Helium flash

A helium flash is the runaway fusion of helium in the core of low mass stars of less than about 2.25 solar masses and greater than about 0.5 solar mass, or on the surface of an accreting white dwarf star. They may also occur in the outer layers of larger stars in shell flashes. A helium flash occurs in these situations because the helium is degenerate, meaning it is supported against gravity by quantum mechanical pressure rather than thermal pressure. Thus an increase in the temperature in the material undergoing fusion does not act to expand the material and by doing so cool, and there is no regulation of the rate of fusion. It ends when the material is heated to the point where thermal pressure again becomes dominant, and the material then expands and cools.

Core helium flash

For a star with a mass less than 2.25 solar masses, the core helium flash occurs when the core runs out of hydrogen, and the thermal pressure is no longer sufficient to counter the gravitational collapse. This causes the star to start contracting. During the contraction the core becomes hotter and hotter until it causes the outer layers to expand outwards initiating the red giant stage. As the star continues contracting due to gravity, it eventually becomes compressed enough that it becomes degenerate matter. This degeneracy pressure is finally sufficient to stop further collapse of the most central material. As the rest of the core continues to contract and the temperature continues to rise, a temperature (~100×10⁶ K) is reached at which the helium can start to fuse, and so helium ignition occurs.

The explosive nature of the helium flash arises from its taking place in degenerate matter. When degeneracy pressure (which is purely a function of density) dominates thermal pressure (proportional to the product of density and temperature), the total pressure is only weakly dependent on temperature. Thus, once the temperature reaches 100 million–200 million kelvins and helium fusion begins using the triple-alpha process, because degenerate matter is a good conductor of heat, the temperature rapidly increases, further raising the helium fusion rate and expanding the reaction region. However, the volume does not increase and pressure does not decrease, 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 increased temperature again renders thermal pressure dominant, eliminating the degeneracy. The core can then expand and cool down and a stable burning of helium will continue.^[1]

Stars with mass greater than about 2.25 solar masses start to burn helium without their core becoming degenerate and so do not exhibit this type of helium flash. For very low mass stars with mass less than about 0.5 solar mass, their cores are 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,^[2] but later star modeling taking neutrino energy loss into account indicates no such mass loss.^[3][4]

Helium flash on binary white dwarfs

When hydrogen gas is accreted onto a white dwarf from a binary companion star, the hydrogen usually fuses to form helium. This helium can build up to form a shell near the surface of the star. When the mass of helium becomes sufficiently large, a helium flash can occur, with runaway fusion causing a nova.

Shell helium flash

Shell helium flashes are a similar helium ignition event, although not necessarily dependent on 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 continues to burn in a thin shell around this core. The shell of helium is not large enough to raise the material above it, and so cannot expand. Thus there is no expansion related cooling of the burning shell, so the temperature rapidly rises. This leads to a thermal pulse, rapidly releasing the energy built and allowing s-process reactions to occur. This pulse may last a few hundred years and are thought to occur periodically every 10,000 to 100,000 years.^[5] Thermal pulses may cause a star to shed circumstellar shells of gas and dust.

See also

• Carbon detonation

References

1. ^ 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. 
2. ^ Two- and three-dimensional numerical simulations of the core helium flash by Deupree, R. G.
3. ^ A Reexamination of the Core Helium Flash by Deupree, R. G.
4. ^ Multidimensional hydrodynamic simulations of the core helium flash in low-mass stars by Mocák, M.
5. ^ 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.