Red giant of helium increases to the point where the star ceases to be fully convective and the remaining hydrogen locked in the core is consumed in only a few billion more years. Depending on mass, the temperature and luminosity continue to increase for a time during hydrogen shell burning, the star can become hotter than the Sun and tens of times more luminous than when it formed although still not as luminous as the Sun. After some billions more years, they start to become less luminous and cooler even though hydrogen shell burning continues. These become cool helium white dwarfs. Very-high-mass
Red supergiant star are uncommon compared to M-type, because they are a short-lived transition stage and somewhat unstable. The K-type stars, especially early or hotter K types, are sometimes described as orange supergiants (e.g. Zeta Cephei), or even as yellow (e.g. yellow hypergiant HR 5171A). Red supergiants are cool and large. They have spectral types of K and M, hence temperatures below 4,100 K. They are typically several hundred to over a thousand times the radius of the Sun, although size is not the primary factor in a star being designated as a supergiant. A bright cool giant star can easily be larger