Transcript: Eventually all main sequence stars must exhaust their hydrogen fuel supply. This is true whether or not they are high mass and live their lives quickly or low mass and live their lives very slowly. The star must then eventually pass through either the red giant or the white dwarf stage. For a star like the Sun and more massive than the Sun it goes to a red giant stage. The Sun will spend roughly a billion years in this phase of evolution. As a star leaves the main sequence the core collapses because there is no pressure support from nuclear reactions. Meanwhile a shell of hydrogen is still fusing into helium. The core releases gravitational energy which drives an expanding envelope. Thus the outer part of the star turns into a huge, thin, and diffuse envelope with a low effective temperature, hence the name red giant, while the core becomes smaller, hotter, and denser than it was before. The size of the red giant envelope is huge, up to a thousand times the size of the Sun itself. If the Sun turns red giant its envelope would extend to the orbit of Jupiter. On the HR diagram stars leave the main sequence and have properties to place them in the upper right of the diagram.