The main sequence: the long, steady middle age of nearly every star
Plot stars by colour against true brightness and most fall along one diagonal band. That band, the main sequence, is where stars spend the bulk of their lives fusing hydrogen in their cores, and a star's mass alone largely fixes where on it the star sits and how long it stays.
The pattern emerged early in the 20th century, once Annie Jump Cannon and Edward Pickering at Harvard had sorted stellar spectra into classes, published in 1901. In Potsdam in 1906, Danish astronomer Ejnar Hertzsprung noticed that the reddest stars split into two groups, far brighter or far fainter than the Sun, which he dubbed giants and dwarfs. Plotting colour against brightness for star clusters, he saw a continuous band and named it the main sequence. At Princeton, Henry Norris Russell independently found that dwarf stars followed a tight link between spectral type and true brightness. In 1933 Bengt Strömgren christened the chart the Hertzsprung-Russell diagram.
A star joins the sequence when a collapsing cloud, roughly 70 percent hydrogen and 28 percent helium, grows hot and dense enough at its core to fuse hydrogen into helium. The outward push of that energy balances gravity. Stars below about 1.5 solar masses, including the Sun, run mainly on the proton-proton chain, while heavier stars rely on the CNO cycle, using carbon, nitrogen and oxygen as go-betweens, and churn their cores by convection. The smallest stars, under 0.4 solar masses, are convective throughout.
Mass dominates the story. Heavier stars burn much faster, so the most massive have the shortest stay. When core hydrogen runs low, a star moves up and to the right on the diagram, becoming a red giant or supergiant or, in some cases, heading straight toward a white dwarf. Stars richer in heavy elements trap heat more effectively in their cores, which speeds fusion and shortens their time on the sequence.
The word dwarf causes confusion. For cool stars, main-sequence red and orange dwarfs really are smaller and dimmer than giants of the same colour, but for the hottest stars the size difference fades, and the labels mainly reflect spectral lines. White dwarfs are not main-sequence stars at all but Earth-sized dead cores. The 1943 Morgan-Keenan scheme classes main-sequence stars as luminosity class V, with temperature types running O, B, A, F, G, K, M. In April 2018 astronomers reported the most distant known ordinary star, Icarus, 9 billion light-years away.
Source: Main sequence
