Reaction atlas
A history of nuclear astrophysics
How we learned what powers the stars: the ideas, the measurements, the observations that tested them, and the laboratories that made the measurements possible.
1920s
Aug 1920
Eddington proposes that stars burn hydrogen into helium
Theory
In an address to the British Association in Cardiff, Arthur Eddington noted that a helium nucleus weighs less than four hydrogen nuclei. He argued that the mass lost when hydrogen turns into helium inside stars would be released as energy, enough to keep them shining for a very long time. How this could happen was not yet known.
1928
Gamow explains how particles cross the Coulomb barrier
Theory
George Gamow used the new quantum mechanics to explain alpha decay. A particle can pass through the electric repulsion between nuclei, the Coulomb barrier, even when its energy is too low to climb over it. This effect, called tunnelling, is what makes nuclear fusion possible in stars.
1929
Atkinson and Houtermans apply tunnelling to the stars
Theory
Robert Atkinson and Fritz Houtermans used Gamow's result to estimate how often light nuclei fuse at the temperatures inside stars. They found that tunnelling makes such reactions frequent enough to power a star. Their paper is often taken as the start of nuclear astrophysics.
1930s
1938
Von Weizsäcker and the carbon–nitrogen cycle
Theory
In two papers in Physikalische Zeitschrift, Carl Friedrich von Weizsäcker discussed which nuclear reactions could power stars. The second paper proposed a cycle in which carbon and nitrogen nuclei help turn hydrogen into helium and are given back at the end. His notes and letters show that after he turned to fission and war research in 1939 he kept thinking about neutron stars and cosmology.
Aug 1938
Bethe and Critchfield describe proton–proton fusion
Theory
Hans Bethe and Charles Critchfield calculated the rate at which two protons fuse into a deuteron, emitting a positron and a neutrino. This very slow weak-interaction process is the first step of the proton–proton chain that powers the Sun.
Mar 1939
Bethe works out the CN cycle in “Energy production in stars”
Theory
Hans Bethe worked out the carbon–nitrogen cycle in detail and estimated its rate. He concluded that carbon and nitrogen act as catalysts that fuse protons into helium in stars. Together with the proton–proton chain, this is still the basis of how we understand hydrogen burning.
1940s
1946
Los Alamos finds atmospheric ignition very unlikely
Theory · Experiment
In 1942 Edward Teller had asked whether a fission bomb could start nitrogen burning in the air. Hans Bethe judged the chance very low, and in 1946 Emil Konopinski, Cloyd Marvin and Teller found that, under their assumptions, the energy lost to radiation always exceeds the energy gained from the fusion of two nitrogen-14 nuclei; they still asked for further work. At Notre Dame, secret accelerator experiments of the 1940s studied how efficiently the air cools the fireball of a nuclear explosion. The experience gained fed into post-war nuclear astrophysics.
Apr 1948
Alpher, Bethe and Gamow on the hot early Universe
Theory
Ralph Alpher and George Gamow proposed that the light elements formed in the first moments of a hot, expanding Universe. Gamow added Bethe's name so that the authors read like the first three Greek letters. The idea became Big Bang nucleosynthesis, although heavier elements turned out to be made in stars.
1950s
1952
Salpeter shows how helium burns to carbon
Theory
Edwin Salpeter showed how helium can burn in red giant stars. Two helium nuclei form unstable 8Be, which lives just long enough to capture a third helium nucleus and become carbon. He did this work at Caltech's Kellogg Radiation Laboratory, with William Fowler, in the summer of 1951.
1952
Technetium is found in red giants
Observation
Paul Merrill found lines of technetium in the spectra of red giants of class S. Technetium has no stable isotope, and even its longest-lived ones last only a few million years, far less than the age of these stars. It must have been made inside them: the first direct evidence that stars build elements, by the slow neutron captures of the s-process.
Nov 1953
The 7.68 MeV state of carbon-12 is found
Experiment
At Caltech, Dunbar, Pixley, Wenzel and Whaling found an excited state of 12C at 7.68 ± 0.03 MeV in the reaction 14N(d,α)12C. Fred Hoyle had pointed out that such a state would make the triple-alpha process fast enough to produce the carbon we see. It is now called the Hoyle state.
1954
Hoyle on the synthesis of elements in hot stars
Theory
Fred Hoyle argued that the elements from carbon to nickel are made at the high temperatures reached inside massive stars late in their lives. The paper set out the idea that stars, not the Big Bang, made most of the elements.
1957
Cook, Fowler and the Lauritsens confirm the Hoyle state
Experiment
At the Kellogg Radiation Laboratory, Cook, Fowler, C. C. Lauritsen and T. Lauritsen studied the decay of 12B to the 7.65 MeV state of 12C and its break-up into three alpha particles. They concluded that the state can act as the resonance Hoyle needed for helium burning in red giants.
