Apart from the main sequence towards the top right of the diagram there is a group of stars that are very bright but that present relatively low surface temperatures. Because of this they appear reddish, and their size and color bestowed them the familiar name of red giants. These are the largest stars n the Universe, and they include Betelgeuse and Antares. The third and final group at the left bottom of the diagram is represented by stars that are dim and very hot. Because of this factor and also due to their appearance, the stars in this group are the familiar white dwarfs.
Nothing lasts forever, and stars are no exception.
We have seen already that the energy generation mechanism of a star is based in the fusion reaction that converts hydrogen to helium. The delicate equilibrium mechanism in a star is the balance between two forces. The outer layers of the star simply fall in due to their own weight. The nuclear reactions taking place in the core produce a gas pressure that equilibrates the weight of the envelope. As hydrogen is depleted in the core of the star and nuclear reactions tend to shift towards the outer layers of the star, there is no more heat to prevent the core from contracting through gravitational attraction.