Helium flash

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A helium flash ( English helium flash ) is the explosive fusion of helium in the triple-alpha process (helium burning) . This can happen in the core of stars of medium mass (up to 2.2  solar masses ), on the surface of white dwarfs or as a burning shell for stars on the asymptotic giant branch .

Explosive helium burning

The basis of a helium flash is the degeneration of a helium-rich layer or the core. The quantum mechanical state of degeneration has the consequence that temperature and pressure in a plasma are independent of each other. Therefore, there is no expansion when the temperature rises . Since the nuclear reaction rate of the three-alpha process, a thermonuclear reaction , is highly temperature-dependent, energy production continues to increase. Thermal expansion can only control the burning of helium when the temperature rises to such an extent that the degeneration is reversed.

Helium flash in the core

In stars with less than 2.2 solar masses, a helium flash begins when the nucleus no longer has any hydrogen available for the proton-proton reaction ( hydrogen burning ). The falling energy production leads to a contraction of the star and thus to an increase in the core temperature. As the nucleus contracts, matter degenerates; H. Density and pressure no longer depend on the temperature, the Fermi energy of the degenerate electron gas is higher than the thermal energy .

When the mass of the star is high enough to reach a core temperature of 100 million Kelvin , the helium flame ignites explosively. While the temperature rises sharply, density and pressure remain almost constant due to the temperature-insensitive state of the matter when it degenerates. Because the density remains the same as the temperature rises, the energy production increases and the temperature continues to rise. The result is an energy production of up to 100 billion solar luminosities over a period of a few seconds. This energy is completely absorbed by the shell that surrounds the core. Therefore, observation of the phenomenon by electromagnetic radiation is not possible.

The helium flash ends with the temperature high enough to reverse the degeneration. The core expands and cools down. Stable helium burning now takes place in it. The only way to prove this event would be via neutrinos , which can leave the star almost unhindered due to their small cross-section .

In stars with more than 2.2 solar masses, the helium burn ignites before the core degenerates. Therefore, a helium flash cannot occur in the core of these stars.

Helium flash on the surface of white dwarfs

With some super-soft X-ray sources , mass is transferred from a companion to a white dwarf, where it is converted into helium in a stable hydrogen burn. The helium reaches the surface of the white dwarf through gravitational separation and collects there.

A second type of source of helium are companions that have already lost their hydrogen-rich outer layer of the atmosphere and are now transferring plasma to the white dwarf, which has already been enriched with helium by burning hydrogen .

When the helium ignites on the white dwarf, it should resemble a classic nova , in which explosive hydrogen burning takes place on the surface of the white dwarf. The helium flash on the surface of a white dwarf is so far only a theoretical scenario.

Helium lightning on the asymptotic giant branch

Medium-mass stars develop into red giants on the asymptotic giant branch in a late phase . They consist of a core of oxygen and carbon , created by burning helium, and an extensive atmosphere. In a thin shell around the core, the helium flame ignites periodically every 10,000 to 100,000 years. The bowl is not big enough to lift the layers above, so the temperature continues to rise (see above). The result is a thermal pulse that runs through the atmosphere. The effects are:

  • the formation of heavy elements through S-processes
  • the transport of the heavy elements to the surface by convection
  • the expansion of the star's diameter with a cooling of the star's surface and subsequent contraction.

The following observations are associated with a helium flash on the asymptotic giant branch:

literature

  • Michael F. Bode, Aneurin Evans (Ed.): Classical Novae . 2nd Edition. Cambridge University Press, Cambridge u. a. 2008, ISBN 978-0-521-84330-0 ( Cambridge astrophysics series 43).
  • Harm J. Habing, Hans Olofson: Asymptotic Giant Branch Stars . Springer, Berlin a. a. 2004, ISBN 0-387-00880-2 (Astronomy and astrophysics library).
  • John R. Percy: Understanding Variable Stars . Cambridge University Press, Cambridge u. a. 2007, ISBN 978-0-521-23253-1 .
  • Dina Prialnik: An Introduction to the Theory of Stellar Structure and Evolution . Cambridge University Press, Cambridge u. a. 2000, ISBN 0-521-65937-X .