Hydrogen and helium burning

In the nebular hypothesis, the majority of the mass of the dust cloud collects at the center. The intense gravitational forces present ultimately lead to nuclear fusion taking place. As most of the matter initially present in the nebula is hydrogen, the process of hydrogen burning takes place.

$\parbox{4.5in}{\vspace*{5pt} H + H $\rightarrow$\space ${}^2$ H + e$... ...n\\ ${}^3$ He + ${}^3$ He $\rightarrow$\space ${}^4$ He + H + H \vspace*{5pt}}$

The net effect of this is to convert protons (Hydrogen nuclei) into ⁴He nuclei, along with energetic particles such as photons which reach us here on earth.

When the hydrogen fuel is depleted, the star will start to collapse again. At some stage helium burning will begin to occur:

$\parbox{4.5in}{\vspace*{5pt} ${}^4$ He + ${}^4$ He + ${}^4$ He $\rightarrow$\space ${}^12$ C + photon \vspace*{5pt}}$

The gravitational collapse is then once more balanced by the heat pressure exerted from these fusion reactions. When the ⁴He fuel runs out, gravitational collapse again takes over. There is a point in this collapse when the star will expand, and the star becomes what is called a red giant. However, as the nuclear fires subside, the star cools and subsequently shrinks. What happens after this depends on the initial mass of the star.