main sequence

When astronomers first plotted thousands of stars by temperature and luminosity, they expected a cloud of dots. Instead, most stars fell along a single sweeping diagonal stripe running from hot-and-brilliant down to cool-and-faint. That stripe is the main sequence, and the fact that the great majority of stars sit on it is one of the most important clues in all of astronomy about how stars work.

A star lands on the main sequence for a simple physical reason: it is steadily fusing hydrogen into helium in its core, and that fusion releases just enough energy to hold the star up against its own gravity in a stable balance. This balance lasts a long time — the Sun has been on the main sequence for about 4.6 billion years and will stay there for roughly 5 billion more. A star's exact spot along the band is set almost entirely by its mass: massive stars sit high and hot (blue, top left), low-mass stars sit low and cool (red, bottom right). Roughly 90 percent of all stars are on the main sequence at any moment, simply because that is the longest phase of a star's life.

The main sequence is therefore not a path stars travel along but a long resting phase they occupy. Because position along it tracks mass, and mass sets brightness through the mass-luminosity relation, the main sequence ties together temperature, luminosity, and mass in one band. When a star finally exhausts the hydrogen in its core, it leaves the main sequence and swells toward the giant branch — and the point where a cluster's stars peel away (the turnoff) reveals the cluster's age.