I would like to start this story by pointing out that solar scientists can be super dramatic in their naming. Astronomers – we’re just bad at it. But for solar scientists, everything is a crisis. I grew up hearing about the solar neutrino crisis, and with that solved, now I’m learning there is a solar abundance crisis. Back in 2009, the Sun’s chemical ingredients list was measured to a level of precision that should have made models of the Sun match what we see beautifully. But they didn’t.

The Sun is a mass of incandescent gas that shakes like liquid on a speaker playing a long low tone. We can see these waves reshaping the Sun’s surface, and the size of the waves is directly related to the volume of the part of the Sun that is resonating and the density of the material inside. This is like how the notes you can play when blowing in a soda bottle are related to the size of the empty part of the bottle, and if the bottle is filled with normal, mostly nitrogen gas or something else.

It turns out that those 2009 solar measurements predicted a Sun that should resonate with a different set of pitches than the ones our Sun’s surface is observed to actually play. And the solar scientists named this mismatch the “Solar Abundance Crisis” because it seemed to imply that our understanding of what is in the Sun’s atmosphere must not be right.

Again, they are dramatic in how they name things.

Turns out, the problem wasn’t in the measurements of the Sun’s ingredients. The problem was in our interpretation of those measurements.

In modeling how stellar atmospheres are heated and how they excite different atoms to absorb or emit light, it has generally been assumed the Sun is able to effectively spread heat through its atmosphere and is in what’s called thermal equilibrium. This implies that even though the Sun’s atmosphere is heated by roiling blobs of rising and falling gas that are all different temperatures, like the convecting blobs in a lava lamp, somehow the atmosphere can be modeled like everything – at least at the local level – is a nice smooth distribution of temperatures.

But in a new paper in Astronomy & Astrophysics with first author Ekaterina Magg, researchers in a team led by Maria Bergemann try modeling the Sun using calculations that don’t assume thermal equilibrium at local scales. This small change makes the maths a lot harder but also makes for a more realistic description of how energy moves in lower-density areas like the Sun’s photosphere.

And it turns out that these changes to the models changed what ratios of ingredients the Sun needs to have to produce the interrupted rainbow of light we see.  According to Magg: The value for the oxygen abundance was almost 15% higher than in previous studies.

This change made the Sun better match both the compositions of primitive meteors that are representative of the early solar system and brought the results from the spectra and the helioseismology all into alignment

As they state in their press release: Crisis resolved.

Bergemann goes on to explain: The new solar models based on our new chemical composition are more realistic than ever before: they produce a model of the Sun that is consistent with all the information we have about the Sun’s present-day structure – sound waves, neutrinos, luminosity, and the Sun’s radius – without the need for non-standard, exotic physics in the solar interior.

At the end of the day, it seems it is a good idea to check your assumptions when maths and observations don’t match. Sometimes, our simplifications create false emergencies, and all it takes is a bit of more complex calculations to see things as they really are.

More Information

MPIA press release

“Observational constraints on the origin of the elements: IV. Standard composition of the Sun,” Ekaterina Magg et al., 2022 May 20, Astronomy & Astrophysics