What a reaction twice as fast would change, one sentence at a time. Each one comes from a one-zone burn of the rate explorer (a simplified model: one parcel of gas at a set temperature and density, with no mixing), says what it leaves out, and opens the exact calculation with “Try it”.
Proton–proton chains
Curiosity·☉ Sun
If 1H(p,e+ν)2H were twice as fast, this model of the Sun's centre would end up with about a sixth less hydrogen (−15%).
with today's rate
with the rate ×2
Biggest knock-on effect: 3He (+26%).
Energy output +62%; the first 1% of the hydrogen burns 46% sooner.
The Sun's slowest step, never measured. A real Sun would cool down to compensate.
One-zone model at 16 MK over 1 billion years. Real stars adjust. The energy is that of this gas at the model's temperature, not the star's luminosity: a real star would adjust its temperature to keep its luminosity. Doubling is a what-if, not the rate's actual uncertainty. How well this rate is known: well known.
If 3He(α,γ)7Be were twice as fast, this model of the Sun's centre would end up with about two fifths more beryllium‑7 (+37%).
with today's rate
with the rate ×2
Energy output +12%; the first 1% of the hydrogen burns 14% sooner.
Beryllium‑7 gives off the Sun's 7Be neutrinos, which Borexino counted on Earth.
One-zone model at 16 MK over 1 billion years. Real stars adjust. The energy is that of this gas at the model's temperature, not the star's luminosity: a real star would adjust its temperature to keep its luminosity. Doubling is a what-if, not the rate's actual uncertainty. How well this rate is known: improving.
If 3He(3He,2p)4He were twice as fast, this model of the Sun's centre would give off about a fifth fewer boron‑8 neutrinos (−21%).
with today's rate
with the rate ×2
Energy output −6%; the first 1% of the hydrogen burns 8% later.
Two helium‑3 nuclei fusing directly skip beryllium‑7, so fewer boron‑8 neutrinos reach Earth.
One-zone model at 16 MK over 1 billion years. Real stars adjust. The neutrinos are those of this gas at the model's temperature: a real star would adjust its temperature to keep its luminosity. Doubling is a what-if, not the rate's actual uncertainty. How well this rate is known: well known.
If 14N(p,γ)15O were twice as fast, this model of the Sun's centre would end up with twice as much oxygen‑15 (×2.0).
with today's rate
with the rate ×2
Biggest knock-on effect: 12C (×2.0).
CNO reactions only, about 1% of the Sun's power (a few per cent at its centre): energy output ×1.9.
Oxygen‑15 gives off CNO neutrinos as it decays; Borexino first detected them in 2020.
One-zone model at 16 MK over 1 billion years. Real stars adjust. The C, N and O start as fresh fuel at today's temperature and density, not as the Sun's processed core. The energy is that of this gas at the model's temperature, not the star's luminosity: a real star would adjust its temperature to keep its luminosity. Doubling is a what-if, not the rate's actual uncertainty. How well this rate is known: improving.
If 13C(p,γ)14N were twice as fast, this model of a 15 M☉ star's core would end up with a carbon‑12 to carbon‑13 ratio twice as high (×2.0).
ratio, today's rate
ratio, rate ×2
Astronomers measure this ratio in red giants; LUNA measured this rate underground in 2023.
One-zone model at 35 MK over 6.4 million years. Real stars adjust. Doubling is a what-if, not the rate's actual uncertainty. How well this rate is known: improving.
If 17O(p,α)14N were twice as fast, this model of a 15 M☉ star's core would end up with an oxygen‑17 to oxygen‑16 ratio half as high (−50%).
ratio, today's rate
ratio, rate ×2
Biggest knock-on effect: 18O (−50%).
Stardust grains in meteorites preserve this oxygen ratio; LUNA measured this rate in 2016.
One-zone model at 35 MK over 6.4 million years. Real stars adjust. Doubling is a what-if, not the rate's actual uncertainty. How well this rate is known: improving.
If 18F(p,α)15O were twice as fast, this model of a carbon–oxygen nova would end up with about two fifths less fluorine‑18 (−39%).
with today's rate
with the rate ×2
Fluorine‑18 powers the 511 keV gamma-ray flash expected in a nova's first hours.
