<P> When the star exhausts the hydrogen fuel in its core , nuclear reactions can no longer continue and so the core begins to contract due to its own gravity . This brings additional hydrogen into a zone where the temperature and pressure are adequate to cause fusion to resume in a shell around the core . The outer layers of the star then expand greatly , thus beginning the red - giant phase of the star 's life . As the star expands , the energy produced in the burning shell of the star is spread over a much larger surface area , resulting in a lower surface temperature and a shift in the star 's visible light output towards the red -- hence it becomes a red giant . At this time , the star is said to be ascending the red - giant branch of the Hertzsprung -- Russell ( H -- R ) diagram . </P>
<P> When the star exhausts the hydrogen fuel in its core , nuclear reactions can no longer continue and so the core begins to contract due to its own gravity . This brings additional hydrogen into a zone where the temperature and pressure are adequate to cause fusion to resume in a shell around the core . The outer layers of the star then expand greatly , thus beginning the red - giant phase of the star 's life . As the star expands , the energy produced in the burning shell of the star is spread over a much larger surface area , resulting in a lower surface temperature and a shift in the star 's visible light output towards the red -- hence it becomes a red giant . At this time , the star is said to be ascending the red - giant branch of the Hertzsprung -- Russell ( H -- R ) diagram . </P>
where are red giant stars found in the h-r diagram
where are red giant stars found in the h-r diagram
the red - giant branch
the red - giant branch