# Triple-alpha process

The triple-alpha process is a set of nuclear fusion reactions by which three helium-4 nuclei (alpha particles) are transformed into carbon.[1][2]

Overview of the triple-alpha process.

## Triple-alpha process in stars

Helium accumulates in the core of stars as a result of the proton–proton chain reaction and the carbon–nitrogen–oxygen cycle. Further nuclear fusion reactions of helium with hydrogen or another alpha particle produce lithium-5 and beryllium-8 respectively. Both products are highly unstable and decay, almost instantly, back into smaller nuclei, unless a third alpha particle fuses with a beryllium before that time to produce a stable carbon-12 nucleus.[3]

When a star runs out of hydrogen to fuse in its core, it begins to collapse until the central temperature rises to 108 K,[4] six times hotter than the sun's core. At this temperature and density, alpha particles can fuse fast enough (the half-life of 5Li is 3.7×10−22 s and that of 8Be is 6.7×10−17 s) to produce significant amounts of carbon and restore thermodynamic equilibrium in the core

 42He + 42He → 84Be (−0.0918 MeV) 84Be + 42He → 126C + 2γ (+7.367 MeV)

The net energy release of the process is 7.275 MeV.

As a side effect of the process, some carbon nuclei fuse with additional helium to produce a stable isotope of oxygen and energy:

12
6
C
+ 4
2
He
16
8
O
+
γ
(+7.162 MeV)

See alpha process for more details about this reaction and further steps in the chain of stellar nucleosynthesis.

This creates a situation in which stellar nucleosynthesis produces large amounts of carbon and oxygen but only a small fraction of those elements are converted into neon and heavier elements. Both oxygen and carbon make up the 'ash' of helium-4 burning.

## Primordial carbon

Because the triple-alpha process is unlikely, it normally needs a long time to produce much carbon. One consequence of this is that no significant amount of carbon was produced in the Big Bang because, within minutes after the Big Bang, the temperature fell below the critical point for nuclear fusion.

## Resonances

Ordinarily, the probability of the triple alpha process is extremely small. However, the beryllium-8 ground state has almost exactly the energy of two alpha particles. In the second step, 8Be + 4He has almost exactly the energy of an excited state of 12C. This "resonance" greatly increases the probability that an incoming alpha particle will combine with beryllium-8 to form carbon. The existence of this resonance was predicted by Fred Hoyle before its actual observation, based on the physical necessity for it to exist, in order for carbon to be formed in stars. The prediction and then discovery of this energy resonance and process gave very significant support to Hoyle's hypothesis of stellar nucleosynthesis, which posited that all chemical elements had originally been formed from hydrogen, the true primordial substance. The anthropic principle has been cited to explain the fact that nuclear resonances are sensitively arranged to create large amounts of carbon and oxygen in the universe.[5][6]

## Nucleosynthesis of heavy elements

With further increases of temperature and density, fusion processes produce nuclides only up to nickel-56 (which decays later to iron); heavier elements (those beyond Ni) are created mainly by neutron capture. The slow capture of neutrons, the s-process, produces about half of elements beyond iron. The other half are produced by rapid neutron capture, the r-process, which probably occurs in core-collapse supernovae and neutron star mergers.[7]

## Reaction rate and stellar evolution

The triple-alpha steps are strongly dependent on the temperature and density of the stellar material. The power released by the reaction is approximately proportional to the temperature to the 40th power, and the density squared.[8] In contrast, the proton–proton chain reaction produces energy at a rate proportional to the fourth power of temperature, the CNO cycle at about the 17th power of the temperature, and both are linearly proportional to the density. This strong temperature dependence has consequences for the late stage of stellar evolution, the red giant stage.

