Giant Stars - Formation

Formation

A star becomes a giant star after all the hydrogen available for fusion at its core has been depleted and, as a result, it has left the main sequence. A star whose initial mass is less than approximately 0.25 solar masses will not become a giant star. For most of their lifetimes, such stars have their interior thoroughly mixed by convection and so they can continue fusing hydrogen for a time in excess of 1012 years, much longer than the current age of the Universe. Eventually, however, they will develop a radiative core, subsequently exhausting hydrogen in the core and burning hydrogen in a shell surrounding the core. (Stars with mass in excess of 0.16 solar masses may expand at this point, but will never become very large.) Shortly thereafter the star's supply of hydrogen will be completely exhausted and it will become a helium white dwarf.

If a star is more massive than 0.25 solar masses, then when it consumes all of the hydrogen in its core available for fusion, the core will begin to contract and hydrogen will begin to fuse to helium in a shell around the helium-rich core, and the portion of the star outside the shell expands and cools. During this portion of its evolution, labeled the subgiant branch on the Hertzsprung–Russell diagram, the luminosity of the star remains approximately constant and its surface temperature decreases. Eventually the star will start to ascend the red giant branch on the Hertzsprung–Russell diagram. At this point, the surface temperature of the star, now typically a red giant, will remain approximately constant as its luminosity and radius increase drastically. The core will continue to contract, raising its temperature., § 5.9.

If the star's mass, when on the main sequence, was below approximately 0.5 solar masses, it is thought that it will never attain the central temperatures necessary to fuse helium., p. 169. It will therefore remain a hydrogen-fusing red giant until it eventually becomes a helium white dwarf., § 4.1, 6.1. Otherwise, when the core temperature reaches approximately 108 K, helium will begin to fuse to carbon and oxygen in the core by the triple-alpha process.,§ 5.9, chapter 6. The energy generated by helium fusion causes the core to expand. This causes the pressure in the surrounding hydrogen-burning shell to decrease, which reduces its energy-generation rate. The luminosity of the star decreases, its outer envelope contracts again, and the star leaves the red giant branch. Its subsequent evolution will depend on its mass. If not very massive, it may be found in the horizontal branch on the Hertzsprung–Russell diagram, or its position in the diagram may move in loops., chapter 6. If the star is not heavier than approximately 8 solar masses, it will eventually exhaust the helium at its core and begin to fuse helium in a shell around the core. It will then increase in luminosity again as, now an AGB star, it ascends the asymptotic giant branch of the Hertzsprung–Russell diagram. After the star sheds most of its mass, its core will remain as a carbon-oxygen white dwarf., § 7.1–7.4.

For main-sequence stars with masses great enough to eventually fuse carbon (approximately 8 solar masses), p. 189, this picture must be modified in many ways, as they will become Blue giants which will continue fusion. These stars do not increase greatly in luminosity after leaving the main sequence, but they will become redder. They may become red supergiants, or mass loss may cause them to become blue supergiants or with insufficient mass become Bright giants., pp. 33–35; Eventually, they will become white dwarfs composed of oxygen and neon, or, if over 9 solar masses, will undergo a core-collapse supernova to form neutron stars, or black holes., § 7.4.4–7.8.