Main sequence

The main sequence is the area on a Hertzsprung-Russell diagram in which most observable stars are located. The Sun is on the main sequence and will remain there for another 5 billion years before becoming a red giant and ending its life as a white dwarf after travelling along the asymptotic branch of the giants.

(Credit: CSIRO Australia 2004)

When they are on the main sequence, stars obtain their energy from two main types of reaction transforming hydrogen into helium. These are the proton-proton cycle, dominant in stars with small masses, like the Sun, and the Bethe cycle also called the CNO cycle (for carbon–nitrogen–oxygen), in stars of several solar masses. The rate of energy production of the proton-proton cycle depends on the temperature in the centre of the star according to a T⁴law, whereas that of the CNO cycle is in T²⁰, these two rates only becoming equal around 1.2 solar masses. The amount of energy released is therefore very sensitive to the temperature and the mass of the star, which consumes its reserves faster the more massive it is.