The Sun mainly consists of hydrogen and helium. Its principle source of energy is the synthesis of helium from hydrogen - the so-called proton-proton cycle. The essence of the reaction is that helium nucleus is formed from four protons, as it releases a pair of positrons and a pair of neutrinos, as well as 26.7 MeV of energy.
The Sun processes around 600 million tons of hydrogen per second. Nuclear fuel reserves are enough to last for five billion years. Once the nuclear fuel is depleted, the Sun will eventually turn into a white dwarf.
Central regions of the Sun will compress and its overall temperature will increase while the heat will be transferred to the outer blanket. The Sun will expand tremendously - in fact, it will become so big that it will absorb Mercury and Venus, and spend its "fuel" at a rate that will be one hundred times higher than the current rate. Our star will turn into a red giant, whose sizes will be comparable to the current distance from the Sun to the Earth. Life on Earth will either vanish, or find refuge on the outer planets of the Solar System.
Of course, the occurrence of such an event will become known a long time in advance, since a transition to the new stage will take approximately 100-200 million years. When the temperature of the central region of the Sun reaches 100 000 000 K, helium will also start burning and transforming into heavy elements. The Sun will enter the stage of complex compression and expansion cycles. Our star will lose its outer blanket during the last stage. Central core will assume a tremendously high density, and its size will be comparable with the size of the Earth. Several more billion years will pass as the Sun will get progressively colder, and gradually turn into a white dwarf.
This model illustrates the four basic variations of the helium synthesis reaction. Press "Run" button to observe one of the four hydrogen cycle reactions under various temperatures. Four protons transform into an alpha particle in each reaction, although each of the four times it occurs in a different fashion. Select the process you would like to see by using an appropriate switch. Pressing "Stop" button halts the animation; pressing "Reset" button will return the model to its initial state.
Protons and anti-protons are depicted as big red and blue balls, neutrons - as gray balls, electrons and positrons - as small blue and red balls, electron neutrinos are white, and gamma quantums are yellow balls.
Most of the reactions covered in this model have several stages to them. The current stage is outlined in red.