Red giant

red giant starred giantsred giant starsred-giantgiant starRedred-giant branchaging starexpected lifetime of the solar systemgiant
A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ) in a late phase of stellar evolution.wikipedia
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Carbon star

carbon starsCCarbon
The appearance of the red giant is from yellow-orange to red, including the spectral types K and M, but also class S stars and most carbon stars.
A carbon star is typically an asymptotic giant branch star, a luminous red giant, whose atmosphere contains more carbon than oxygen.

Stellar evolution

evolvedevolvingevolution
A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ) in a late phase of stellar evolution.
This process causes the star to gradually grow in size, passing through the subgiant stage until it reaches the red giant phase.

Arcturus

ArcturiansArcturianAlpha Boo
The K0 RGB star Arcturus is 36 light-years away, and Gamma Crucis is the nearest M-class giant at 88 light-years' distance.
Relatively close at 36.7 light-years from the Sun, Arcturus is a red giant of spectral type K0III—an ageing star around 7.1 billion years old that has used up its core hydrogen and moved off the main sequence.

Red clump

red clump giantclump giantclump giants
Metal-rich helium-fusing stars instead lie on the so-called red clump in the H–R diagram.
The red clump is a clustering of red giants in the Hertzsprung–Russell diagram at around 5,000 K and absolute magnitude (M V ) +0.5, slightly hotter than most red-giant-branch stars of the same luminosity.

Red-giant branch

red giant branchred-giant-branchred giant branch (RGB)
When the envelope of the star cools sufficiently it becomes convective, the star stops expanding, its luminosity starts to increase, and the star is ascending the red-giant branch of the Hertzsprung–Russell (H–R) diagram.
Red giants were identified early in the 20th century when the use of the Hertzsprung–Russell diagram made it clear that there were two distinct types of cool stars with very different sizes: dwarfs, now formally known as the main sequence; and giants.

Giant star

giantorange giantgiants
A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ) in a late phase of stellar evolution.
This strong convection also increases the transport of energy to the surface, the luminosity increases dramatically, and the star moves onto the red-giant branch where it will stably burn hydrogen in a shell for a substantial fraction of its entire life (roughly 10% for a Sun-like star).

Asymptotic giant branch

AGBAGB starthermal pulse
This puts the star onto the asymptotic giant branch, a second red-giant phase.
Observationally, an asymptotic-giant-branch star will appear as a bright red giant with a luminosity ranging up to thousands of times greater than the Sun.

Sun

solarSolThe Sun
They have radii tens to hundreds of times larger than that of the Sun.
When hydrogen fusion in its core has diminished to the point at which the Sun is no longer in hydrostatic equilibrium, its core will undergo a marked increase in density and temperature while its outer layers expand, eventually transforming the Sun into a red giant.

Stellar nucleosynthesis

hydrogen burningnucleosynthesisstellar fusion
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.
Stars evolve because of changes in their composition (the abundance of their constituent elements) over their lifespans, first by burning hydrogen (main sequence star), then helium (red giant star), and progressively burning higher elements.

Dredge-up

dredged upfirst dredge-updredge up
Among the asymptotic-giant-branch stars belong the carbon stars of type C-N and late C-R, produced when carbon and other elements are convected to the surface in what is called a dredge-up.
The first dredge-up occurs when a main-sequence star enters the red-giant branch.

Triple-alpha process

helium fusionhelium burningtriple alpha process
Once the core is degenerate, it will continue to heat until it reaches a temperature of roughly 10 8 K, hot enough to begin fusing helium to carbon via the triple-alpha process.
This strong temperature dependence has consequences for the late stage of stellar evolution, the red giant stage.

Subgiant

subgiant staryellow subgiantsubgiant branch
This process of cooling and expanding is the subgiant star.
Luminosity-class-IV stars are the subgiants, located between main-sequence stars (luminosity class V) and red giants (luminosity class III).

Planetary nebula

planetary nebulaeplanetarycentral stars of planetary nebulae
Instead, at the end of the asymptotic-giant-branch phase the star will eject its outer layers, forming a planetary nebula with the core of the star exposed, ultimately becoming a white dwarf.
A planetary nebula, abbreviated as PN or plural PNe, is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives.

