Hertzsprung-Russell Diagram

Luminosity class V is the long, narrow strip running diagonally across the diagram, and it is called the main sequence. The Sun lies on the main sequence, as do 90% of all stars. Stars on the main sequence are stable and healthy, shining as a result of nuclear fusion reactions in their cores that convert their hydrogen to helium. Stars On the H-R diagram, the Sun is a main sequence star. Main sequence of stars run from extremely bright, hot stars in upper left-hand corner to faint, cool stars in lower right-hand corner. Illustration by K. Lee Lerner and Argosy. The Gale Group. spend most of their lives on the main sequence, so it is not surprising that most stars are found there.

The main sequence slopes from upper left to lower right on the H-R diagram. Therefore, the hotter main sequence stars are, the brighter they are. Main sequence O stars, or O V stars (using the luminosity class numeral), are extremely hot and blaze away with the brightness of 10,000 or more Suns. At the other end of the main sequence are the little M V stars, shining with a dull glow, only 1% as bright as the Sun.

For main sequence stars, there are also relationships between surface temperature, radius, mass, and lifetime. Hotter main sequence stars are both larger (greater radius) and more massive than cooler ones. So not only are O V stars brighter than the Sun, they are also physically larger and may be 20 or more times as massive. M V stars may be only a tenth as massive as the Sun. However, the brilliant O stars have to consume their hydrogen fuel thousands of times faster than their cooler cousins. Therefore, they live for a very short time—no more than a few million years—while stars like the Sun may remain on the main sequence for 10 billion years. And the tiny, faint M stars, though not very impressive, will remain shining faintly on for hundreds of billions of years.