Main
sequence stars are stable spheres of gas that change little over lifetimes of millions or billions of years. When their
temperatures and luminosities are plotted on a graph, known as the Hertzsprung-Russell diagram, they all fall together on a line that
extends from hot, blue stars to cool, red stars, a line called the main
sequence. The majority of observed stars, including the Sun, are on this line, which extends from hot blue stars to cool, red stars. The most massive are at the bright, high-temperature end of the sequence, and the least massive are at the faint, cool end.