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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 (M☉) and 2.0 M☉[1]) during their red giant phase (the Sun is predicted to experience a flash 1.2 billion years after it leaves the main sequence). A much rarer runaway helium fusion process can also occur on the surface of accreting white dwarf stars.

Low mass stars do not produce enough gravitational pressure to initiate normal helium fusion. As the hydrogen in the core is exhausted, some of the helium left behind is instead compacted into degenerate matter, supported against gravitational collapse by quantum mechanical pressure rather than thermal pressure. This increases the density and temperature of the core until it reaches approximately 100 million kelvin, which is hot enough to cause helium fusion (or "helium burning") in the core.

However, a fundamental quality of degenerate matter is that increases in temperature do not produce an increase in volume of the matter until the thermal pressure becomes so very high that it exceeds degeneracy pressure. In main sequence stars, thermal expansion regulates the core temperature, but in degenerate cores this does not occur. Helium fusion increases the temperature, which increases the fusion rate, which further increases the temperature in a runaway reaction. This produces a flash of very intense helium fusion that lasts only a few minutes, but briefly emits energy at a rate comparable to the entire Milky Way galaxy.

In the case of normal low mass stars, the vast energy release causes much of the core to come out of degeneracy, allowing it to thermally expand, however, consuming as much energy as the total energy released by the helium flash, and any left-over energy is absorbed into the star's upper layers. Thus the helium flash is mostly undetectable to observation, and is described solely by astrophysical models. After the core's expansion and cooling, the star's surface rapidly cools and contracts in as little as 10,000 years until it is roughly 2% of its former radius and luminosity. It is estimated that the electron-degenerate helium core weighs about 40% of the star mass and that 6% of the core is converted into carbon.[2]

Red giants
Sakurai's Object is a white dwarf undergoing a helium flash.[3]

During the red giant phase of stellar evolution in stars with less than 2.0 M☉ the nuclear fusion of hydrogen ceases in the core as it is depleted, leaving a helium-rich core. While fusion of hydrogen continues in the star's shell causing a continuation of the accumulation of helium ash in the core, making the core denser, the temperature still is unable to reach the level required for helium fusion, as happens in more massive stars. Thus the thermal pressure from fusion is no longer sufficient to counter the gravitational collapse and create the hydrostatic equilibrium found in most stars. This causes the star to start contracting and increasing in temperature until it eventually becomes compressed enough for the helium core to become degenerate matter. This degeneracy pressure is finally sufficient to stop further collapse of the most central material but the rest of the core continues to contract and the temperature continues to rise until it reaches a point (≈1×108 K) at which the helium can ignite and start to fuse.[4][5][6]

The explosive nature of the helium flash arises from its taking place in degenerate matter. Once the temperature reaches 100 million–200 million kelvin and helium fusion begins using the triple-alpha process, the temperature rapidly increases, further raising the helium fusion rate and, because degenerate matter is a good conductor of heat, widening the reaction region.

However, since degeneracy pressure (which is purely a function of density) is dominating thermal pressure (proportional to the product of density and temperature), the total pressure is only weakly dependent on temperature. Thus, the dramatic increase in temperature only causes a slight increase in pressure, so there is no stabilizing cooling expansion of the core.

This runaway reaction quickly climbs to about 100 billion times the star's normal energy production (for a few seconds) until the temperature increases to the point that thermal pressure again becomes dominant, eliminating the degeneracy. The core can then expand and cool down and a stable burning of helium will continue.[7]

A star with mass greater than about 2.25 M☉ starts to burn helium without its core becoming degenerate, and so does not exhibit this type of helium flash. In a very low-mass star (less than about 0.5 M☉), the core is never hot enough to ignite helium. The degenerate helium core will keep on contracting, and finally becomes a helium white dwarf.

The helium flash is not directly observable on the surface by electromagnetic radiation. The flash occurs in the core deep inside the star, and the net effect will be that all released energy is absorbed by the entire core, leaving the degenerate state to become nondegenerate. Earlier computations indicated that a nondisruptive mass loss would be possible in some cases,[8] but later star modeling taking neutrino energy loss into account indicates no such mass loss.[9][10]

In a one solar mass star, the helium flash is estimated to release about 5×1041 J,[11] or about 0.3% of the energy release of a 1.5×1044 J type Ia supernova,[12] which is triggered by an analogous ignition of carbon fusion in a carbon–oxygen white dwarf.
Binary white dwarfs

When hydrogen gas is accreted onto a white dwarf from a binary companion star, the hydrogen can fuse to form helium for a narrow range of accretion rates, but most systems develop a layer of hydrogen over the degenerate white dwarf interior. This hydrogen can build up to form a shell near the surface of the star. When the mass of hydrogen becomes sufficiently large, runaway fusion causes a nova. In a few binary systems where the hydrogen fuses on the surface, the mass of helium built up can burn in an unstable helium flash. In certain binary systems the companion star may have lost most of its hydrogen and donate helium-rich material to the compact star. Note that similar flashes occur on neutron stars.
Shell helium flash

