Deuterium burning
Encyclopedia
Deuterium burning is a nuclear fusion
reaction that occurs in stars and some substellar object
s, in which a deuterium
nucleus
and a proton
combine to form a helium-3
nucleus. It occurs as the second stage of the proton–proton chain reaction, in which a deuterium nucleus formed from two proton
s fuses with a further proton, but can also proceed from primordial
deuterium.
s, and burning in the center of protostars can proceed when temperatures exceed 106 K
. The reaction rate is so sensitive to temperature that the temperature does not rise very much above this. Deuterium burning drives convection, which carries the heat generated to the surface.
If there were no deuterium burning, then there should be no stars with masses more than about two or three times the mass of the Sun
in the pre-main-sequence phase because hydrogen burning would occur while the object was still accreting matter. Deuterium burning prevents this by acting as a thermostat that stops the central temperature rising above about one million degrees, which is not hot enough for hydrogen burning. Only after energy transport switches from convective to radiative, forming a radiative barrier around a deuterium exhausted core, does central deuterium burning stop. Then the central temperature of the protostar can increase.
The matter surrounding the radiative zone is still rich in deuterium and burning proceeds in a shell that gradually moves outwards as the star becomes more and more radiative. The generation of nuclear energy in these low-density
outer regions causes the protostar to swell, delaying the gravitational contraction of the object and postponing its arrival onto the main sequence. The total energy available by deuterium burning is comparable to that released by gravitational contraction.
Due to the scarcity of deuterium in the universe
, a protostar's supply of it is limited. After a few million years it will have effectively been completely consumed.
s, and have masses between about 13 and 80 times the mass of Jupiter
. Brown dwarfs may shine for a hundred million years at most before their deuterium supply is gone.
, tritium
, or (more rarely) helium-4
, or with helium to form various isotope
s of lithium
.
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
reaction that occurs in stars and some substellar object
Substellar object
A substellar object, sometimes called a lump, is an astronomical object whose mass is smaller than the smallest mass, approximately 0.08 solar masses, at which a star can sustain hydrogen fusion...
s, in which a deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
nucleus
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
and a proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
combine to form a helium-3
Helium-3
Helium-3 is a light, non-radioactive isotope of helium with two protons and one neutron. It is rare on Earth, and is sought for use in nuclear fusion research...
nucleus. It occurs as the second stage of the proton–proton chain reaction, in which a deuterium nucleus formed from two proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s fuses with a further proton, but can also proceed from primordial
Big Bang nucleosynthesis
In physical cosmology, Big Bang nucleosynthesis refers to the production of nuclei other than those of H-1 during the early phases of the universe...
deuterium.
In protostars
Deuterium is the most easily fused nucleus available to accreting protostarProtostar
A protostar is a large mass that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. The protostellar phase is an early stage in the process of star formation. For a one solar-mass star it lasts about 100,000 years...
s, and burning in the center of protostars can proceed when temperatures exceed 106 K
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...
. The reaction rate is so sensitive to temperature that the temperature does not rise very much above this. Deuterium burning drives convection, which carries the heat generated to the surface.
If there were no deuterium burning, then there should be no stars with masses more than about two or three times the mass of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
in the pre-main-sequence phase because hydrogen burning would occur while the object was still accreting matter. Deuterium burning prevents this by acting as a thermostat that stops the central temperature rising above about one million degrees, which is not hot enough for hydrogen burning. Only after energy transport switches from convective to radiative, forming a radiative barrier around a deuterium exhausted core, does central deuterium burning stop. Then the central temperature of the protostar can increase.
The matter surrounding the radiative zone is still rich in deuterium and burning proceeds in a shell that gradually moves outwards as the star becomes more and more radiative. The generation of nuclear energy in these low-density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
outer regions causes the protostar to swell, delaying the gravitational contraction of the object and postponing its arrival onto the main sequence. The total energy available by deuterium burning is comparable to that released by gravitational contraction.
Due to the scarcity of deuterium in the universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...
, a protostar's supply of it is limited. After a few million years it will have effectively been completely consumed.
In substellar objects
Since hydrogen burning requires much higher temperatures and pressures than deuterium burning does, there are objects massive enough to burn deuterium but not massive enough to burn hydrogen. These objects are called brown dwarfBrown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...
s, and have masses between about 13 and 80 times the mass of Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
. Brown dwarfs may shine for a hundred million years at most before their deuterium supply is gone.
Other reactions
Though fusion with a proton is the dominant method of consuming deuterium, other reactions are possible. These include fusion with another deuterium nucleus to form helium-3Helium-3
Helium-3 is a light, non-radioactive isotope of helium with two protons and one neutron. It is rare on Earth, and is sought for use in nuclear fusion research...
, tritium
Tritium
Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...
, or (more rarely) helium-4
Helium-4
Helium-4 is a non-radioactive isotope of helium. It is by far the most abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on earth. Its nucleus is the same as an alpha particle, consisting of two protons and two neutrons. Alpha decay of heavy...
, or with helium to form various 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...
s of lithium
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
.