Jun 1957
Cameron's lectures on nucleogenesis
Theory
Alastair Cameron, at Chalk River in Canada, published an independent account of how the elements are made in stars. Among other results he named the two reactions that supply neutrons for slow neutron capture, 13C(α,n)16O and 22Ne(α,n)25Mg.
Oct 1957
The B2FH paper
Theory
Margaret Burbidge, Geoffrey Burbidge, William Fowler and Fred Hoyle published a 104-page review of how the elements are made in stars. It sorted the processes by the nuclear reactions involved, including slow and rapid neutron capture, and compared them with the measured Solar System abundances. It is known by the authors' initials, B2FH.
1960s
1961
The s-process as a chain of neutron captures
Theory
Donald Clayton, William Fowler, Thomas Hull and Barbara Zimmerman worked out how abundances build up along a long chain of slow neutron captures and compared the result with the observed abundances of the heavy elements, for the slow process that B2FH had named.
1964
Bahcall proposes a solar neutrino experiment with chlorine
Theory
John Bahcall calculated the rate at which solar neutrinos would turn chlorine into argon in a large underground tank. The rate was dominated by the rare high-energy neutrinos from 8B decay, and it was large enough to make the experiment practical. Raymond Davis described the detector in a companion paper in the same journal.
May 1968
Davis detects solar neutrinos, and the solar neutrino problem begins
Observation
Raymond Davis and colleagues counted argon atoms produced by solar neutrinos in a large tank of chlorine-containing liquid in the Homestake mine, South Dakota. In the same issue, John Bahcall and collaborators showed that solar models predicted a rate about 2.5 times higher than Davis's limit. The gap became known as the solar neutrino problem.
1980s
1983
Nobel Prize for Fowler and Chandrasekhar
Experiment
William Fowler shared the Nobel Prize in Physics with Subrahmanyan Chandrasekhar. Fowler's half recognized his experimental and theoretical studies of the nuclear reactions that form the chemical elements, much of it done at the Kellogg Radiation Laboratory at Caltech.
1984
Radioactive 26Al is seen in the Galaxy
Observation
The germanium γ-ray spectrometer on the HEAO 3 satellite detected the 1809 keV line emitted when 26Al decays. It was the first γ-ray line from a radioactive nucleus found in the Galaxy. The flux implied about three solar masses of 26Al in the interstellar medium, all of it made within the last few million years.
1987
Neutrinos from supernova SN 1987A
Observation
A star exploded in the Large Magellanic Cloud, and the Kamiokande-II detector in Japan recorded eleven neutrino events in about 13 seconds. The IMB detector in the United States saw the burst too. It was the first detection of neutrinos from a star other than the Sun.
1987
Stardust from AGB stars in a meteorite
Observation
Grains of silicon carbide less than a micrometre across were found in the Murray meteorite, with isotopic compositions unlike anything formed in the Solar System. They condensed around red giants before the Sun was born. The heavy elements trapped in such grains carry the s-process pattern of a single star, measured atom by atom in the laboratory.
1989
Kamiokande-II sees 8B solar neutrinos
Observation
The Kamiokande-II water detector measured high-energy solar neutrinos from the decay of 8B in real time. Over 450 days it found about half of the flux predicted by the standard solar model, confirming the deficit seen at Homestake with a different technique.
1990s
Dec 1991
SAGE, the first gallium experiment
Observation
The Soviet–American Gallium Experiment at Baksan used about 30 tonnes of gallium, which captures the low-energy neutrinos from proton–proton fusion. Its first result was well below the solar-model prediction.
1992
GALLEX observes pp neutrinos at Gran Sasso
Observation
The GALLEX gallium detector at Gran Sasso measured 83 ± 19 (stat.) ± 8 (syst.) SNU. The collaboration presented this as the first observation of the neutrinos from proton–proton fusion, the reaction that starts the Sun's energy production.
Oct 1994
LUNA, the first underground accelerator
LUNA
A small 50 kV accelerator was installed deep under the Gran Sasso mountain in Italy, where the rock stops most cosmic rays. It had a windowless gas target and was built to measure 3He(3He,2p)4He at the energies where the reaction takes place in the Sun. The facility paper describing it appeared in 1994.
1996
A storage ring shows that bare rhenium-187 decays in years
Experiment
Rhenium-187 lives about 42 billion years as a neutral atom. Fully stripped of its electrons in the GSI storage ring, it decayed with a half-life of 32.9 years, because its electron can then be created in an empty atomic orbit. Inside stars, where atoms are ionized, laboratory half-lives can therefore change completely; the result also affects the rhenium–osmium clock used to date the Galaxy.
