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A super-AGB star is a star with a mass intermediate between those that end their lives as a white dwarf and those that end with a core collapse supernova, and properties intermediate between asymptotic giant branch (AGB) stars and red supergiants. They have initial masses of 7.5–9.25 M☉ in stellar-evolutionary models, but have exhausted their core hydrogen and helium, left the main sequence, and expanded to become large, cool, and luminous.
HR diagram

Super-AGB stars occupy the top-right of the Hertzsprung–Russell diagram (HR diagram), and have cool temperatures between 3,000 and 4,000 K, which is similar to normal AGB stars and red supergiant stars (RSG stars).[1] These cool temperatures allow molecules to form in their photospheres and atmospheres.[2] Super-AGB stars emit most of their light in the infra-red spectrum because of their extremely cool temperatures.
The Chandrasekhar limit and their life

A super-AGB star's core may grow to the Chandrasekhar mass because of continued hydrogen (H) and helium (He) shell burning, ending as core-collapse supernovae.[1][3] The most massive super-AGB stars (at around 9 M☉) are theorized to end in electron capture supernovae. The error in this determination due to uncertainties in the third dredge-up efficiency and AGB mass-loss rate could lead to about a doubling of the number of electron-capture supernovae, which also supports the theory that these stars make up 66% of the supernovae detected by satellites.

These stars are at a similar stage in life to red giant stars, such as Aldeberan, Mira, and Chi Cygni, and are at a stage where they start to brighten, and their brightness tends to vary, along with their size and temperature.

These stars represent a transition to the more massive supergiant stars that undergo full fusion of elements heavier than helium. During the triple-alpha process, some elements heavier than carbon are also produced: mostly oxygen, but also some magnesium, neon, and even heavier elements, gaining an oxygen-neon (ONe) core. Super-AGB stars develop partially degenerate carbon–oxygen cores that are large enough to ignite carbon in a flash analogous to the earlier helium flash. The second dredge-up is very strong in this mass range and that keeps the core size below the level required for burning of neon as occurs in higher-mass supergiants.[citation needed]
References

Groenewegen, M. A. T.; Sloan, G. C. (2018). "Luminosities and mass-loss rates of Local Group AGB stars and red supergiants". Astronomy & Astrophysics. 609: A114.arXiv:1711.07803. Bibcode:2018A&A...609A.114G. doi:10.1051/0004-6361/201731089. S2CID 59327105.
Levesque, Emily M.; Massey, Philip; Olsen, K. A. G.; Plez, Bertrand; Josselin, Eric; Maeder, Andre; Meynet, Georges (2005). "The Effective Temperature Scale of Galactic Red Supergiants: Cool, but Not as Cool as We Thought". The Astrophysical Journal. 628 (2): 973–985.arXiv:astro-ph/0504337. Bibcode:2005ApJ...628..973L. doi:10.1086/430901. S2CID 15109583.

Poelarends, A. J. T.; Herwig, F.; Langer, N.; Heger, A. (2008). "The Supernova Channel of Super‐AGB Stars". The Astrophysical Journal. 675 (1): 614–625.arXiv:0705.4643. Bibcode:2008ApJ...675..614P. doi:10.1086/520872. S2CID 18334243.

attribution contains text copied from Asymptotic giant branch available under CC-BY-SA-3.0

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Stars
Formation

Accretion Molecular cloud Bok globule Young stellar object
Protostar Pre-main-sequence Herbig Ae/Be T Tauri FU Orionis Herbig–Haro object Hayashi track Henyey track

Evolution

Main sequence Red-giant branch Horizontal branch
Red clump Asymptotic giant branch
super-AGB Blue loop Protoplanetary nebula Planetary nebula PG1159 Dredge-up OH/IR Instability strip Luminous blue variable Blue straggler Stellar population Supernova Superluminous supernova / Hypernova

Spectral classification

Early Late Main sequence
O B A F G K M Brown dwarf WR OB Subdwarf
O B Subgiant Giant
Blue Red Yellow Bright giant Supergiant
Blue Red Yellow Hypergiant
Yellow Carbon
S CN CH White dwarf Chemically peculiar
Am Ap/Bp HgMn Helium-weak Barium Extreme helium Lambda Boötis Lead Technetium Be
Shell B[e]

Remnants

White dwarf
Helium planet Black dwarf Neutron
Radio-quiet Pulsar
Binary X-ray Magnetar Stellar black hole X-ray binary
Burster

Hypothetical

Blue dwarf Green Black dwarf Exotic
Boson Electroweak Strange Preon Planck Dark Dark-energy Quark Q Black Gravastar Frozen Quasi-star Thorne–Żytkow object Iron Blitzar

Stellar nucleosynthesis

Deuterium burning Lithium burning Proton–proton chain CNO cycle Helium flash Triple-alpha process Alpha process Carbon burning Neon burning Oxygen burning Silicon burning S-process R-process Fusor Nova
Symbiotic Remnant Luminous red nova

Structure

Core Convection zone
Microturbulence Oscillations Radiation zone Atmosphere
Photosphere Starspot Chromosphere Stellar corona Stellar wind
Bubble Bipolar outflow Accretion disk Asteroseismology
Helioseismology Eddington luminosity Kelvin–Helmholtz mechanism

Properties

Designation Dynamics Effective temperature Luminosity Kinematics Magnetic field Absolute magnitude Mass Metallicity Rotation Starlight Variable Photometric system Color index Hertzsprung–Russell diagram Color–color diagram

Star systems

Binary
Contact Common envelope Eclipsing Symbiotic Multiple Cluster
Open Globular Super Planetary system

Earth-centric
observations

Sun
Solar System Sunlight Pole star Circumpolar Constellation Asterism Magnitude
Apparent Extinction Photographic Radial velocity Proper motion Parallax Photometric-standard

Lists

Proper names
Arabic Chinese Extremes Most massive Highest temperature Lowest temperature Largest volume Smallest volume Brightest
Historical Most luminous Nearest
Nearest bright With exoplanets Brown dwarfs White dwarfs Milky Way novae Supernovae
Candidates Remnants Planetary nebulae Timeline of stellar astronomy

Related articles

Substellar object
Brown dwarf Sub-brown dwarf Planet Galactic year Galaxy Guest Gravity Intergalactic Planet-hosting stars Tidal disruption event

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