Helium flash
Encyclopedia
A helium flash is the runaway fusion
of helium
in the core of low mass star
s of less than about 2.25 solar mass
es 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.
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 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 kelvin
s 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.
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, but later star modeling taking neutrino energy loss into account indicates no such mass loss.
.
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.
Thermal pulses may cause a star to shed circumstellar shells of gas and dust.
Stellar nucleosynthesis
Stellar nucleosynthesis is the collective term for the nuclear reactions taking place in stars to build the nuclei of the elements heavier than hydrogen. Some small quantity of these reactions also occur on the stellar surface under various circumstances...
of helium
Triple-alpha process
The triple alpha process is a set of nuclear fusion reactions by which three helium-4 nuclei are transformed into carbon.Older stars start to accumulate helium produced by the proton–proton chain reaction and the carbon–nitrogen–oxygen cycle in their cores...
in the core of low mass star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s of less than about 2.25 solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es and greater than about 0.5 solar mass, or on the surface of an accreting
Accretion (astrophysics)
In astrophysics, the term accretion is used for at least two distinct processes.The first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc. Accretion discs are common around smaller stars or stellar remnants...
white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
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
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
is degenerate
Degenerate matter
Degenerate matter is matter that has such extraordinarily high density that the dominant contribution to its pressure is attributable to the Pauli exclusion principle. The pressure maintained by a body of degenerate matter is called the degeneracy pressure, and arises because the Pauli principle...
, meaning it is supported against gravity by quantum mechanical
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
pressure rather than thermal pressure
Ideal gas law
The ideal gas law is the equation of state of a hypothetical ideal gas. It is a good approximation to the behavior of many gases under many conditions, although it has several limitations. It was first stated by Émile Clapeyron in 1834 as a combination of Boyle's law and Charles's law...
. 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 starStar
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
with a mass less than 2.25 solar masses, the core helium flash occurs when the core runs out of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
, and the thermal pressure is no longer sufficient to counter the gravitational collapse
Gravitational collapse
Gravitational collapse is the inward fall of a body due to the influence of its own gravity. In any stable body, this gravitational force is counterbalanced by the internal pressure of the body, in the opposite direction to the force of gravity...
. 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
Red giant
A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...
stage. As the star continues contracting due to gravity, it eventually becomes compressed enough that it becomes degenerate matter
Degenerate matter
Degenerate matter is matter that has such extraordinarily high density that the dominant contribution to its pressure is attributable to the Pauli exclusion principle. The pressure maintained by a body of degenerate matter is called the degeneracy pressure, and arises because the Pauli principle...
. 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 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
Degenerate matter
Degenerate matter is matter that has such extraordinarily high density that the dominant contribution to its pressure is attributable to the Pauli exclusion principle. The pressure maintained by a body of degenerate matter is called the degeneracy pressure, and arises because the Pauli principle...
. 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 kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
s and helium fusion
Helium fusion
Helium fusion is a kind of nuclear fusion, with the nuclei involved being helium.The fusion of helium-4 nuclei is known as the triple-alpha process, because fusion of just two helium nuclei only produces beryllium-8, which is unstable and breaks back down to two helium nuclei with a half-life of...
begins using the triple-alpha process
Triple-alpha process
The triple alpha process is a set of nuclear fusion reactions by which three helium-4 nuclei are transformed into carbon.Older stars start to accumulate helium produced by the proton–proton chain reaction and the carbon–nitrogen–oxygen cycle in their cores...
, 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.
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, 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 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 novaNova
A nova is a cataclysmic nuclear explosion in a star caused by the accretion of hydrogen on to the surface of a white dwarf star, which ignites and starts nuclear fusion in a runaway manner...
.
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 BranchAsymptotic Giant Branch
The asymptotic giant branch is the region of the Hertzsprung-Russell diagram populated by evolving low to medium-mass stars. This is a period of stellar evolution undertaken by all low to intermediate mass stars late in their lives....
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
S-process
The S-process or slow-neutron-capture-process is a nucleosynthesis process that occurs at relatively low neutron density and intermediate temperature conditions in stars. Under these conditions the rate of neutron capture by atomic nuclei is slow relative to the rate of radioactive beta-minus decay...
reactions to occur. This pulse may last a few hundred years and are thought to occur periodically every 10,000 to 100,000 years.
Thermal pulses may cause a star to shed circumstellar shells of gas and dust.