1998
Super-Kamiokande finds neutrino oscillations
Observation
Super-Kamiokande found fewer muon neutrinos from the atmosphere than expected, and the deficit depended on how far the neutrinos had travelled through the Earth. The data fit neutrinos changing type, or oscillating, on the way. Oscillation requires neutrinos to have mass.
1998
Solar Fusion I
Theory
A group of experimentalists and theorists reviewed the nuclear reactions that produce the Sun's energy and its neutrinos. They gave recommended low-energy S-factors with uncertainties and named the experiments and calculations needed to improve them.
1999
The NACRE compilation
Theory
The NACRE collaboration compiled the measured cross sections of the charged-particle reactions of stellar hydrogen, helium and carbon burning. It turned them into thermonuclear reaction rates in a common format, which stellar modellers used for many years.
1999
LUNA measures 3He + 3He inside the solar Gamow peak
LUNA
With the 50 kV accelerator, LUNA measured the fusion of two 3He nuclei down to 16.5 keV, an energy at which it occurs in the Sun. No narrow resonance was found. Such a resonance had been suggested as a nuclear explanation of the solar neutrino problem.
2000s
2000
A compilation of stellar neutron-capture cross sections
Experiment
Bao, Beer, Käppeler, Voss, Wisshak and Rauscher surveyed the measured neutron-capture cross sections of the nuclei from hydrogen to bismuth and recommended stellar values at a thermal energy of 30 keV, with calculations where no measurement existed. It became the reference for s-process models, continued today by the KADoNiS database.
2001
Three lead stars are discovered
Observation
Van Eck, Goriely, Jorissen and Plez found three old, iron-poor stars more enriched in lead than in any other element heavier than iron. With fewer iron seeds for the same neutrons, the s-process in such stars runs through to lead, as the models of AGB stars had predicted.
2002
SNO shows that solar neutrinos change flavour
Observation
The Sudbury Neutrino Observatory in Canada used heavy water to measure both the electron neutrinos and the total number of neutrinos of all types from 8B decay in the Sun. The total agreed with solar models, but only about a third arrived as electron neutrinos. The solar neutrino problem was due to neutrino physics, not to the Sun.
Dec 2002
Nobel Prize for Davis and Koshiba
Observation
Raymond Davis and Masatoshi Koshiba received the Nobel Prize in Physics for their work on neutrinos from space. Over 25 years the Homestake experiment captured about 2200 solar neutrinos, one third of the predicted number. Kamiokande confirmed the deficit and detected the neutrinos from SN 1987A.
2003
LUNA's 400 kV accelerator is installed
LUNA
A second, larger accelerator was installed at Gran Sasso. Its proton beam energy is known to about 300 eV, its energy spread is below 100 eV, and it drifts by only about 5 eV per hour. This stability allows the long runs needed to measure very small cross sections.
2004
LUNA measures 14N(p,γ)15O underground
LUNA
LUNA measured the slowest reaction of the CNO cycle at energies lower than surface laboratories could reach, with a solid target and a germanium detector. The extrapolated S-factor at zero energy, 1.7 ± 0.2 keV b, was lower than in earlier compilations, so the CNO cycle runs more slowly than had been assumed.
2004
A radioactive s-process branch point measured at n_TOF
Experiment
At CERN's n_TOF facility the neutron-capture cross section of samarium-151, which lives about 95 years, was measured on a radioactive sample from 1 eV to 1 MeV. The stellar value turned out well above the theoretical predictions used until then.
2006
14N(p,γ)15O down to 70 keV
LUNA
With a nitrogen gas target and a large detector surrounding it, LUNA measured 14N(p,γ)15O down to 70 keV. There the cross section is only 0.24 picobarn, closer to solar energies than any earlier experiment.
2007
12C + 12C fusion measured near the Gamow window
Experiment
At the Bochum Dynamitron, Spillane and colleagues measured carbon fusion down to 2.1 MeV, using carbon targets almost free of hydrogen. They found a strong resonance at 2.14 MeV that raises the rate of one branch about fivefold near 0.8 GK, and showed how uncertain any extrapolation to lower energies is.
Aug 2007
Borexino detects 7Be solar neutrinos in real time
Observation
The Borexino liquid scintillator detector at Gran Sasso measured the 0.862 MeV neutrinos from 7Be electron capture in the Sun, one event at a time. It was the first real-time measurement of solar neutrinos below 1 MeV.
2010s
2011
Solar Fusion II
Theory
A group of experimentalists and theorists reviewed the cross sections of the proton–proton chain and the CNO cycles and recommended values for solar models. The review followed an earlier one from 1998.
