Brief thermal runaway nuclear fusion in the core of low mass stars
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. Subsequent hydrogen shell fusion further increases the mass of the core until it reaches approximately 100 million kelvin, which is hot enough to initiate 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 the pressure 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 which quickly spans the entire core. This produces a flash of very intense helium fusion that lasts only a few minutes,[2] but during that time, produces energy at a rate comparable to the entire Milky Way galaxy.[2]
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. This consumes most of the total energy released by the helium flash,[2] and any left-over energy is absorbed into the star's upper layers. Thus the helium flash is mostly undetectable by 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]
^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.
^ abcdTaylor, David. "The End Of The Sun". Northwestern University. almost all the energy of the flash is absorbed by the titanic weight-lifting necessary to lift the core out of its white-dwarf condition.
A heliumflash is a very brief thermal runaway nuclear fusion of large quantities of helium into carbon through the triple-alpha process in the core of...
less dramatic event in the normal evolution of solar-mass stars, the "heliumflash". Chemical reactions involving thermal runaway are also called thermal...
sufficient helium in its core to ignite it. In stars around the mass of the Sun, this begins at the tip of the red giant branch with a heliumflash from a...
so-called heliumflash. In more-massive stars, the collapsing core will reach 108 K before it is dense enough to be degenerate, so helium fusion will...
brightest nova of this millennium, reaching magnitude 3.3. A helium nova (undergoing a heliumflash) is a proposed category of nova events that lacks hydrogen...
pressure, helium fusion will ignite on a timescale of days in a heliumflash. In the nondegenerate cores of more massive stars, the ignition of helium fusion...
stars originally similar to the Sun which have undergone a heliumflash and are now fusing helium in their cores. Red clump stellar properties vary depending...
up and eventually the helium shell ignites explosively, a process known as a helium shell flash. The power of the shell flash peaks at thousands of times...
as their core hydrogen is depleted and they begin to burn helium in core in a heliumflash; they develop a degenerate carbon-oxygen core later on the...
expands, producing an event called heliumflash. Non-degenerate cores initiate fusion more smoothly, without a flash. The output of this event is absorbed...
An extreme helium star (abbreviated EHe) is a low-mass supergiant that is almost devoid of hydrogen, the most common chemical element of the Universe....
helium core during a red giant phase, undergo a heliumflash before fusing helium on the horizontal branch, evolve along the AGB while burning helium...
but much happens. First, the core (full of degenerate helium) ignites violently in the heliumflash; it is estimated that 6% of the core—itself 40% of the...
processes occur, preventing any supernova explosions. Once the Sun stops fusing helium in its core and ejects its layers in a planetary nebula in about 8 billion...
to do so until it accumulates a large enough degenerate helium core to ignite the heliumflash. It has likely exhausted the hydrogen from its core and...
allow for energy production through thermonuclear fusion of hydrogen into helium. This energy in turn counterbalances the mass of the star pressing inward;...
A helium star is a class O or B star (blue), which has extraordinarily strong helium lines and weaker than normal hydrogen lines, indicating strong stellar...
stars on the red-giant branch (RGB) have an inert helium core and remain on the RGB until a heliumflash moves them to the horizontal branch. However, stars...
the star expands so that the helium fusion ceases, and the hydrogen shell burning restarts. During these shell heliumflashes, the mass loss from the star...
surface. The Sun is composed primarily of the chemical elements hydrogen and helium; they account for 74.9% and 23.8%, respectively, of the mass of the Sun...
dwarf is a substellar object not massive enough to ever fuse hydrogen into helium, but still massive enough to fuse deuterium – less than about 0.08 M☉ and...
later become a main-sequence star at the onset of hydrogen fusion producing helium. The modern picture of protostars, summarized above, was first suggested...