For lower mass stars, the helium accumulating in the core is prevented from further collapse only by electron degeneracy pressure. As the temperature rises, increased pressure in the core would normally result in an expansion, reduction of density, and thus reduction in reaction rate. However, due to the high pressure at the center of the star this does not occur and energy production continues unmoderated. As a consequence, the temperature increases, causing an increased reaction rate in a positive feedback cycle that becomes a runaway reaction. This process, known as the helium flash, lasts a matter of seconds but burns 60–80% of the helium in the core. During the core flash, the star's energy production can reach approximately 1011 solar luminosities which is comparable to the luminosity of a whole galaxy,[9] although no effects will be immediately observed at the surface, as it is hidden by the star's overlying layers.

For higher mass stars, carbon collects in the core, displacing the helium to a surrounding shell where helium burning occurs. In this helium shell, the pressures are lower and the mass is not supported by electron degeneracy. Thus, as opposed to the center of the star, the shell is able to expand in response to increased thermal pressure in the helium shell. Expansion cools this layer and slows the reaction, causing the star to contract again. This process continues cyclically, and stars undergoing this process will have periodically variable radius and power production. These stars will also lose material from their outer layers as they expand and contract.

## Discovery

The triple alpha process is highly dependent on carbon-12 and beryllium-8 having resonances with slightly more energy than helium-4, and before 1952, no such energy levels were known for carbon. The astrophysicist Fred Hoyle used the fact that carbon-12 is abundant in the universe as evidence for the existence of a carbon-12 resonance. The only way Hoyle could find that would produce an abundance of both carbon and oxygen is through a triple alpha process with a carbon-12 resonance near 7.68 MeV.[10]

Hoyle went to nuclear physicist William Alfred Fowler's lab at Caltech and said that there had to be a resonance of 7.68 MeV in the carbon-12 nucleus. (There had been reports of an excited state at about 7.5 MeV.[10]) Fred Hoyle's audacity in doing this is remarkable, and initially the nuclear physicists in the lab were skeptical. Finally, a junior physicist, Ward Whaling, fresh from Rice University, who was looking for a project decided to look for the resonance. Fowler gave Whaling permission to use an old Van de Graaff generator that was not being used. Hoyle was back in Cambridge when his prediction was verified a few months later. The nuclear physicists put Hoyle as first author on a paper delivered by Whaling at the Summer meeting of the American Physical Society. A long and fruitful collaboration between Hoyle and Fowler soon followed, with Fowler even coming to Cambridge.[11] By 1952, Fowler had noted the beryllium-8 resonance, and Edwin Salpeter calculated the reaction rate taking this resonance into account.[12][13]

This helped to explain the rate of the process, but the rate calculated by Salpeter seemed too low at the temperatures expected in supernovas.[10] When Fowler's lab discovered a carbon-12 resonance near 7.65 MeV it eliminated the discrepancy between the nuclear theory and the theory of stellar evolution.

The final reaction product lies in a 0+ state (spin 0 and positive parity). Since the Hoyle state was predicted to be either a 0+ or a 2+ state, electron–positron pairs or gamma rays were expected to be seen. However, when experiments were carried out, the gamma emission reaction channel was not observed, and this meant the state must be a 0+ state. This state completely suppresses single gamma emission, since single gamma emission must carry away at least 1 unit of angular momentum. Pair production from an excited 0+ state is possible because their combined spins (0) can couple to a reaction that has a change in angular momentum of 0.[14]

## Improbability and fine-tuning

Carbon is a necessary component of all life that we know. 12C, a stable isotope of carbon, is abundantly produced in stars due to three factors:

1. The decay lifetime of a 8Be nucleus is four orders of magnitude larger than the time for two 4He nuclei (alpha particles) to scatter.[15]
2. An excited state of the 12C nucleus exists a little (0.3193 MeV) above the energy level of 8Be + 4He. This is necessary because the ground state of 12C is 7.3367 MeV below the energy of 8Be + 4He. Therefore, a 8Be nucleus and a 4He nucleus cannot reasonably fuse directly into a ground-state 12C nucleus. The excited Hoyle state of 12C is 7.656 MeV above the ground state of 12C. This allows 8Be and 4He to use the kinetic energy of their collision to fuse into the excited 12C, which can then transition to its stable ground state. According to one calculation, the energy level of this excited state must be between about 7.3 and 7.9 MeV to produce sufficient carbon for life to exist, and must be further "fine-tuned" to between 7.596 MeV and 7.716 MeV in order to produce the abundant level of 12C observed in nature.[16]
3. In the reaction 12C + 4He → 16O there is an excited state of oxygen which, if it were slightly higher, would provide a resonance and speed up the reaction. In that case insufficient carbon would exist in nature; it would almost all have converted to oxygen.[15]