Aug 2014
Borexino sees pp neutrinos
Observation
Borexino measured the spectrum of the neutrinos from proton–proton fusion, the first reaction of the Sun's main chain. The collaboration estimated that this chain produces about 99% of the Sun's power.
2015
Nobel Prize for Kajita and McDonald
Observation
Takaaki Kajita (Super-Kamiokande) and Arthur McDonald (SNO) shared the Nobel Prize in Physics for showing that neutrinos change type as they travel. This means that neutrinos have mass.
2017
First beam at CASPAR
Facility
An accelerator moved from Notre Dame to the 4850 ft level of the Sanford Underground Research Facility in South Dakota delivered its first beam. It is the first underground accelerator for nuclear astrophysics in the United States, in the mine where Davis had counted solar neutrinos.
2017
A review of 12C(α,γ)16O
Theory
deBoer and colleagues collected the measurements of 12C(α,γ)16O, the reaction that sets how much carbon and oxygen helium burning leaves behind, and fitted them together with an R-matrix analysis. They also followed the effect of its uncertainty on stellar models.
Aug 2017
A neutron-star merger makes heavy elements
Observation
On 17 August 2017 the LIGO and Virgo detectors recorded gravitational waves from two merging neutron stars. Telescopes then found the glow of the explosion, a kilonova. Its spectra showed elements made by rapid neutron capture, the r-process.
May 2018
Trojan Horse resonances in carbon fusion, and a debate
Experiment
Using the indirect Trojan Horse method at the LNS in Catania, Tumino and colleagues reported resonances in carbon fusion at the energies of stellar cores, raising the rate more than 25-fold near 0.5 GK. The analysis was soon challenged, and direct measurements in 2020 at Orsay and Notre Dame did not see such a rise.
Jul 2019
The Felsenkeller underground accelerator laboratory opens
Facility
HZDR and TU Dresden inaugurated a 5 MV accelerator under 45 m of rock in the Felsenkeller tunnels in Dresden. The rock reduces the cosmic-ray muon flux by a factor of 40. With an active muon veto, the background is about twice that of a deep underground laboratory.
2020s
Nov 2020
LUNA measures deuterium burning for cosmology
LUNA
LUNA measured d(p,γ)3He at the energies of Big Bang nucleosynthesis with high precision. With this rate, the observed amount of primordial deuterium gives the density of ordinary matter in the Universe to 1.6%, in agreement with the cosmic microwave background.
Nov 2020
Borexino detects CNO neutrinos
Observation
Borexino reported the first experimental evidence of neutrinos from the CNO cycle in the Sun. It shows directly that the cycle runs in stars. In the Sun it gives about one per cent of the energy.
2022
A 3.5 MV accelerator is installed under Gran Sasso
LUNA
A 3.5 MV Singletron accelerator, built by High Voltage Engineering Europa, was installed underground at Gran Sasso: the MV machine of the Bellotti Ion Beam Facility (BIBF). It delivers proton, helium and carbon beams, so that helium and carbon burning can be studied underground.
Oct 2022
JUNA measures 19F(p,γ)20Ne underground
Experiment
The JUNA accelerator, under about 2400 m of rock at the China Jinping Underground Laboratory, measured 19F(p,γ)20Ne down to 186 keV and found a resonance at 225 keV. Near 0.1 GK the new rate is up to 7.4 times the previous one. The reaction lets material leak out of the CNO cycle, and the faster rate may explain the calcium in one of the oldest known stars.
Oct 2023
The Bellotti Ion Beam Facility is inaugurated
LUNA
The underground accelerator laboratory was inaugurated and named after Enrico Bellotti, the first director of the Gran Sasso laboratory, who started underground accelerator physics there. It runs as a user facility, with beam time given through yearly calls.
Nov 2024
The stellar decay of thallium-205 is measured
Experiment
Bare thallium-205 ions circulating in the GSI storage ring were seen to decay into lead-205 by bound-state β decay, with a half-life of 291 days. In stars this decay makes lead-205, a radioactive nucleus whose traces in meteorites help date the birth of the Sun; the long-sought measurement fixes a key nuclear input of that clock.
2025
Solar Fusion III
Theory
A new review collected the data and theory for hydrogen burning in the Sun and other stars that appeared after Solar Fusion II. It gives recommended S-factors with uncertainties for the proton–proton chain and the CNO cycles.
Aug 2026
Neutron capture on radioactive niobium-94 is measured
Experiment
Stardust grains older than the Sun hold more molybdenum-94 than earlier models predicted. Its production depends on niobium-94, a radioactive branch point of the s-process, where neutron capture competes with decay. At CERN's n_TOF its capture cross section was measured for the first time. The result confirms the general scale of the theoretical estimates; used in fully coupled models of low-mass giant stars, together with their improved physics, it brings them into agreement with the grains.