Giant star

A giant star is much larger and brighter than a main-sequence star—also called a dwarf—of the same surface temperature. On the Hertzsprung–Russell diagram, giants appear above the main sequence (luminosity class V) and are assigned to luminosity classes II and III. The distinction between giant and dwarf stars, based on large differences in brightness despite similar temperatures (especially for K and M types), was introduced by Ejnar Hertzsprung around 1905 or 1906.

Giant stars can have radii hundreds of times that of the Sun and luminosities more than ten times solar. Stars even more luminous are classified as supergiants or hypergiants. A hot, bright main-sequence star is sometimes called a giant, but strictly speaking any main-sequence star is a dwarf, regardless of its size or luminosity.

**Formation**

A star becomes a giant when it exhausts the hydrogen available for fusion in its core and leaves the main sequence. What happens next depends largely on the star’s mass.

**Intermediate-mass stars** For stars with masses above about 0.25 solar masses, the hydrogen-depleted core contracts and heats up, triggering hydrogen fusion in a shell around it. The outer layers expand and cool, but luminosity increases only slightly, and the star becomes a subgiant. The inert helium core grows and heats as it accumulates helium from the shell, but in stars up to about 10–12 solar masses, it does not get hot enough to ignite helium burning (more massive stars become supergiants and evolve differently). After a few million years, the core reaches the Schönberg–Chandrasekhar limit, collapses rapidly, and may become degenerate. This causes the outer layers to expand further and creates a strong convective zone that brings heavy elements to the surface (the first dredge-up). The increased energy transport raises luminosity dramatically, and the star moves onto the red-giant branch, where it burns hydrogen in a shell stably for a substantial portion of its life (roughly 10% for a Sun-like star). The core continues to gain mass, contract, and heat up, while some mass is lost from the outer layers.

If the star’s main-sequence mass was below about 0.4 solar masses, it will never reach the central temperature needed to fuse helium. It remains a hydrogen-fusing red giant until its hydrogen runs out, then becomes a helium white dwarf. According to stellar evolution theory, no star of such low mass has yet evolved to that stage within the age of the Universe.

In stars above about 0.4 solar masses, the core eventually reaches 100 million Kelvin, and helium begins fusing into carbon and oxygen via the triple-alpha process. If the core is degenerate, helium fusion starts explosively, but most energy goes into lifting degeneracy, and the core becomes convective. The energy from helium fusion reduces pressure in the surrounding hydrogen-burning shell, lowering its energy output. The star’s overall luminosity drops, its outer envelope contracts, and it moves from the red-giant branch to the horizontal branch.

When the core helium is exhausted, a star up to about 8 solar masses has a carbon–oxygen core that becomes degenerate, and helium burning begins in a shell. As with the earlier core collapse, this triggers convection in the outer layers, a second dredge-up, and a dramatic increase in size and luminosity. This is the asymptotic giant branch (AGB), similar to the red-giant branch but brighter, with a hydrogen-burning shell providing most of the energy. Stars remain on the AGB for only about a million years, becoming increasingly unstable until they exhaust their fuel, pass through a planetary nebula phase, and end as a carbon–oxygen white dwarf.

**High-mass stars** Main-sequence stars above about 12 solar masses are already very luminous. When they leave the main sequence, they move horizontally across the HR diagram, briefly becoming blue giants before expanding further into blue supergiants. They start core-helium burning before the core becomes degenerate and smoothly evolve into red supergiants without a strong luminosity increase. At this stage, their luminosities are comparable to bright AGB stars, though they have much higher masses. They will continue to brighten as they burn heavier elements and eventually become supernovae.

Stars in the 8–12 solar mass range have intermediate properties and are called super-AGB stars. They largely follow the paths of lighter stars through the red-giant branch, horizontal branch, and AGB phases, but are massive enough to ignite core carbon burning and even some neon burning. They form oxygen–magnesium–neon cores, which may collapse in an electron-capture supernova or leave behind an oxygen–neon white dwarf.

O-class main-sequence stars are already highly luminous. For them, the giant phase is a brief period of slightly increased size and brightness before they develop a supergiant spectral class. Type O giants can be more than a hundred thousand times as luminous as the Sun, outshining many supergiants. Classification is complex, with small differences between luminosity classes and a continuous range of intermediate forms. The most massive stars develop giant or supergiant spectral features while still burning hydrogen in their cores, due to mixing of heavy elements to the surface and high luminosity, which produces a powerful stellar wind and causes the star’s atmosphere to expand.

**Low-mass stars** A star whose initial mass is less than about 0.4 solar masses will never fuse helium and will remain a red giant until its hydrogen is exhausted, then become a helium white dwarf.