For a star with at least 0.5 M ☉, when the hydrogen supply in its core is exhausted and it expands to become a red giant, it can start to fuse helium atoms to form carbon. The energy output of the helium fusion process per unit mass is only about a tenth the energy output of the hydrogen process, and the luminosity of the star increases. This results in a much shorter length of time in this stage compared to the main-sequence lifetime. (For example, the Sun is predicted to spend 130 million years burning helium, compared to about 12 billion years burning hydrogen.) Thus, about 90% of the observed stars above 0.5 M ☉ will be on the main sequence. On average, main-sequence stars are known to follow an empirical mass–luminosity relationship. The luminosity (L) of the star is roughly proportional to the total mass (M) as the following power law :