helium flash
The onset of runaway helium burning in
the core of a low-mass star (such as the Sun). The helium flash happens
in the hydrogen-exhausted core of a star that has become a red
giant. When gravitational pressure has raised the temperature of the
dormant helium core to a temperature of about 100 million K, the helium
nuclei start to undergo thermonuclear reactions. Once the helium burning
has started, the temperature climbs rapidly (without a cooling, stabilizing
expansion), and the extreme sensitivity of the nuclear reaction rate to
temperature causes the helium-burning process to accelerate. This in turn
raises the temperature, which further accelerates the helium burning, until
a point is reached at which the thermal pressure expands the core and thereby
limits the flash. The helium flash can only occur when the helium core is
less than the 1.4-solar-mass Chandrasekhar
limit and thus it is restricted to fairly low-mass stars.
Related entry
• helium shell
flash
Related category
• STELLAR
ASTROPHYSICS
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