Pair-instability supernova
Encyclopedia
Pair production
Pair production refers to the creation of an elementary particle and its antiparticle, usually from a photon . For example an electron and its antiparticle, the positron, may be created...
, the production of free electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s and positron
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...
s in the collision between atomic nuclei and energetic gamma rays, reduces thermal pressure inside a supermassive star
Hypergiant
A hypergiant is a star with a tremendous mass and luminosity, showing signs of a very high rate of mass loss.-Characteristics:...
's core. This pressure drop leads to a partial collapse, then greatly accelerated burning in a runaway
Thermal runaway
Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result...
thermonuclear explosion which blows the star completely apart without leaving a black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
remnant behind. Pair-instability supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
e can only happen in stars with a mass range from around 130 to 250 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 low to moderate metallicity
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
(low abundance of elements other than hydrogen and helium, a situation common in Population III stars). The recently observed objects SN 2006gy
SN 2006gy
SN 2006gy was an extremely energetic supernova, sometimes referred to as a hypernova or quark-nova, that was discovered on September 18, 2006. It was first observed by Robert Quimby and P. Mondol, and then studied by several teams of astronomers using facilities that included the Chandra, Lick, and...
and SN 2007bi
SN 2007bi
SN 2007bi was an extremely energetic supernova discovered early in 2007 by the international Nearby Supernova Factory based at the U.S. Department of Energy's Lawrence Berkeley National Laboratory. The precursor star is estimated to have had 200 solar masses at the time of its formation and around...
are hypothesized to have been pair-instability supernovae.
Photon pressure
In very large hot stars, pressure from fusion reaction gamma rays in the stellar core keeps the upper layers of the star supported against gravitational pull from the core. If the stream of gamma rays is reduced, then the outer layers of the star will start to be pulled inwards in a gravitational collapse.Pair creation
Pair creation results from gamma-atomic nucleus reactions interacting via the coulomb force (see Pair production by gamma raysPair production
Pair production refers to the creation of an elementary particle and its antiparticle, usually from a photon . For example an electron and its antiparticle, the positron, may be created...
). The pair creation cross section for a given material is strongly dependent on the energy of the gamma ray photon – as the gamma rays get more energetic, they are more likely to interact with the atoms they pass through. From Einstein's equation
, gamma rays must have more energy than the mass of the electron-positron pairs to produce these pairs.As described in the introduction, the results of pair creation interactions are pairs of electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s and positron
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...
s. These particles are released into the star's core and usually recombine (releasing another gamma ray) in very short time periods.
Even though the energy is typically re-released rapidly by the recombination of the electron and positron, the speed at which energy (radiation) transfers through a gas is highly dependent on the average distance between interactions. A photon that is absorbed in pair creation interactions effectively is stopped, and then re-radiated in a random direction.
Gamma ray production
Gamma rays are produced directly by some of the thermonuclear reactions in stars, and emitted as part of the black body spectrum light emission from the hot gas in the stellar core. Total energy emitted by a material is proportional to the fourth power of the temperature (Stefan-Boltzmann law), and its peak wavelength decreases with temperature as well (Wien's displacement lawWien's displacement law
Wien's displacement law states that the wavelength distribution of thermal radiation from a black body at any temperature has essentially the same shape as the distribution at any other temperature, except that each wavelength is displaced on the graph...
). The hotter the material, the brighter it is, and the more high energy photons (gamma rays) will be produced.
Gamma ray absorption
The average distance that gamma rays can travel through matter before they are absorbed (optical thickness) depends on the material (hydrogen has a low cross section; metals much more) and energy of the gamma ray. At low energy, the photoelectric effectPhotoelectric effect
In the photoelectric effect, electrons are emitted from matter as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as photoelectrons...
and Compton scattering
Compton scattering
In physics, Compton scattering is a type of scattering that X-rays and gamma rays undergo in matter. The inelastic scattering of photons in matter results in a decrease in energy of an X-ray or gamma ray photon, called the Compton effect...
dominate. As gamma rays get more energetic, the photoelectric and Compton effects are reduced, and the gammas can travel farther on the average. Eventually, as gamma energy increases, pair production starts to become significant.
Pair-instability
As described above, the hotter a star's core becomes, the higher the energy of the gamma rays it produces. Once these reach gamma energies where pair production starts to become the dominant mechanism in gamma ray capture in the gas, then the distance that gamma rays travel in the gas starts to decrease instead of increasing. This decrease in the distance that gamma rays travel is an instabilityInstability
In numerous fields of study, the component of instability within a system is generally characterized by some of the outputs or internal states growing without bounds...
, and causes a feedback loop: as gamma travel distance decreases, the temperature at the core increases, and this increases the gamma energy and further decreases the distance that gammas can travel.
