Stellar nucleosynthesis is the process by which the natural abundances of the
chemical elements within stars change due to nuclear fusion reactions in the
cores and overlying mantles of stars. Stars are said to evolve with changes in
the abundances of the elements within. Core fusion increases the atomic weight
of its gaseous elements, causing pressure loss and contraction
accompanied by increase of temperature. Stars lose most of their mass when it is
ejected late in their stellar lifetimes, thereby increasing the abundance of
elements heavier than helium in the interstellar medium. The term supernova
nucleosynthesis is used to describe the creation of elements during the
evolution and explosion of a presupernova star, as Fred Hoyle
advocated presciently in 1954. One stimulus to the development of the
theory of nucleosynthesis was the variations in the abundances of elements
found in the universe. Those abundances, when plotted on a graph as a function of
atomic number of the element, have a jagged sawtooth shape that varies by
factors of tens of millions. This suggested a natural process other than a
random distribution. Such a graph of the abundances can be seen at History of
nucleosynthesis theory. Stellar nucleosynthesis is the dominating
contributor to several processes that also occur under the collective term
nucleosynthesis. A second stimulus to understanding the
processes of stellar nucleosynthesis occurred during the 20th century, when
it was realized that the energy released from nuclear fusion reactions accounted
for the longevity of the Sun as a source of heat and light. The fusion of nuclei
in a star, starting from its initial hydrogen and helium abundance, provides
that energy and synthesizes new nuclei as a byproduct of that fusion process.
This became clear during the decade prior to World War II. The fusion
product nuclei are restricted to those only slightly heavier than the fusing
nuclei; thus they do not contribute heavily to the natural abundances of the
elements. Nonetheless, this insight raised the plausibility of explaining
all of the natural abundances of elements in this way. The prime energy
producer in the sun is the fusion of hydrogen to form helium, which occurs at
a solar-core temperature of 14 million kelvin.
History In 1920, Arthur Eddington, on the basis
of the precise measurements of atomic masses by F.W. Aston and a preliminary
suggestion by Jean Perrin, proposed that stars obtained their energy from nuclear
fusion of hydrogen to form helium and raised the possibility that the heavier
elements are produced in stars. This was a preliminary step toward the idea of
nucleosynthesis. In 1928, George Gamow derived what is now called the Gamow
factor, a quantum-mechanical formula that gave the probability of bringing
two nuclei sufficiently close for the strong nuclear force to overcome the
Coulomb barrier. The Gamow factor was used in the decade that followed by
Atkinson and Houtermans and later by Gamow himself and Edward Teller to
derive the rate at which nuclear reactions would proceed at the high
temperatures believed to exist in stellar interiors.
In 1939, in a paper entitled "Energy Production in Stars", Hans Bethe
analyzed the different possibilities for reactions by which hydrogen is fused
into helium. He defined two processes that he believed to be the sources of
energy in stars. The first one, the proton–proton chain reaction, is the
dominant energy source in stars with masses up to about the mass of the Sun.
The second process, the carbon-nitrogen-oxygen cycle, which was
also considered by Carl Friedrich von Weizsäcker in 1938, is most important in
more massive stars. These works concerned the energy generation capable
of keeping stars hot. A clear physical description of the p-p chain and of the
CNO cycle appears in a 1968 textbook. Bethe's two papers did not address the
creation of heavier nuclei, however. That theory was begun by Fred Hoyle in
1946 with his argument that a collection of very hot nuclei would assemble into
iron. Hoyle followed that in 1954 with a large paper describing how advanced
fusion stages within stars would synthesize elements between carbon and
iron in mass. This is the dominant work in stellar nucleosynthesis. It provided
the roadmap to how the most abundant elements on earth had been synthesized
from initial hydrogen and helium, making clear how those abundant elements
increased their galactic abundances as the galaxy aged.
Quickly, Hoyle's theory was expanded to other processes, beginning with the
publication of a celebrated review paper in 1957 by Burbidge, Burbidge, Fowler
and Hoyle. This review paper collected and refined earlier research into a
heavily cited picture that gave promise of accounting for the observed relative
abundances of the elements; but it did not itself enlarge Hoyle's 1954 picture
for the origin of primary nuclei as much as many assumed, except in the
understanding of nucleosynthesis of those elements heavier than iron.
Significant improvements were made by Alastair GW Cameron and by Donald D.
Clayton. Cameron presented his own independent approach of nucleosynthesis.
He introduced computers into time-dependent calculations of evolution
of nuclear systems. Clayton calculated the first time-dependent models of the
S-process and of the R-process, as well as of the burning of silicon into the
abundant alpha-particle nuclei and iron-group elements, and discovered
radiogenic chronologies for determining the age of the elements. The entire
research field expanded rapidly in the 1970s.
