Red giant star

In Astronomy the term red giant refers to a period after a star exhausts the Hydrogen available for Hydrogen fusion, in its core. It is the energy from Hydrogen fusion that postpones a star's gravitational collapse. After the Hydrogen available for fusion is exhausted gravity forces the star to continue to shrink, which, in turn, raises the temperature and pressure within the Helium in its core.

A star requires a certain minimum mass to reach a high enough temperature and pressure for Helium fusion to begin. When Helium fusion begins inu a star's dense core, its outer layer massively expands, causing the star's diameter to balloon. Red dwarf stars are not massive enough, and will never become red giants. All stars more massive than Sol, our sun, go through a red giant phase. yellow dwarf stars, like Sol, go through a red giant phase. Orange dwarf stars, like nearby Tau Ceti, will go through a red giant phase, although the Universe is not old enough for any of them to have exhausted its available Hydrogen.

Hydrogen fusion produces much more energy, per atom, than Helium fusion, so stars remain on the main sequence much longer than they remain red giant stars.

Helium fuses into Carbon or Oxygen. A red giant star with a mass of four times that of Sol will, eventually, have a core hot and dense enough to fuse Carbon. Carbon fusion releases relatively little energy, and the Xarbon usion period of a star's lifetime is measured in mere hundreds of years. The Oxygen fusion period of a star lasts around one year, or less. Oxygen fusion results in Silicon, Sulphur, Argon and Calcium. The period when massive stars fuse Silicon and Sulfur is even more brief, around one day, and results in Iron.

Under a high enough temperature and pressure Iron, and elements heavier than Iron, will fuse. But Iron fusion does not release energy. Iron fusion requires enormous energy. When a star's core is hot and dense enough for Iron to fuse, it takes so much energy that the core begins a catastrophic core-collapse, which, in turn, triggers a Supernova.