A red giant is a luminous, expanded star that has evolved beyond the main sequence and developed a large cool outer envelope surrounding a compact core. Red giants form when a star has substantially exhausted hydrogen in its central core. The inert helium rich core contracts under gravity, while hydrogen fusion continues in a shell surrounding the core. The contraction and shell burning cause the outer layers to expand dramatically.
As the radius increases, the surface cools, even though the total luminosity can rise because the emitting surface area becomes enormous. The star therefore moves towards the upper right of the Hertzsprung Russell diagram, where relatively cool temperatures coexist with high luminosities. Red giants can be tens to hundreds of times the Suns radius, and some evolved giant stars can become even larger. The term red giant refers primarily to this evolutionary and structural state rather than to one narrow spectral class.
The formation of a red giant begins near the end of core hydrogen burning. During the main sequence, hydrogen is converted into helium in the central region. Eventually the central hydrogen concentration becomes too low to sustain the same fusion structure. The helium rich core contracts and heats, while hydrogen burning shifts into a shell around the core. The increase in shell energy generation contributes to expansion of the outer envelope.
The surface temperature decreases as the envelope expands, producing a redder appearance. Despite the cooler surface, the star can become substantially more luminous because its radius increases enormously. This is an important example of why stellar temperature and luminosity cannot be treated as the same property. A star can cool at its surface while simultaneously increasing its total energy output.
In low and intermediate mass stars, the red giant phase leads eventually to helium ignition in the core. The precise conditions depend on stellar mass and electron degeneracy. In stars similar to the Sun, the contracting helium core can become strongly electron degenerate before reaching the temperature required for helium fusion. When helium ignition finally begins under these conditions, the resulting helium flash rapidly changes the core structure, although the process does not blow the star apart.
The star then settles into a more stable phase of core helium burning, becoming a horizontal branch star or, depending on composition and classification system, occupying a related region of the Hertzsprung Russell diagram. More massive stars can ignite helium without a strongly degenerate core and follow somewhat different evolutionary paths.
Red giant envelopes can become highly convective and extended. Convection transports energy through large scale movements of plasma and can mix material between deeper layers and the surface. This can alter surface chemical abundances and bring products of nuclear processing to observable layers, a process known as dredge up. During later giant phases, repeated thermal pulses can cause additional mixing and nucleosynthesis in asymptotic giant branch stars. Red giant winds can also remove substantial amounts of mass from the star.
These winds enrich the surrounding interstellar medium with material processed inside the star, contributing to the chemical evolution of galaxies. The outer envelope is therefore not merely an enlarged version of a main sequence atmosphere; it is a dynamic region affected by convection, pulsation, radiation pressure, dust formation and mass loss.
Red giant stars vary considerably in size, temperature and luminosity because red giant evolution encompasses several related stages. A star can move from the subgiant branch onto the red giant branch as hydrogen shell burning strengthens. Later, after core helium burning, lower and intermediate mass stars can develop into asymptotic giant branch stars, which are more evolved and can undergo thermal pulses and strong mass loss. Some red giants are variable stars because their extended envelopes can pulsate. Long period variables, Mira variables and semiregular variables occur among highly evolved cool giant and supergiant stars. The exact terminology matters because red giant, red giant branch star, asymptotic giant branch star and red supergiant describe related but distinct evolutionary categories.
Red giants are important sources of elements and dust returned to the interstellar medium. During their evolution, nuclear reactions produce and rearrange elements, while convection and stellar winds can transport processed material outward. On the asymptotic giant branch, slow neutron capture processes can produce many elements heavier than iron, while carbon and other products can be brought to the surface and released through winds.
The expelled material eventually becomes part of the gas and dust from which later generations of stars and planets can form. Planetary nebulae often develop when evolved low and intermediate mass stars lose their outer envelopes, although the nebula is not the red giant itself. The chemical enrichment associated with giant stars is therefore an important part of galactic chemical evolution.
Red giant expansion has major consequences for surrounding planetary systems. When a Sun like star becomes a red giant, its radius will expand enormously and its luminosity will increase, altering the radiation environment of its planetary system. The exact fate of individual planets depends on orbital evolution, stellar mass loss, tidal interactions and the degree of stellar expansion. Planets that remain outside the expanded envelope can still experience extreme heating and atmospheric changes. Similar effects occur around other evolved stars. Red giants can therefore provide important information about the future evolution of planetary systems and about the late stages of stars that began their lives as ordinary main sequence objects.
A common misconception is that red giants are simply very old versions of red dwarf stars. The terms describe entirely different physical categories. A red dwarf is a low mass, cool main sequence star undergoing hydrogen fusion in its core, whereas a red giant is an evolved star with an expanded envelope and a compact core after substantial main sequence evolution. Another misconception is that red giants are cool throughout their interiors.
Their surfaces are relatively cool, often a few thousand kelvin, but their cores can reach tens of millions of kelvin or more, depending on the evolutionary stage. The defining feature of a red giant is therefore the combination of a compact hot interior, shell burning and a greatly expanded cool envelope, not simply a red colour.
