Main sequence stars fuse hydrogen into helium in their cores.

The defining characteristic of a main sequence star is the thermonuclear fusion of hydrogen atoms into helium atoms occurring within its core. This process, known as the proton-proton chain or the CNO cycle (depending on the star's mass), releases an immense amount of energy in the form of photons and neutrinos. This outward flow of energy creates a pressure that counteracts the inward gravitational collapse, establishing a hydrostatic equilibrium that stabilizes the star for billions of years.