Some scholars argue the 7.656 MeV Hoyle resonance, in particular, is unlikely to be the product of mere chance. Fred Hoyle argued in 1982 that the Hoyle resonance was evidence of a "superintellect";[10] Leonard Susskind in The Cosmic Landscape rejects Hoyle's intelligent design argument.[17] Instead, some scientists believe that different universes, portions of a vast "multiverse", have different fundamental constants:[18] according to this controversial fine-tuning hypothesis, life can only evolve in the minority of universes where the fundamental constants happen to be fine-tuned to support the existence of life. Other scientists reject the hypothesis of the multiverse on account of the lack of independent evidence.[19]

## References

1. ^ Appenzeller; Harwit; Kippenhahn; Strittmatter; Trimble, eds. (1998). Astrophysics Library (3rd ed.). New York: Springer.
2. ^ Carroll, Bradley W. & Ostlie, Dale A. (2007). An Introduction to Modern Stellar Astrophysics. Addison Wesley, San Francisco. ISBN 978-0-8053-0348-3.
3. ^ G. Audia,§, O. Bersillonb, J. Blachotb and A.H. Wapstrac, "Archived copy" (PDF). Archived from the original on 2008-09-23. Retrieved 2010-10-07.CS1 maint: Archived copy as title (link) The NUBASE evaluation of nuclear and decay properties, (2001)
4. ^ Wilson, Robert (1997). "Chapter 11: The Stars- their Birth, Life, and Death". Astronomy through the ages the story of the human attempt to understand the universe. Basingstoke: Taylor & Francis. ISBN 9780203212738.
5. ^ For example John Barrow; Frank Tipler (1986). The Anthropic Cosmological Principle.
6. ^ Fred Hoyle, "The Universe: Past and Present Reflections." Engineering and Science, November, 1981. pp. 8–12
7. ^ Pian, E.; d'Avanzo, P.; Benetti, S.; Branchesi, M.; Brocato, E.; Campana, S.; Cappellaro, E.; Covino, S.; d'Elia, V.; Fynbo, J. P. U.; Getman, F.; Ghirlanda, G.; Ghisellini, G.; Grado, A.; Greco, G.; Hjorth, J.; Kouveliotou, C.; Levan, A.; Limatola, L.; Malesani, D.; Mazzali, P. A.; Melandri, A.; Møller, P.; Nicastro, L.; Palazzi, E.; Piranomonte, S.; Rossi, A.; Salafia, O. S.; Selsing, J.; et al. (2017). "Spectroscopic identification of r-process nucleosynthesis in a double neutron-star merger". Nature. 551 (7678): 67–70. arXiv:1710.05858. doi:10.1038/nature24298. PMID 29094694.
8. ^ Carroll, Bradley W.; Ostlie, Dale A. (2006). An Introduction to Modern Astrophysics (2nd ed.). Addison-Wesley, San Francisco. pp. 312–313. ISBN 978-0-8053-0402-2.
9. ^ Carroll, Bradley W.; Ostlie, Dale A. (2006). An Introduction to Modern Astrophysics (2nd ed.). Addison-Wesley, San Francisco. pp. 461–462. ISBN 978-0-8053-0402-2.
10. ^ a b c d Kragh, Helge (2010) When is a prediction anthropic? Fred Hoyle and the 7.65 MeV carbon resonance. http://philsci-archive.pitt.edu/5332/
11. ^ Fred Hoyle, A Life in Science, Simon Mitton, Cambridge University Press, 2011, pages 205–209.
12. ^ Salpeter, E. E. (1952). "Nuclear Reactions in Stars Without Hydrogen". The Astrophysical Journal. 115: 326–328. Bibcode:1952ApJ...115..326S. doi:10.1086/145546.
13. ^ Salpeter, E. E. (2002). "A Generalist Looks Back". Annu. Rev. Astron. Astrophys. 40: 1–25. Bibcode:2002ARA&A..40....1S. doi:10.1146/annurev.astro.40.060401.093901.
14. ^ Cook, CW; Fowler, W.; Lauritsen, C.; Lauritsen, T. (1957). "12B, 12C, and the Red Giants". Physical Review. 107 (2): 508–515. Bibcode:1957PhRv..107..508C. doi:10.1103/PhysRev.107.508.
15. ^ a b Uzan, Jean-Philippe (April 2003). "The fundamental constants and their variation: observational and theoretical status". Reviews of Modern Physics. 75 (2): 403–455. arXiv:hep-ph/0205340. Bibcode:2003RvMP...75..403U. doi:10.1103/RevModPhys.75.403.
16. ^ Livio, M.; Hollowell, D.; Weiss, A.; Truran, J. W. (27 July 1989). "The anthropic significance of the existence of an excited state of 12C". Nature. 340 (6231): 281–284. Bibcode:1989Natur.340..281L. doi:10.1038/340281a0.
17. ^ Peacock, John (2006). "A Universe Tuned for Life". American Scientist. 94 (2): 168–170. JSTOR 27858743.
18. ^ "Stars burning strangely make life in the multiverse more likely". New Scientist. 1 September 2016. Retrieved 15 January 2017.
19. ^ Barnes, Luke A. "The fine-tuning of the universe for intelligent life." Publications of the Astronomical Society of Australia 29.4 (2012): 529–564.
Alpha process

