The Balancing Act

A main-sequence star is in hydrostatic equilibrium — the outward push of radiation from fusion exactly balances the inward crush of gravity. This equilibrium can last millions or billions of years depending on the star's mass. Our Sun has been on the main sequence for 4.6 billion years and has roughly 5 billion more to go.

The Hertzsprung-Russell diagram plots stars by temperature and luminosity, revealing a clear diagonal band where most stars sit. This is the main sequence. Stars on the upper left are hot, blue, and massive. Stars on the lower right are cool, red, and small.