red giant stars are found in the red - giant branch in the h-r diagram.
where are red giant stars found in the h-r diagram?
the red - giant branch
other stars, not strictly horizontal branch stars, can lie in the same region of the h-r diagram. stars too massive to develop a degenerate helium core on the red giant branch will ignite helium before the tip of the red giant branch and perform a blue loop. for stars only a little more massive than the sun, around , the blue loop is very short and at a luminosity similar to the red clump giants. these stars are an order of magnitude less common than sun-like stars, even rarer compared to the sub-solar stars that can form red clump giants, and the duration of the blue loop is far less than the time spent by a red clump giant on the horizontal branch. this means that these imposters are much less common in the h-r diagram, but still detectable.
red clump
the evolutionary path the star takes as it moves along the red-giant branch depends on the mass of the star. for the sun and stars of less than about the core will become dense enough that electron degeneracy pressure will prevent it from collapsing further. once the core is degenerate, it will continue to heat until it reaches a temperature of roughly 10 k, hot enough to begin fusing helium to carbon via the triple-alpha process. once the degenerate core reaches this temperature, the entire core will begin helium fusion nearly simultaneously in a so-called helium flash. in more-massive stars, the collapsing core will reach 10 k before it is dense enough to be degenerate, so helium fusion will begin much more smoothly, and produce no helium flash. the core helium fusing phase of a star's life is called the horizontal branch in metal-poor stars, so named because these stars lie on a nearly horizontal line in the h–r diagram of many star clusters. metal-rich helium-fusing stars instead lie on the so-called red clump in the h–r diagram.
red giant