Red giants

Evolution

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 astrophysics, this refers to all elements other than hydrogen and helium). These elements are all uniformly mixed throughout the star. The star "enters" the main sequence when its core reaches a temperature (several million kelvins) high enough to begin fusing hydrogen-1 (the predominant isotope), and establishes hydrostatic equilibrium. (In astrophysics, stellar fusion is often referred to as "burning", with hydrogen fusion sometimes termed "hydrogen burning".) Over its main sequence life, the star slowly fuses 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 their fuel disproportionately faster and so have a shorter lifetime than less massive stars.