Hertzsprung-Russell diagram

Hertzsprung-Russell diagrams are used by astronomers to place the size, luminosity and surface temperatures of stars into context. The diagram plots surface temperature along the X-axis, and luminosity along the Y-axis.

The vast majority of stars are relatively small dim stars with low surface temperatures, known as red dwarfs. They range in mass from approximately half the mass of Sol, our sun, to slightly less than ten percent the mass of Sol.

A star's luminosity is inversely related to the cube of the star's mass. The frugal rate at which the small, dim red dwarfs turn their Hydrogen to Helium make them long lived, and the Universe is too young for any red dwarf to have consumed all its Hydrogen fuel. The smallest red dwarfs are expected to keep shining for one trillion years.

Sol is 4.5 billion years old, and is expected to have exhausted all its accessable Hydrogen in another 5 billion years, or so. Sirius, the brightest star in the sky, is slightly more than twice as massive as Sol, and it expected to exhaust its Hydrogen fuel within one billion years.

When red dwarf stars eventually exhaust their Hydrogen fuel they will shrink, and grow dim. Larger stars, including Sol, are massive enough that, as they shrink there is enough pressure in their core to start fusing Helium into higher elements.

All but the most massive stars that are still shining from energy derived from Hydrogen fusion are said to be on the main sequence of the Hertzsprung-Russell diagram.