Shell helium flashes are a somewhat analogous but much less violent, nonrunaway helium ignition event, taking place in the absence of degenerate matter. They occur periodically in asymptotic giant branch stars in a shell outside the core. This is late in the life of a star in its giant phase. The star has burnt most of the helium available in the core, which is now composed of carbon and oxygen. Helium fusion continues in a thin shell around this core, but then turns off as helium becomes depleted. This allows hydrogen fusion to start in a layer above the helium layer. After enough additional helium accumulates, helium fusion is reignited, leading to a thermal pulse which eventually causes the star to expand and brighten temporarily (the pulse in luminosity is delayed because it takes a number of years for the energy from restarted helium fusion to reach the surface[13]). Such pulses may last a few hundred years, and are thought to occur periodically every 10,000 to 100,000 years.[13] After the flash, helium fusion continues at an exponentially decaying rate for about 40% of the cycle as the helium shell is consumed.[13] Thermal pulses may cause a star to shed circumstellar shells of gas and dust.
In fiction

In the science-fiction novella The Wandering Earth (Chinese: 流浪地球) written in 2000 by Liu Cixin, the prediction of a helium flash is what drives the plot to escape the solar system. This plot element was not in the 2019 movie based on the novella.
See also

Carbon detonation

References

Pols, Onno (September 2009). "Chapter 9: Post-main sequence evolution through helium burning" (PDF). Stellar Structure and Evolution (lecture notes). Archived from the original (PDF) on 20 May 2019.
Taylor, David. "The End Of The Sun". North Western.
"White Dwarf Resurrection". Retrieved 3 August 2015.
Hansen, Carl J.; Kawaler, Steven D.; Trimble, Virginia (2004). Stellar Interiors - Physical Principles, Structure, and Evolution (2 ed.). Springer. pp. 62–5. ISBN 978-0387200897.
Seeds, Michael A.; Backman, Dana E. (2012). Foundations of Astronomy (12 ed.). Cengage Learning. pp. 249–51. ISBN 978-1133103769.
Karttunen, Hannu; Kröger, Pekka; Oja, Heikki; Poutanen, Markku; Donner, Karl Johan, eds. (2007-06-27). Fundamental Astronomy (5 ed.). Springer. p. 249. ISBN 978-3540341437.
Deupree, R. G.; R. K. Wallace (1987). "The core helium flash and surface abundance anomalies". Astrophysical Journal. 317: 724–732. Bibcode:1987ApJ...317..724D. doi:10.1086/165319.
Deupree, R. G. (1984). "Two- and three-dimensional numerical simulations of the core helium flash". The Astrophysical Journal. 282: 274. Bibcode:1984ApJ...282..274D. doi:10.1086/162200.
Deupree, R. G. (1996-11-01). "A Reexamination of the Core Helium Flash". The Astrophysical Journal. 471 (1): 377–384. Bibcode:1996ApJ...471..377D. CiteSeerX 10.1.1.31.44. doi:10.1086/177976.
Mocák, M (2009). Multidimensional hydrodynamic simulations of the core helium flash in low-mass stars (PhD. Thesis). Technische Universität München. Bibcode:2009PhDT.........2M.
Edwards, A. C. (1969). "The Hydrodynamics of the Helium Flash". Monthly Notices of the Royal Astronomical Society. 146 (4): 445–472. Bibcode:1969MNRAS.146..445E. doi:10.1093/mnras/146.4.445.
Khokhlov, A.; Müller, E.; Höflich, P. (1993). "Light curves of Type IA supernova models with different explosion mechanisms". Astronomy and Astrophysics. 270 (1–2): 223–248. Bibcode:1993A&A...270..223K.

Wood, P. R.; D. M. Zarro (1981). "Helium-shell flashing in low-mass stars and period changes in mira variables". Astrophysical Journal. 247 (Part 1): 247. Bibcode:1981ApJ...247..247W. doi:10.1086/159032.

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White dwarf
Formation

Chandrasekhar limit PG 1159 star Stellar evolution Hertzsprung–Russell diagram Mira variable

Fate

Black dwarf Type Ia supernova
Candidates Neutron star
Pulsar Magnetar Related links Stellar black hole
Related links Compact star
Quark star Exotic star Extreme helium star Subdwarf B star Helium planet

In binary
systems

Nova
Remnant List Dwarf nova Symbiotic nova Cataclysmic variable star
AM CVn star Polar Intermediate polar X-ray binary
Super soft X-ray source Binary pulsar
X-ray pulsar List Helium flash Carbon detonation

Properties

Pulsating Urca process Electron-degenerate matter Quasi-periodic oscillations

Related

Planetary nebula
List RAMBOs White dwarf luminosity function Timeline of white dwarfs, neutron stars, and supernovae

List-Class article List Category Category Commons page WikiCommons

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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

Physics Encyclopedia

World

Index

Hellenica World - Scientific Library

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