This diagram shows the primary evolutionary stages of a solar-mass star. The axes are scaled to show the logarithm of the stellar Luminosity (L) versus the surface Temperature (T). In this diagram the Sun currently has a log(L) of zero and lies along the diagonal, main-sequence line.

The star begins as a Bok globule (a) that contracts to form a condensing protostar. When it begins burning Hydrogen about 30 million years later, it stabilizes into a zero-age main sequence star. Thereafter it remains a main sequence star for about the next 10 billion years. The proportion of Helium at the core of this star steadily increases over this period, causing a gradual rise in luminosity.

As the Hydrogen at the core becomes exhausted, the star blossoms into a massive red giant (b). The star increases in luminosity, burning Hydrogen in a shell around a degenerate Helium core. Finally the Helium flash triggers Helium fusion to begin at the core.