Astronomy characteristics of the resulting star depend primarily upon its starting mass. The more massive the star, the greater its luminosity, and the more rapidly it fuses its hydrogen fuel into helium in its core. Over time, this hydrogen fuel is completely converted into helium, and the star begins to evolve. The fusion of helium requires a higher core temperature. A star with a high enough core temperature will push its outer layers outward while increasing its core density. The resulting red giant formed by the expanding outer layers enjoys a brief life span, before the helium fuel in the core
Red supergiant star than a hotter supergiant. For example, Alpha Herculis is classified as a giant star with a radius of while Epsilon Pegasi is a K2 supergiant of only . Although red supergiants are much cooler than the Sun, they are so much larger that they are highly luminous, typically . There is an upper limit to the luminosity of a red supergiant at around . Stars above this luminosity would be too unstable and simply don't form. Red supergiants have masses between about and . Main-sequence stars more massive than about do not expand and cool to become red supergiants. Red