Question: explain what causes a dying star to expand to a red giant

title: Red giant
text: 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 .