The main sequence

The long, stable middle age a star spends fusing hydrogen in its core.

Plot stars by brightness against temperature and most fall along one diagonal band. That band is not a sequence in time but a state: it is where a star sits while fusing hydrogen into helium in its core, which is the overwhelming majority of its life. Where on the band it sits is set almost entirely by its mass. The Sun has about five billion years of main sequence left; a star ten times heavier burns through in tens of millions, which is why massive stars are always young. The bottom end of the band is where the universe is still building. A star of a tenth of a solar mass burns so slowly that none has yet had time to leave the main sequence — the universe is not old enough — so every red dwarf ever formed is still on it, and they are about three quarters of all the stars there are.