White dwarf

white dwarfswhite dwarf starcentral star
Instead, at the end of the asymptotic-giant-branch phase the star will eject its outer layers, forming a planetary nebula with the core of the star exposed, ultimately becoming a white dwarf.
, § 1. After the hydrogen-fusing period of a main-sequence star of low or medium mass ends, such a star will expand to a red giant during which it fuses helium to carbon and oxygen in its core by the triple-alpha process.

Variable star

variablevariable starsvariability
Another noteworthy feature of red giants is that, unlike Sun-like stars whose photospheres have a large number of small convection cells (solar granules), red-giant photospheres, as well as those of red supergiants, have just a few large cells, the features of which cause the variations of brightness so common on both types of stars.
These are red giants or supergiants.

Helium flash

flashshell helium flashflashes
Once the degenerate core reaches this temperature, the entire core will begin helium fusion nearly simultaneously in a so-called helium flash.
A helium flash is a very brief thermal runaway nuclear fusion of large quantities of helium into carbon through the triple-alpha process in the core of low mass stars (between 0.8 solar masses and 2.0 ) during their red giant phase (the Sun is predicted to experience a flash 1.2 billion years after it leaves the main sequence).

Aldebaran

Alpha Tauribrightest starRohini
Aldebaran, designated α Tauri (Latinized to Alpha Tauri, abbreviated Alpha Tau, α Tau), is a red giant star about 65 light-years from the Sun in the zodiac constellation Taurus.

Mira

Omicron Cetiο Cetiο Ceti
Mira, designation Omicron Ceti (ο Ceti, abbreviated Omicron Cet, ο Cet), is a red giant star estimated to be 200–400 light-years from the Sun in the constellation Cetus.

Planetary system

planetary systemssolar systemssystem
Although traditionally it has been suggested the evolution of a star into a red giant will render its planetary system, if present, uninhabitable, some research suggests that, during the evolution of a star along the red-giant branch, it could harbor a habitable zone for several billion years at 2 astronomical units (AU) out to around 100 million years at 9 AU out, giving perhaps enough time for life to develop on a suitable world.
As stars evolve and turn into red giants, asymptotic giant branch stars, and planetary nebulae they engulf the inner planets, evaporating or partially evaporating them depending on how massive they are.

Main sequence

main-sequencemain-sequence starmain sequence dwarf
Red giants are evolved from main-sequence stars with masses in the range from about to around.
After the hydrogen fuel at the core has been consumed, the star evolves away from the main sequence on the HR diagram, into a supergiant, red giant, or directly to a white dwarf.

Supernova

supernovaecore-collapse supernovasupernovas
These usually end their life as a type II supernova.
The larger of the two stars is the first to evolve off the main sequence, and it expands to form a red giant.

Circumstellar habitable zone

habitable zonehabitable zonesGoldilocks zone
Although traditionally it has been suggested the evolution of a star into a red giant will render its planetary system, if present, uninhabitable, some research suggests that, during the evolution of a star along the red-giant branch, it could harbor a habitable zone for several billion years at 2 astronomical units (AU) out to around 100 million years at 9 AU out, giving perhaps enough time for life to develop on a suitable world.
Even while stars are on the main sequence, though, their energy output steadily increases, pushing their habitable zones farther out; our Sun, for example, was 75% as bright in the Archaean as it is now, and in the future, continued increases in energy output will put Earth outside the Sun's habitable zone, even before it reaches the red giant phase.

Chi Cygni

χ Cygχ Cygni
χ Cygni is an asymptotic giant branch star, a very cool and luminous red giant nearing the end of its life.

Convection zone

convectionconvectiveconvective zone
Very-low-mass stars are fully convective and may continue to fuse hydrogen into helium for up to a trillion years until only a small fraction of the entire star is hydrogen.
In red giant stars, and particularly during the asymptotic giant branch phase, the surface convection zone varies in depth during the phases of shell burning.

Roche lobe

donor starRoche-lobemass transfer
The growth in planet mass could be partly due to accretion from stellar wind, although a much larger effect would be Roche lobe overflow causing mass-transfer from the star to the planet when the giant expands out to the orbital distance of the planet.
Mass transfer due to Roche-lobe overflow is responsible for a number of astronomical phenomena, including Algol systems, recurring novae (binary stars consisting of a red giant and a white dwarf that are sufficiently close that material from the red giant dribbles down onto the white dwarf), X-ray binaries and millisecond pulsars.