen-oxygen cycle.

Now these cycles are accepted as main sources of energy in stars. Which of them is more effective depends on the temperature of the stars.