A helium flash is the runaway fusion of helium in the core of low mass stars of less than about 2.25 solar masses (M☉) and greater than about 0.5 M☉, or on the surface of an accreting white dwarf star. 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 it, and there is no regulation of the rate of fusion. The very high density also speeds up the fusion rate. The subsequent runaway nuclear reaction emits energy at a rate comparable to the whole Milky Way, but only for a few seconds. In the case of normal low mass stars, the energy is absorbed by the star and not visible from outside. The process ends when the material is heated to the point where thermal pressure again becomes dominant, and the material then expands and cools. The helium flash, undetected by observation, is described by astrophysical models.
Core helium flash
During the red giant phase of stellar evolution, hydrogen burning ceases in the core as hydrogen is depleted, leaving a helium-rich core. Hydrogen burning continues in the star’s shell, producing helium ash that falls into the core. As the core’s density and temperature rises, helium burning becomes a possibility depending on the star’s mass. For stars of greater than 2.25 M☉, when the temperature reaches ≈1×108 K, helium burning generates enough energy to prevent further contraction of the core. A hydrostatic equilibrium is reached where the heat from helium fusion expands the core, subsequently cooling and reducing the helium burning, thereby regulating the fusion to maintain a sustainable and stable process.
For stars less than 2.25 M☉, the temperature does not reach the level required for helium burning in a regulated manner. Thermal pressure is no longer sufficient to counter the gravitational collapse. This causes the star to start contracting and increasing in temperature. As the core continues contracting due to gravity, it eventually becomes compressed enough that it becomes degenerate matter as in a white dwarf. 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 (≈1×108 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. Once the temperature reaches 100 million–200 million kelvins 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.
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, but later star modeling taking neutrino energy loss into account indicates no such mass loss.
Helium flash on 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). Such pulses may last a few hundred years, and are thought to occur periodically every 10,000 to 100,000 years. After the flash, helium fusion continues at an exponentially decaying rate for about 40% of the cycle as the helium shell is consumed. Thermal pulses may cause a star to shed circumstellar shells of gas and dust.
- nuclear fusion of hydrogen
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