The alpha process, also known as the alpha ladder, is one of two classes of nuclear fusion reactions by which stars convert helium into heavier elements, the other being the triple-alpha process. The triple-alpha process consumes only helium, and produces carbon. After enough carbon has accumulated, the reactions below take place, all consuming only helium and the product of the previous reaction.

${\displaystyle {\begin{array}{ll}{\ce {_6^{12}C + _2^4He -> _{8}^{16}O + \gamma}}&E=7.16\ \mathrm {MeV} \\{\ce {_8^{16}O + _2^4He -> _{10}^{20}Ne + \gamma}}&E=4.73\ \mathrm {MeV} \\{\ce {_{10}^{20}Ne + _2^4He -> _{12}^{24}Mg + \gamma}}&E=9.32\ \mathrm {MeV} \\{\ce {_{12}^{24}Mg + _2^4He -> _{14}^{28}Si + \gamma}}&E=9.98\ \mathrm {MeV} \\{\ce {_{14}^{28}Si + _2^4He -> _{16}^{32}S + \gamma}}&E=6.95\ \mathrm {MeV} \\{\ce {_{16}^{32}S + _2^4He -> _{18}^{36}Ar + \gamma}}&E=6.64\ \mathrm {MeV} \\{\ce {_{18}^{36}Ar + _2^4He -> _{20}^{40}Ca + \gamma}}&E=7.04\ \mathrm {MeV} \\{\ce {_{20}^{40}Ca + _2^4He -> _{22}^{44}Ti + \gamma}}&E=5.13\ \mathrm {MeV} \\{\ce {_{22}^{44}Ti + _2^4He -> _{24}^{48}Cr + \gamma}}&E=7.70\ \mathrm {MeV} \\{\ce {_{24}^{48}Cr + _2^4He -> _{26}^{52}Fe + \gamma}}&E=7.94\ \mathrm {MeV} \\{\ce {_{26}^{52}Fe + _2^4He -> _{28}^{56}Ni + \gamma}}&E=8.00\ \mathrm {MeV} \end{array}}}$

E  is the energy produced by the reaction, released primarily as gamma rays (γ).

