4.1 The main sequence

The main sequence is the most prominent feature on the diagram precisely because most stars spend the vast majority of their lifetime in this stable state. Any large and random sample of stars will contain mostly main-sequence stars, with relatively fewer in the other, shorter-lived, phases of their lifecycles.

This stability is the result of the nuclear reactions in the cores of main-sequence stars. Whatever their mass, temperature or luminosity, all main-sequence stars are powered by the conversion of hydrogen into helium by nuclear fusion. The equilibrium between gravity and the outward pressure of energy production in the core can be maintained as long as there is sufficient hydrogen available to fuel the reactions. For most of the main-sequence life of a star these reactions run at a steady rate, but eventually the hydrogen will run out and the star will come to the end of its stable existence on the main sequence.