Main sequence stars

For most of its lifetime, a star is a main sequence star. It is stable, with balanced forces keeping it the same size all the time. During this period:

  • gravitational attraction tends to collapse the star
  • radiation pressure from the fusion reactions tends to expand the star
  • forces caused by gravitational attraction and fusion energy are balanced

The Sun is expected to be a main sequence star for billions of years.

Fusion reactions

In a main sequence star, hydrogen nuclei fuse together to form helium nuclei. This happens in several steps, but one way to simplify the overall change is:

_{1}^{2}\textrm{H} +~ _{1}^{3}\textrm{H} \rightarrow _{2}^{4}\textrm{He} +~ _{0}^{1}\textrm{n}

Two hydrogen nuclei fuse to produce a helium nucleus and a neutron.