<P> Another noteworthy feature of red giants is that, unlike Sun - like stars whose photospheres have a large number of small convection cells (solar granules), red - giant photospheres, as well as those of red supergiants, have just a few large cells, the features of which cause the variations of brightness so common on both types of stars . </P> <P> Red giants are evolved from main - sequence stars with masses in the range from about 0.3 M to around 8 M. When a star initially forms from a collapsing molecular cloud in the interstellar medium, it contains primarily hydrogen and helium, with trace amounts of "metals" (in stellar structure, this simply refers to any element that is not hydrogen or helium i.e. atomic number greater than 2). These elements are all uniformly mixed throughout the star . The star reaches the main sequence when the core reaches a temperature high enough to begin fusing hydrogen (a few million kelvin) and establishes hydrostatic equilibrium . Over its main sequence life, the star slowly converts the hydrogen in the core into helium; its main - sequence life ends when nearly all the hydrogen in the core has been fused . For the Sun, the main - sequence lifetime is approximately 10 billion years . More - massive stars burn disproportionately faster and so have a shorter lifetime than less massive stars . </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> <P> The evolutionary path the star takes as it moves along the red - giant branch, that ends finally with the complete collapse of the core, depends on the mass of the star . For the Sun and stars of less than about 2 M 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 . </P>

Where are red giant stars found in the h-r diagram