One-zone model cooling from 200 MK over 17 minutes. Real stars adjust. Doubling is a what-if, not the rate's actual uncertainty. This rate has no atlas page yet: see its source.
If 14N(p,γ)15O were twice as fast, this model of a 15 M☉ star's core would make about twice the power (×1.9) and run out of hydrogen in half the time (−49%).
with today's rate
with the rate ×2
Biggest knock-on effect: 16O (×2.0).
The CNO cycle's slowest step sets the temperature at which a massive star burns hydrogen.
One-zone model at 35 MK over 6.4 million years. Real stars adjust. The energy is that of this gas at the model's temperature, not the star's luminosity: a real star would adjust its temperature to keep its luminosity. Doubling is a what-if, not the rate's actual uncertainty. How well this rate is known: improving.
If 15N(p,γ)16O were twice as fast, this model of a 15 M☉ star's core would end up with twice as much oxygen‑17 (×2.0).
with today's rate
with the rate ×2
Biggest knock-on effect: 18O (×2.0).
A rare leak from the CN cycle feeds oxygen‑16, and oxygen‑16 makes oxygen‑17.
One-zone model at 35 MK over 6.4 million years. Real stars adjust. Doubling is a what-if, not the rate's actual uncertainty. How well this rate is known: improving.
If 22Ne(p,γ)23Na were twice as fast, this model of an AGB star's hot envelope would end up with about a fifth more sodium (+21%).
with today's rate
with the rate ×2
Sodium varies from star to star in globular clusters; LUNA measured this rate in 2018.
One-zone model at 80 MK over 10 years. Real stars adjust. Doubling is a what-if, not the rate's actual uncertainty. How well this rate is known: improving.
If 23Na(p,α)20Ne were twice as fast, this model of an AGB star's hot envelope would end up with about half as much sodium (−46%).
with today's rate
with the rate ×2
This reaction closes the NeNa cycle; its rival, (p,γ), lets sodium leak towards magnesium.
One-zone model at 80 MK over 10 years. Real stars adjust. Doubling is a what-if, not the rate's actual uncertainty. How well this rate is known: uncertain.
If 22Na(p,γ)23Mg were twice as fast, this model of an oxygen–neon nova would end up with about two fifths less sodium‑22 (−38%).
with today's rate
with the rate ×2
Sodium‑22's 1275 keV gamma-ray line has not yet been seen from a nova.
One-zone model cooling from 250 MK over 17 minutes. Real stars adjust. Doubling is a what-if, not the rate's actual uncertainty. This rate has no atlas page yet: see its source.
If 20Ne(p,γ)21Na were twice as fast, this model of a 25 M☉ star's core would end up with about one and a half times as much sodium (+54%).
with today's rate
with the rate ×2
Biggest knock-on effect: 22Ne (+41%).
Neon‑20 starts the NeNa cycle; sodium, three captures later, follows its pace.
One-zone model at 38 MK over 6.7 million years. Real stars adjust. Doubling is a what-if, not the rate's actual uncertainty. How well this rate is known: improving.
If 25Mg(p,γ)26Al were twice as fast, this model of an AGB star's hot envelope would end up with about two fifths more aluminium‑26 (+38%).
with today's rate
with the rate ×2
Biggest knock-on effect: 27Al (+17%).
Aluminium‑26's 1809 keV glow maps the Milky Way.
One-zone model at 80 MK over 10 years. Real stars adjust. This model counts all the aluminium‑26 made, including a short-lived form that never glows, so here it is an upper limit. Doubling is a what-if, not the rate's actual uncertainty. How well this rate is known: improving.
If 25Mg(p,γ)26Al were twice as fast, this model of a 25 M☉ star's core would end up with slightly less aluminium‑26 (−4%).
with today's rate
with the rate ×2
Faster is not more: magnesium‑25 runs out anyway, and aluminium‑26 decays while it waits.
One-zone model at 38 MK over 6.7 million years. Real stars adjust. Doubling is a what-if, not the rate's actual uncertainty. How well this rate is known: improving.