It is a common misconception that the above sequence ends at ${\displaystyle \mathrm {_{28}^{56}Ni} }$ (or ${\displaystyle \mathrm {_{26}^{56}Fe} }$, which is a decay product of ${\displaystyle \mathrm {_{28}^{56}Ni} }$) because it is the most stable element - i.e., it has the highest nuclear binding energy per nucleon, and production of heavier nuclei requires energy (is endothermic) instead of releasing it (exothermic). 28 62 N i {\displaystyle \mathrm {_{28}^{62}Ni} } (Nickel-62) is actually the most stable element. However, the sequence ends at ${\displaystyle \mathrm {_{28}^{56}Ni} }$ because conditions in the stellar interior cause the competition between photodisintegration and the alpha process to favor photodisintegration around iron, leading to more ${\displaystyle \mathrm {_{28}^{56}Ni} }$ being produced than ${\displaystyle \mathrm {_{28}^{62}Ni} }$.

All these reactions have a very low rate at the temperatures and densities in stars and therefore do not contribute significantly to a star's energy production; with elements heavier than neon (atomic number > 10), they occur even less easily due to the increasing Coulomb barrier.

Alpha process elements (or alpha elements) are so-called since their most abundant isotopes are integer multiples of four, the mass of the helium nucleus (the alpha particle); these isotopes are known as alpha nuclides. Stable alpha elements are: C, O, Ne, Mg, Si, S, Ar, Ca. They are synthesized by alpha capture prior to the silicon fusing process, a precursor to Type II supernovae. Silicon and calcium are purely alpha process elements. Magnesium can be burned by proton capture reactions. As for oxygen, some authors[which?] consider it an alpha element, while others do not. Oxygen is surely an alpha element in low-metallicity population II stars. It is produced in Type II supernovas and its enhancement is well correlated with an enhancement of other alpha process elements. Sometimes carbon and nitrogen are considered alpha process elements, since they are synthesized in nuclear alpha-capture reactions.

The abundance of alpha elements in stars is usually expressed in a logarithmic manner:

${\displaystyle [\alpha /{\ce {Fe}}]=\log _{10}{\left({\frac {N_{\alpha }}{N_{{\ce {Fe}}}}}\right)_{Star}}-\log _{10}{\left({\frac {N_{\alpha }}{N_{{\ce {Fe}}}}}\right)_{Sun}}}$,

Here ${\displaystyle N_{\alpha }}$ and ${\displaystyle N_{{\ce {Fe}}}}$ are the number of alpha elements and iron nuclei per unit volume. Theoretical galactic evolution models predict that early in the universe there were more alpha elements relative to iron. Type II supernovae mainly synthesize oxygen and the alpha-elements (Ne, Mg, Si, S, Ar, Ca and Ti) while Type Ia supernovae mainly produce elements of the iron peak (Ti, V, Cr, Mn, Fe, Co and Ni) but also alpha-elements.

Attosecond

An attosecond is 1×10−18 of a second (one quintillionth of a second). For context, an attosecond is to a second what a second is to about 31.71 billion years.The word "attosecond" is formed by the prefix atto and the unit second. Atto- was derived from the Danish word for eighteen (atten). Its symbol is as.

An attosecond is equal to 1000 zeptoseconds, or ​1⁄1000 of a femtosecond. Because the next higher SI unit for time is the femtosecond (10−15 seconds), durations of 10−17 s and 10−16 s will typically be expressed as tens or hundreds of attoseconds:

Times which can be expressed in attoseconds:

1 attosecond: the time it takes for light to travel the length of two hydrogen atoms

24 attoseconds: the atomic unit of time

43 attoseconds: the shortest pulses of laser light yet created

53 attoseconds: the second-shortest pulses of laser light created

84 attoseconds: the approximate half-life of a neutral pion

100 attoseconds: fastest-ever view of molecular motion

200 attoseconds (approximately): half-life of beryllium-8, maximum time available for the triple-alpha process for the synthesis of carbon and heavier elements in stars

