Archive of Astronomy Questions and Answers

What is the Main Sequence?

Around the turn of the century, Henry Norris Russel and Einar Hertzsprung independently discovered that if you plotted a large number of stars on a diagram with luminosity running vertically, and temperature horizontally, most of the stars would be found in a narrow diagonal band. Later, once astronomers figured out that stars shine by converting hydrogen into helium in their cores, the explanation for this 'Main Sequence' was found. It represents the locus of stars in the diagram which are fusing hydrogen to helium either by the 'CNO-cycle' for the massive, Upper Main Sequence stars more massive than the Sun, or through the 'proton-proton cycle' for stars like our Sun or less massive. The luminosity of a star depends on the fourth power of its mass, so a 10 solar mass star is about 10,000 times as luminous as the sun, and is located much higher up on the main sequence than the cooler Sun is.

Hydrogen is converted to helium to produce the luminosity of the star and to keep it stable under the force of gravity. Stars do this for the majority of their lives because there is so much hydrogen in their cores to use as fuel. During its 11 billion years of projected life, the Sun will spend nearly 10 billion of these years on the Main Sequence, before evolving off the Main Sequence to become a giant star.