Red supergiant star to individual objects, but it may become useful for analysis of galactic structure and discovery of otherwise obscured red supergiant stars. Surface abundances of red supergiants are dominated by hydrogen even though hydrogen at the core has been completely consumed. In the latest stages of mass loss before a star explodes, surface helium may become enriched to levels comparable with hydrogen. In theoretical extreme mass loss models, sufficient hydrogen may be lost that helium becomes the most abundant element at the surface. When pre-red supergiant stars leave the main sequence, oxygen is more abundant than carbon at the surface, and
Red-giant branch core becomes degenerate. The core still supports its own weight thermodynamically with the help of energy from the hydrogen shell, but is no longer in thermal equilibrium. It shrinks and heats causing the hydrogen shell to become thinner and the stellar envelope to inflate. This combination decreases luminosity as the star cools towards the foot of the RGB. Before the core becomes degenerate, the outer hydrogen envelope becomes opaque which causes the star to stop cooling, increases the rate of fusion in the shell, and the star has entered the RGB. In these stars, the subgiant phase occurs within a