320 attoseconds: estimated time it takes electrons to transfer between atoms

BPM 37093

BPM 37093 (V886 Centauri) is a variable white dwarf star of the DAV, or ZZ Ceti, type, with a hydrogen atmosphere and an unusually high mass of approximately 1.1 times the Sun's. It is about 50 light-years from Earth, in the constellation Centaurus, and vibrates; these pulsations cause its luminosity to vary. Like other white dwarfs, BPM 37093 is thought to be composed primarily of carbon and oxygen, which are created by thermonuclear fusion of helium nuclei in the triple-alpha process.

Beryllium-8

Beryllium-8 is an isotope of beryllium with 4 neutrons and 4 protons, and four electrons when its oxidation state is 0. It is one of the radionuclides.

Carbon-12

Carbon-12 (12C) is the more abundant of the two stable isotopes of carbon (carbon-13 being the other), amounting to 98.93% of the element carbon; its abundance is due to the triple-alpha process by which it is created in stars. Carbon-12 is of particular importance in its use as the standard from which atomic masses of all nuclides are measured, thus, its atomic mass is exactly 12 daltons by definition. Carbon-12 is composed of 6 protons, 6 neutrons, and 6 electrons.

Carbon-burning process

The carbon-burning process or carbon fusion is a set of nuclear fusion reactions that take place in the cores of massive stars (at least 8 M ⊙ {\displaystyle {\begin{smallmatrix}M_{\odot }\end{smallmatrix}}} at birth) that combines carbon into other elements. It requires high temperatures (> 5×108 K or 50 keV) and densities (> 3×109 kg/m3).

These figures for temperature and density are only a guide. More massive stars burn their nuclear fuel more quickly, since they have to offset greater gravitational forces to stay in (approximate) hydrostatic equilibrium. That generally means higher temperatures, although lower densities, than for less massive stars. To get the right figures for a particular mass, and a particular stage of evolution, it is necessary to use a numerical stellar model computed with computer algorithms. Such models are continually being refined based on nuclear physics experiments (which measure nuclear reaction rates) and astronomical observations (which include direct observation of mass loss, detection of nuclear products from spectrum observations after convection zones develop from the surface to fusion-burning regions – known as dredge-up events – and so bring nuclear products to the surface, and many other observations relevant to models).

HD 149382

HD 149382 is a star in the constellation of Ophiuchus with an apparent visual magnitude of 8.943. This is too faint to be seen with the naked eye even under ideal conditions, although it can be viewed with a small telescope. Based upon parallax measurements, this star is located at a distance of about 240 light-years (74 parsecs) from the Earth.

This is the brightest known B-type subdwarf star with a stellar classification of B5 VI. It is generating energy through the thermonuclear fusion of helium at its core (triple-alpha process). The effective temperature of the star's outer envelope is about 35,500 K, giving it the characteristic blue-white hue of a B-type star. HD 149382 has a visual companion located at an angular separation of 1 arcsecond.In 2009, a substellar companion, perhaps even a superjovian planet, was announced orbiting the star. This candidate object was estimated to have nearly half the mass of the Sun. In 2011, this discovery was thrown into doubt when an independent team of astronomers were unable to confirm the detection. Their observations rule out a companion with a mass greater than Jupiter orbiting with a period of less than 28 days.

Helium flash

A helium flash is a very brief thermal runaway nuclear fusion of large quantities of helium into carbon through the triple-alpha process in the core of low mass stars (between 0.8 solar masses (M☉) and 2.0 M☉) during their red giant phase (the Sun is predicted to experience a flash 1.2 billion years after it leaves the main sequence). A much rarer runaway helium fusion process can also occur on the surface of accreting white dwarf stars.