Stellar susceptibility
Stars which are rotating fast enough, or which have enough metallicityMetallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
, probably do not collapse in pair-instability supernova due to other effects. Pair-instability happens in low metallicity stars, with low to moderate rotation rates. Stars formed by collision mergers having a metallicity Z between 0.02 and 0.001 may end their lives as pair-instability supernovae if their mass is in the appropriate range.
Very large high metallicity stars are probably unstable due to the Eddington limit, and would tend to shed mass during the formation process.
Stellar behavior
Several sources describe the stellar behavior for large stars in pair-instability conditions.Below 100 solar masses
For lower-mass stars (up to about 100 solar masses) the gamma rays are not energetic enough to produce electron-positron pairs. If a supernovaSupernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
destroys such a star, pair production will not be involved.
100 to 130 solar masses
For stars between 100 and around 130 solar masses, pressure and temperature effects allow larger partial collapses and pressure pulses to occur, initiated by pair production instability in the core, which are too small to fully disrupt the star. These pulses are damped out; they create temporary increased rates of thermonuclear burning, but the star gradually returns to a stable equilibrium. These pulses are expected to lead to ejections of parts of the outer layers of the star, similarly to what happened to the star Eta Carinae in 1843, though that may have had a different underlying mechanism. The pulsing mechanism is thought to cause stars in this mass range to shed mass until their remaining core is small enough to collapse in a normal supernova.130 to 250 solar masses
For very high mass stars, with mass at least 130 and up to perhaps roughly 250 solar masses, a true pair-instability supernova can occur. In these stars, the first time that conditions support pair creation instability, the situation runs out of control. The collapse proceeds to efficiently compress the star's core; the overpressure is sufficient to allow runaway nuclear fusion to burn it in a few seconds, creating a thermonuclear explosion. With more thermal energy released than the stars' gravitational binding energyGravitational binding energy
The gravitational binding energy of an object consisting of loose material, held together by gravity alone, is the amount of energy required to pull all of the material apart, to infinity...
, it is completely disrupted; no black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
or other remnant is left behind.
In addition to the immediate energy release, a large fraction of the star's core is transformed to nickel-56
Isotopes of nickel
Naturally occurring nickel is composed of five stable isotopes; , , , and with being the most abundant . 58Ni may decay by double beta-plus decay to 58Fe. 26 radioisotopes have been characterised with the most stable being with a half-life of 76,000 years, with a half-life of 100.1 years,...
, a radioactive isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
which decays with a half-life of 6.1 days into cobalt-56
Isotopes of cobalt
Naturally occurring cobalt is composed of 1 stable isotope, 59Co. 28 radioisotopes have been characterized with the most stable being 60Co with a half-life of 5.2714 years, 57Co with a half-life of 271.79 days, 56Co with a half-life of 77.27 days, and 58Co with a half-life of 70.86 days...
. Cobalt-56
Isotopes of cobalt
Naturally occurring cobalt is composed of 1 stable isotope, 59Co. 28 radioisotopes have been characterized with the most stable being 60Co with a half-life of 5.2714 years, 57Co with a half-life of 271.79 days, 56Co with a half-life of 77.27 days, and 58Co with a half-life of 70.86 days...
has a half-life of 77 days and then further decays to the stable isotope iron-56
Isotopes of iron
Naturally occurring iron consists of four isotopes: 5.845% of 54Fe , 91.754% of 56Fe, 2.119% of 57Fe and 0.282% of 58Fe. There are 24 known radioactive isotopes and their half-lives are shown below...
(see Supernova nucleosynthesis
Supernova nucleosynthesis
Supernova nucleosynthesis is the production of new chemical elements inside supernovae. It occurs primarily due to explosive nucleosynthesis during explosive oxygen burning and silicon burning...
). For the hypernova SN 2006gy
SN 2006gy
SN 2006gy was an extremely energetic supernova, sometimes referred to as a hypernova or quark-nova, that was discovered on September 18, 2006. It was first observed by Robert Quimby and P. Mondol, and then studied by several teams of astronomers using facilities that included the Chandra, Lick, and...
, studies indicate that perhaps 40 solar masses of the original star were released as Ni-56, almost the entire mass of the star's core regions. Collision between the exploding star core and gas it ejected earlier, and radioactive decay, release most of the visible light.
250 solar masses or more
A different reaction mechanism, photodisintegrationPhotodisintegration
Photodisintegration is a physical process in which an extremely high energy gamma ray interacts with an atomic nucleus and causes it to enter an excited state, which immediately decays by emitting a subatomic particle. A single proton or neutron is effectively knocked out of the nucleus by the...
, results after collapse starts in stars of at least 250 solar masses. This endothermic reaction (energy-absorbing) causes the star to continue collapse into a black hole rather than exploding due to thermonuclear reactions.