mainsequence

The main sequence is a continuous and distinctive band on the Hertzsprung-Russell diagram in which stars spend most of their lifetimes fusing hydrogen in their cores. On this sequence, solar-type stars and many of the Galaxy’s brightest stars lie, spanning roughly spectral types O through M. A star on the main sequence maintains hydrostatic equilibrium as thermonuclear energy generated by hydrogen fusion in its core balances gravitational contraction.

In cooler, lower-mass stars the dominant fusion pathway is the proton-proton chain; in hotter, more massive stars the CNO cycle dominates. Energy transport through the star is mainly radiative in the inner regions, with convection in the outer layers for many low- and intermediate-mass stars. The location of a star on the main sequence depends primarily on its mass and chemical composition.

The main sequence is bounded at its hot end by the most massive, luminous stars (roughly a few tens of solar masses and above), and at its cool end by red dwarfs near the hydrogen-burning limit (about 0.08 solar masses). The duration a star spends on the main sequence, its lifetime, increases sharply as mass decreases. The Sun’s main-sequence lifetime is about 10 billion years; low-mass stars can live for trillions of years, while the most massive stars live only a few million years.

Starting from the zero-age main sequence, stars gradually evolve off the main sequence as hydrogen in their cores is depleted, becoming subgiants and eventually giants. The main sequence thus provides a benchmark for stellar populations and galactic evolution.