Low mass stars do not produce enough gravitational pressure to initiate normal helium fusion. As the hydrogen in the core is exhausted, some of the helium left behind is instead compacted into degenerate matter, supported against gravitational collapse by quantum mechanical pressure rather than thermal pressure. This increases the density and temperature of the core until it reaches approximately 100 million kelvin, which is hot enough to cause helium fusion (or "helium burning") in the core.

However, a fundamental quality of degenerate matter is that changes in temperature do not produce a change of volume of the matter until the thermal pressure becomes so very high that it exceeds degeneracy pressure. In main sequence stars, thermal expansion regulates the core temperature, but in degenerate cores this does not occur. Helium fusion increases the temperature, which increases the fusion rate, which further increases the temperature in a runaway reaction. This produces a flash of very intense helium fusion that lasts only a few minutes, but briefly emits energy at a rate comparable to the entire Milky Way galaxy.

In the case of normal low mass stars, the vast energy release causes much of the core to come out of degeneracy, allowing it to thermally expand, however, consuming as much energy as the total energy released by the helium flash, and any left-over energy is absorbed into the star's upper layers. Thus the helium flash is mostly undetectable to observation, and is described solely by astrophysical models. After the core's expansion and cooling, the star's surface rapidly cools and contracts in as little as 10,000 years until it is roughly 2% of its former radius and luminosity. It is estimated that the electron-degenerate helium core weighs about 40% of the star mass and that 6% of the core is converted into carbon.

Horizontal branch

The horizontal branch (HB) is a stage of stellar evolution that immediately follows the red giant branch in stars whose masses are similar to the Sun's. Horizontal-branch stars are powered by helium fusion in the core (via the triple-alpha process) and by hydrogen fusion (via the CNO cycle) in a shell surrounding the core. The onset of core helium fusion at the tip of the red giant branch causes substantial changes in stellar structure, resulting in an overall reduction in luminosity, some contraction of the stellar envelope, and the surface reaching higher temperatures.

Infrared dark cloud

An infrared dark cloud (IRDC) is a cold, dense region of a giant molecular cloud. They can be seen in silhouette against the bright diffuse mid-infrared emission from the galactic plane.

A lead star is a low-metallicity star with an overabundance of lead and bismuth as compared to other products of the S-process.

Nuclear reaction

In nuclear physics and nuclear chemistry, a nuclear reaction is semantically considered to be the process in which two nuclei, or else a nucleus of an atom and a subatomic particle (such as a proton, neutron, or high energy electron) from outside the atom, collide to produce one or more nuclides that are different from the nuclide(s) that began the process. Thus, a nuclear reaction must cause a transformation of at least one nuclide to another. If a nucleus interacts with another nucleus or particle and they then separate without changing the nature of any nuclide, the process is simply referred to as a type of nuclear scattering, rather than a nuclear reaction.

In principle, a reaction can involve more than two particles colliding, but because the probability of three or more nuclei to meet at the same time at the same place is much less than for two nuclei, such an event is exceptionally rare (see triple alpha process for an example very close to a three-body nuclear reaction). "Nuclear reaction" is a term implying an induced changing in a nuclide, and thus it does not apply to any type of radioactive decay (which by definition is a spontaneous process).Natural nuclear reactions occur in the interaction between cosmic rays and matter, and nuclear reactions can be employed artificially to obtain nuclear energy, at an adjustable rate, on demand. Perhaps the most notable nuclear reactions are the nuclear chain reactions in fissionable materials that produce induced nuclear fission, and the various nuclear fusion reactions of light elements that power the energy production of the Sun and stars.

Oxygen-16

Oxygen-16 (16O) is a stable isotope of oxygen, having 8 neutrons and 8 protons in its nucleus. It has a mass of 15.99491461956 u. Oxygen-16 is the most abundant isotope of oxygen and accounts for 99.762% of oxygen's natural abundance. The relative and absolute abundance of 16O are high because it is a principal product of stellar evolution and because it is a primordial isotope, meaning it can be made by stars that were initially made exclusively of hydrogen. Most 16O is synthesized at the end of the helium fusion process in stars; the triple-alpha process creates 12C, which captures an additional 4He to make 16O. The neon-burning process creates additional 16O.

