After two decades of discussion, scientists are getting closer and closer to figuring out exactly what the sun — and therefore the entire universe — is made of.
The sun is mostly composed of hydrogen and helium. There are also heavier elements, such as oxygen and carbon, but how controversial it is. New observations of ghostly subatomic particles known as neutrinos suggest that the sun has a wide supply of “metals,” a term astronomers use for all elements heavier than hydrogen and helium, the researchers reported on May 31 on arXiv.org.
The results are “fully compatible [a] great metallicity ”for the sun, says Livia Ludhova, a physicist at the Jülich Research Center in Germany.
Elements that are heavier than hydrogen and helium are essential for the creation of iron-and-stone planets like Earth and for sustaining life forms like humans. The most abundant of these elements in the universe is oxygen, followed by carbon, neon, and nitrogen.
But astronomers do not know exactly how much of these elements are present in relation to hydrogen, the most common element in the cosmos. This is because astronomers typically use the sun as a reference point to measure the abundance of elements in other stars and galaxies, and because the two techniques involve very different chemical compositions of our stars.
One technique uses the internal vibrations of the sun to infer its internal structure and favors its high metal content. The second technique determines the composition of the sun by how its atoms on the surface absorb certain wavelengths of light. Two decades ago, the use of this second technique suggested that the levels of oxygen, carbon, neon, and nitrogen in the sun were 26 to 42 percent lower than in a previously found determination, creating the current conflict.
Another technique that may have long been debated has now emerged: the use of solar neutrinos.
These particles are formed from nuclear reactions that convert hydrogen into helium in the solar nucleus. About 1 percent of the sun’s energy comes from reactions involving carbon, nitrogen, and oxygen, which convert hydrogen into helium but do not deplete it in the process. So the more carbon, nitrogen and oxygen the sun has, the more neutrinos this CNO cycle should emit.
In 2020, scientists predicted that an Italian underground detector for Borexin would detect these CNO neutrinos (SN: 20/6/24). Now Ludhova and her colleagues have recorded enough neutrinos to estimate that the carbon and nitrogen atoms together are as abundant as the hydrogen atoms of the sun by about 0.06 percent, the first use of neutrinos to determine the makeup of the sun.
And while that number may seem small, it is even higher than what astronomers who support the high metal sun allow. And it’s a 70 percent higher-than-expected amount of low-metal sunscreen.
“This is a great result,” says Marc Pinsonneault, an astronomer at Ohio State University in Columbus who has long advocated for a high-metal sun. “I have been able to prove to Iroki that the current low metallicity solution does not match the data.”
However, due to uncertainties in the number of observed and predicted neutrinos, Borexino cannot completely rule out the low-metal sun, Ludhova says.
Gaël Buldgen, a Swiss astrophysicist at the University of Geneva in Switzerland, says the new work is a “significant improvement” in favor of a low-metal sun. But the predicted numbers of CNO neutrinos come from models of the sun that criticize the oversimplification. These models rule out the rotation of the sun, which can cause chemical elements to mix up during their lifetime and change the amount of carbon, nitrogen and oxygen near the center of the sun, thereby changing the number of CNO neutrinos projected, Buldgen says.
Additional neutrino observations are needed for a final judgment, Ludhova says. Borexin was closed in 2021, but future experiments may fill the gap.
The stakes are high. “We’re debating what the universe is all about,” says Pinsonneault, “because the sun is the reference point for all our studies.”
So if the sun contains much more carbon, nitrogen, and oxygen than we think today, so does the entire universe. “This changes our understanding of how chemical elements are made. It changes our understanding of how stars evolve and how they live and die, “says Pinsonneault.