Researchers have recalculated how much silver the Sun holds, and the answer is about 55 percent higher than the figure astronomers have used for the past decade. The new solar silver abundance came out of better modeling rather than any new observation.
The correction resolves most of a mismatch that has nagged at researchers for years. Silver appeared far less abundant in the Sun’s atmosphere than in primitive meteorites. Yet both formed from the same cloud of gas and dust.
The missing silver
Both formed roughly 4.6 billion years ago out of one collapsing cloud, so their compositions should match element for element. Across most of the periodic table they do.
The value adopted in 2015 for the solar photosphere – the thin visible layer where sunlight escapes – left silver looking oddly scarce. Primitive meteorites appeared to carry about 78 percent more of it than the Sun did.
That gap survived a full census of solar chemistry published in 2021, which singled out silver as one of the few elements where the Sun and the meteorites clearly disagreed. Nobody could say whether the difference was real.
Sema Caliskan, who led the work as a doctoral researcher at Uppsala University in Sweden and is now a postdoctoral fellow in Belgium, went after the problem from the atomic side rather than the observational one. She rebuilt the silver atom itself.
Why silver matters
The stakes of that rebuild go beyond a single number. Asked by Earth.com why the revision matters, Caliskan explained that the Sun, the planets, and the meteorites all condensed from the same material. In principle, their original chemistry should match.
“Silver, however, was a notable outlier: the accepted solar abundance was significantly lower than the meteoritic value,” she said in her reply to Earth.com.
The open question was whether that gap traced a real process in the young Solar System or a flaw in how sunlight was being read.
Roughly 80 percent of the solar system’s silver was built by rapid neutron capture inside dying stars. Silver also sits in a stretch of the periodic table where that process appears to run in two separate modes, which makes it a useful tracer of the weaker one.
A survey of 71 stars published in 2012 found silver and europium moving in opposite directions across the Galaxy’s oldest stars. Europium comes from the main branch of rapid neutron capture, the r-process, so the mismatch marked silver as a product of something else.
Where the old assumption broke down
Every earlier measurement of solar silver leaned on a simplifying assumption. It treated the gas in the Sun’s outer layers and the light passing through it as being in balance, so that temperature alone decided how many atoms sat at each energy level.
The new calculations show the Sun does not work that way. Ultraviolet light streaming up from hotter gas below keeps knocking silver atoms out of their lowest energy state, and collisions carry that excess further up the ladder, stripping many atoms of an electron entirely.
Fewer silver atoms are left in the one state that can absorb at the wavelengths astronomers watch. The absorption marks come out shallower than the balanced calculation predicts.
Through the old assumption, a shallow mark means little silver. The correct reading gives an ordinary amount of silver with fewer of its atoms in a position to absorb.
The team confirmed that the two silver features were driving the effect themselves. They switched off the lines’ absorbing power inside the calculation, and the atomic populations settled back close to balance.
Building a better model
Building that calculation required a full map of the silver atom, covering 57 energy levels and 183 transitions between them. Measurements existed for only seven.
Caliskan’s group calculated 53 more from theory alone and drew the remainder from published work. Nobody had ever worked out what happens when a hydrogen atom slams into a silver one, so those rates were built from scratch.
The team used copper as a stand-in because it sits directly above silver in the periodic table and has a near-identical structure. That single ingredient remains the largest source of uncertainty in the final answer.
Two lines in the ultraviolet
Silver leaves only two usable marks in sunlight, at wavelengths of 328 and 338 nanometers. Both fall in the near-ultraviolet, invisible to the eye and crowded with spectral lines from other elements.
That crowding is the second half of the problem. The 2015 analysis measured the depth of the two silver marks directly off a photographic atlas of the solar spectrum.
Caliskan’s team rebuilt each line’s whole neighborhood instead, calculating every element that contributes there and fitting the result against the real spectrum. One contributor changed the answer.
An iron line that theory says should exist falls almost exactly on top of the 328 nanometer silver feature. Counting it cut the absorption that could be credited to silver by about a fifth.
The payoff shows up in how well the two silver lines now agree. They used to disagree by roughly 50 percent. With the new treatment they land within a few percent of each other.
The authors are careful not to claim too much from that. Nobody has ever measured the iron line’s strength, and predictions of that kind can be off by large factors.
Adding up the corrections
Moving from the simplest treatment to the full three-dimensional one, with the balance assumption dropped, nearly doubles the amount of silver the two lines imply. That is a 91 percent increase.
Most of it comes from abandoning the balance assumption. Modeling the churning solar surface in three dimensions, rather than as a single averaged column, pushes in the same direction by a smaller amount.
The two effects also appear to amplify each other. Caliskan and her colleagues suggest the reason is that sharper temperature contrasts inside the three-dimensional model strengthen the ultraviolet light driving silver atoms out of their lowest state, though that explanation has not been tested directly.
Pulling the other way, the revised line strengths bring the answer back down. What survives is the 55 percent net increase over the 2015 figure.
Bringing the Sun into line
What struck Caliskan was that the shift needed no coaxing. “We did not adjust the model to reproduce the meteoritic abundance, and did not use any fine-tuning,” she told Earth.com.
The team built everything from first principles, not knowing at the outset whether the solar silver figure would climb or fall. That the physics alone pulled the Sun into line with the meteorites is what made the result striking to her.
Measured against CI chondrites – the most primitive meteorites, closest in makeup to the cloud the solar system condensed from – the gap shrinks from roughly 78 percent to roughly 15 percent. That is close enough to sit inside the uncertainty on both numbers.
Silver condenses out of a cooling gas at around 800 degrees Fahrenheit (427 degrees Celsius), which is relatively cool by the standards of rock-forming elements. It belongs to a middling group that neither freezes out early nor stays gaseous.
A small residue is left over, and it is not unique to silver. A recent paper on solar sulfur turned up something similar for that element. Whether the pattern reflects a real chemical difference between the Sun and the meteorites, or one more modeling artifact, is still argued over.
A benchmark for other stars
The clearest result is that the Sun’s silver was never missing. It was there the whole time, and the faulty instrument was the measurement model.
Solar abundances are the yardstick against which every other star is measured, which is why getting the underlying physics right carries so far.
Speaking with Earth.com, Caliskan framed the wider lesson: to work out what a star is made of, researchers need to know not just how its atmosphere moves but how single atoms trade energy with light and particles.
Silver drove that home, she said, because its spectral marks are so sensitive to the three-dimensional motion and off-balance conditions in the Sun that ignoring them badly undercounted the element.
Looking beyond the Sun
Alongside the new number, the group released correction tables covering a grid of model atmospheres for dwarf and giant stars. Other researchers can apply the fix to their own silver measurements without running three-dimensional simulations themselves.
Those corrections should grow larger in the Galaxy’s oldest stars, which formed before much iron existed to soak up ultraviolet light. That bears on a puzzle open since 2011, when dwarf stars and giant stars of similar composition were found to give silver readings differing by roughly a factor of three.
The immediate consequence is a revised curve tracing silver against iron across the Milky Way’s history. Astronomers use that record to work out where the weaker neutron-capture route ran and how often. The new model predicts a steeper curve than the one in use today. Redrawing it is the group’s next job.
The study is published in Astronomy & Astrophysics.
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