Photometric-standard star

Photometric-standard stars are a series of stars that have had their light output in various passbands of photometric system measured very carefully. Other objects can be observed using CCD cameras or photoelectric photometers connected to a telescope, and the flux, or amount of light received, can be compared to a photometric-standard star to determine the exact brightness, or stellar magnitude, of the object.A current set of photometric-standard stars for UBVRI photometry was published by Arlo U. Landolt in 1992 in the Astronomical Journal.

Q star

A Q-Star, also known as a grey hole, is a hypothetical type of a compact, heavy neutron star with an exotic state of matter. The Q stands for a conserved particle number. A Q-Star may be mistaken for a stellar black hole.

Red giant

A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses (M☉)) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around 5,000 K (4,700 °C; 8,500 °F) or lower. The appearance of the red giant is from yellow-orange to red, including the spectral types K and M, but also class S stars and most carbon stars.

The most common red giants are stars on the red-giant branch (RGB) that are still fusing hydrogen into helium in a shell surrounding an inert helium core. Other red giants are the red-clump stars in the cool half of the horizontal branch, fusing helium into carbon in their cores via the triple-alpha process; and the asymptotic-giant-branch (AGB) stars with a helium burning shell outside a degenerate carbon–oxygen core, and a hydrogen burning shell just beyond that.

Starfield (astronomy)

A starfield refers to a set of stars visible in an arbitrarily-sized field of view, usually in the context of some region of interest within the celestial sphere. For example: the starfield surrounding the stars Betelgeuse and Rigel could be defined as encompassing some or all of the Orion constellation.

Tip of the red-giant branch

Tip of the red-giant branch (TRGB) is a primary distance indicator used in astronomy. It uses the luminosity of the brightest red-giant-branch stars in a galaxy as a standard candle to gauge the distance to that galaxy. It has been used in conjunction with observations from the Hubble Space Telescope to determine the relative motions of the Local Cluster of galaxies within the Local Supercluster. Ground-based, 8 meter class telescopes like the VLT are also able to measure the TRGB distance within reasonable observation times in the local universe.

The Hertzsprung–Russell diagram (HR diagram) is a plot of stellar luminosity versus surface temperature for a population of stars. During the core hydrogen burning phase of a Sun-like star's lifetime, it will appear on the HR diagram at a position along a diagonal band called the main sequence. When the hydrogen at the core is exhausted, energy will continue to be generated by hydrogen fusion in a shell around the core. The center of the star will accumulate the helium "ash" from this fusion and the star will migrate along an evolutionary branch of the HR diagram that leads toward the upper right. That is, the surface temperature will decrease and the total energy output (luminosity) of the star will increase as the surface area increases.At a certain point, the helium at the core of the star will reach a pressure and temperature where it can begin to undergo nuclear fusion through the triple-alpha process. For a star with less than 1.8 times the mass of the Sun, this will occur in a process called the helium flash. The evolutionary track of the star will then carry it toward the left of the HR diagram as the surface temperature increases under the new equilibrium. The result is a sharp discontinuity in the evolutionary track of the star on the HR diagram. This discontinuity is called the tip of the red-giant branch.

When distant stars at the TRGB are measured in the I-band (in the infrared), their luminosity is somewhat insensitive to their composition of elements heavier than helium (metallicity) or their mass; they are a standard candle with an I-band absolute magnitude of –4.0±0.1. This makes the technique especially useful as a distance indicator. The TRGB indicator uses stars in the old stellar populations (Population II).

Yellow giant

A yellow giant is a luminous giant star of low or intermediate mass (roughly 0.5–11 solar masses (M)) in a late phase of its stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature as low as 5,200-7500 K. The appearance of the yellow giant is from white to yellow, including the spectral types F and G. About 10.6 percent of all giant stars are yellow giants.

decay
Stellar
nucleosynthesis
Other
processes

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