Once a protostar starts burning hydrogen in its core it becomes a main sequence star (on the main sequnce on a Hertzsprung-Russell diagram)
During the majority of a "light" stars life, the center of the star has a high enough temperature and pressure that hydrogen atoms fuse to form helium. This process releases a great deal of energy (hence the appeal of fusion for generating electricity on Earth), which further increases the temperature and pressure in the center of the star. Eventually, this energy will work its way out of to the edges of the star, and will escape as light.
The star eventually runs out of hydrogen to "burn" in its center, and, if the temperature and pressure are high enough, helium will fuse into carbon, nitrogen and oxygen. CNO Cycle
- The CNO Cycle is not the process of fusing helium into carbon, nitrogen and oxygen!!! The CNO cycle describes how hydrogen is fused to helium, using carbon as a sort of catalysator.
When the star can no longer support fusion in its core, its internal energy source is depleted, and it will slowly cool to be a white dwarf. One famous example of a white dwarf is Sirius B, companion of Sirius. It is believed that Sirius B was more massive than Sirius itself. However, Sirius B now has as much mass as the Sun but is the size of Earth. Sirius B is very likely a new white dwarf, because it is one of the hottest known white dwarfs, with a surface temperature of about 26000 K. For comparison, the Sun's surface temperature is about 5770 K. In some other cases, the star will collapse and create an enormous explosion called supernova. Supernovae are extremely rare, for it only happens with a star at least 10 times bigger than our own Sun.
A white dwarf is believed to maintain its existence by electron degeneracy pressure. Pauli's Exclusion Principle, which states that no two electrons can be in the exact same state, provides a degeneracy pressure, which keeps the star from collapsing under its own gravity. The critical mass for a white dwarf is about 1.4 × Msun. This is mass is known as the Chandrasekhar Limit, which no white dwarf can exceed.
Where is the mean electron molecular weight.