Key reactions The most important reactions in stellar
nucleosynthesis: Hydrogen fusing:
Deuterium burning The proton-proton chain
The carbon-nitrogen-oxygen cycle Helium burning:
The triple-alpha process The alpha process
Burning of heavier elements: Lithium burning: a process found most
commonly in brown dwarfs Carbon-burning process
Neon-burning process Oxygen-burning process
Silicon-burning process Production of elements heavier than
iron: Neutron capture:
The R-process The S-process
Proton capture: The Rp-process
Photo-disintegration: The P-process
= Hydrogen burning= "Hydrogen burning" is an expression that
astronomers sometimes use for the stellar process that results in the
nuclear fusion of four protons to form a nucleus of helium-4. There are two
predominant processes by which stellar hydrogen burning occurs.
In the cores of lower mass main sequence stars such as the Sun, the dominant
process is the proton-proton chain reaction. This creates a helium-4
nucleus through a sequence of chain reactions that begin with the fusion of
two protons to form a nucleus of deuterium. The subsequent process of
deuterium burning will consume any pre-existing deuterium found at the
core. The pp-chain reaction cycle is relatively insensitive to temperature,
so this hydrogen burning process can occur in up to a third of the star's
radius and occupy half the star's mass. As a result, for stars above 35% of the
Sun's mass, the energy flux toward the surface is sufficiently low that the
core region remains a radiative zone, rather than becoming convective. In each
complete fusion cycle, the p-p chain reaction releases about 26.2 MeV.
In higher mass stars, the dominant process is the CNO cycle, which is a
catalytic cycle that uses nuclei of carbon, nitrogen and oxygen as
intermediaries to produce a helium nucleus. During a complete CNO cycle,
25.0 MeV of energy is released. The difference in energy compared to the p-p
chain reaction is accounted for by the energy lost through neutrino emission.
The CNO cycle is very temperature sensitive, so it is strongly
concentrated at the core. About 90% of the CNO cycle energy generation occurs
within the inner 15% of the star's mass. This results in an intense outward
energy flux that can not be sustained by radiative transfer. As a result, the
core region becomes a convection zone, which stirs the hydrogen burning region
and keeps it well mixed with the surrounding proton-rich region. This
core convection occurs in stars where the CNO cycle contributes more than 20%
of the total energy. As the star ages and the core temperature increases, the
region occupied by the convection zone slowly shrinks from 20% of the mass down
to the inner 8% of the mass. The type of hydrogen burning process
that dominates inside a star is determined by the temperature dependency
differences between the two reactions. The pp-chain reaction starts at
temperatures around 7006400000000000000♠4×106 K, making it
the dominant mechanism in smaller stars. A self-maintaining CNO chain requires a
higher temperature of approximately 7007150000000000000♠15×106 K, but
thereafter it increases more rapidly in efficiency than the pp-chain reaction as
the temperature grows. Above approximately 7007170000000000000♠17×106
K, the CNO cycle becomes the dominant source of energy. This temperature is
achieved in the cores of main sequence stars with at least 1.3 times the mass
of the Sun. The Sun itself has a core temperature of around
7007157000000000000♠15.7×106 K and only 6999800000000000000♠0.8% of the energy
being produced in the Sun comes from the CNO cycle. As a main sequence star ages,
the core temperature will rise, resulting in a steadily increasing
contribution from its CNO cycle. Once a star with about 0.5–10 times the
mass of the Sun has consumed nearly all the hydrogen at its core, it begins to
evolve up the red giant branch. Hydrogen burning occurs in a shell surrounding an
inert helium core until the steadily increasing core temperature exceeds
7008100000000000000♠1×108 K. At that point helium burning begins with a
thermal runaway process called the helium flash with hydrogen burning
continuing in a thin shell surrounding the now active helium core.
References Further reading
Bethe, H. A.. "Energy Production in Stars". Physical Review 55: 103.
Bibcode:1939PhRv...55..103B. doi:10.1103/PhysRev.55.103.
Bethe, H. A.. "Energy Production in Stars". Physical Review 55: 434–456.
Bibcode:1939PhRv...55..434B. doi:10.1103/PhysRev.55.434.
Hoyle, F.. "On Nuclear Reactions occurring in very hot stars: Synthesis
of elements from carbon to nickel". Astrophys. J. 1: 121–146.
Bibcode:1954ApJS....1..121H. doi:10.1086/190005.
Clayton, Donald D.. Principles of Stellar Evolution and Nucleosynthesis.
New York: McGraw-Hill. Ray. "Stars as thermonuclear reactors:
Their fuels and ashes". arXiv:astro-ph/0405568 [astro-ph].
G. Wallerstein; I. Iben Jr.; P. Parker; A.M. Boesgaard; G.M. Hale; A. E.
Champagne; et al.. "Synthesis of the elements in stars: forty years of
progress". Reviews of Modern Physics 69: 995–1084. Bibcode:1997RvMP...69..995W.
doi:10.1103/RevModPhys.69.995. Retrieved 2006-08-04.
Woosley, S. E.; A. Heger; T. A. Weaver. "The evolution and explosion of massive
stars". Reviews of Modern Physics 74: 1015–1071. Bibcode:2002RvMP...74.1015W.
doi:10.1103/RevModPhys.74.1015. Clayton, Donald D.. Handbook of Isotopes
in the Cosmos. Cambridge: Cambridge University Press. ISBN 0-521-82381-1.
External links How the Sun Shines by John N. Bahcall
Nucleosynthesis in NASA's Cosmicopia