If 24Mg(p,γ)25Al were twice as fast, this model of an AGB star's hot envelope would end up with about a fifth more aluminium‑26 (+19%).
with today's rate
with the rate ×2
Biggest knock-on effect: 27Al (+10%).
Magnesium‑24 feeds magnesium‑25, which makes aluminium‑26: an effect two steps away.
One-zone model at 80 MK over 10 years. Real stars adjust. This model counts all the aluminium‑26 made, including a short-lived form that never gives off the 1809 keV line, so here it is an upper limit. Doubling is a what-if, not the rate's actual uncertainty. How well this rate is known: well known.
If 13C(α,n)16O were twice as fast, this model of an AGB star's 13C pocket would end up with almost the same amount of barium‑138 (+1%).
with today's rate
with the rate ×2
Biggest knock-on effect: 86Kr (×2.0).
The same energy, released in half the time: power ×2.0.
All the carbon‑13 burns anyway: how much there is matters, not how fast it burns.
One-zone model at 90 MK over 78 000 years. Real stars adjust. Krypton‑86 comes through the branch at krypton‑85, modelled without its short-lived isomer, so this model makes too much of it. The energy is that of this gas at the model's temperature, not the star's luminosity: a real star would adjust its temperature to keep its luminosity. Doubling is a what-if, not the rate's actual uncertainty. How well this rate is known: improving.
If 22Ne(α,n)25Mg were twice as fast, this model of an AGB thermal pulse would end up with three times as much krypton‑86 (×3.1).
with today's rate
with the rate ×2
Biggest knock-on effect: 70Ge (×5.8).
In the short hot pulse, neon‑22 burns only partly: its rate sets the neutrons.
One-zone model at 270 MK over 5 years. Real stars adjust. This pulse gives the nuclei several times the neutron exposure of a real one: read the size of the change with care. Krypton‑86 comes through the branch at krypton‑85, modelled without its short-lived isomer, so this model makes too much of it. Doubling is a what-if, not the rate's actual uncertainty. How well this rate is known: uncertain.
If 22Ne(α,γ)26Mg were twice as fast, this model of a massive star's helium core would end up with about two fifths less strontium‑88 (−42%).
with today's rate
with the rate ×2
Biggest knock-on effect: 86Sr (−43%).
This rival burns neon‑22 without freeing a neutron, so fewer neutrons reach the iron.
One-zone model at 250 MK over 83 000 years. Real stars adjust. For this site the sizes are upper limits: read the direction of the change. Doubling is a what-if, not the rate's actual uncertainty. How well this rate is known: uncertain.
If 25Mg(n,γ)26Mg were twice as fast, this model of a massive star's helium core would end up with about a third as much strontium‑88 (−69%).
with today's rate
with the rate ×2
Biggest knock-on effect: 86Sr (−71%).
Magnesium‑25 is a neutron poison: it swallows neutrons before the iron seeds can.
One-zone model at 250 MK over 83 000 years. Real stars adjust. For this site the sizes are upper limits: read the direction of the change. Doubling is a what-if, not the rate's actual uncertainty. How well this rate is known: improving.
If 56Fe(n,γ)57Fe were twice as fast, this model of an AGB star's 13C pocket would end up with about half as much barium‑138 (−53%).
with today's rate
with the rate ×2
Biggest knock-on effect: 120Sn (−71%).
Iron grabs more of the neutrons, so each nucleus climbs less far towards barium.
One-zone model at 90 MK over 78 000 years. Real stars adjust. Doubling is a what-if, not the rate's actual uncertainty. How well this rate is known: improving.
The numbers are read from the explorer's precomputed tables (every rate ×2 at each site's own conditions) and checked against a fresh run of the same solver. They are rounded to two significant figures. ×2 is a what-if: how well each rate is really known is given beside it, from the reaction atlas. Where a change reaches further, a card adds its biggest knock-on effect, on a nucleus the reaction neither burns nor makes, and what it does to the energy: the power of one gram of gas at the model's temperature, not the star's luminosity.