After two decades of debate, scientists are getting closer and closer to understanding exactly what the sun is made of, and therefore the entire universe.
The sun is mainly composed of hydrogen and helium. There are also heavier elements like oxygen and carbon, but how much is controversial. New observations of spectral subatomic particles known as neutrinos suggest that the sun has a large supply of “metals,” the term astronomers use for all elements heavier than hydrogen and helium, the researchers report May 31 on arXiv.org.
The results “are fully compatible with [a] high metallicity ”for the sun, says Livia Ludhova, physicist at the Jülich Research Center in Germany.
Elements heavier than hydrogen and helium are critical for creating rock-iron planets like Earth and supporting life forms like humans. By far the most abundant of these elements in the universe is oxygen, followed by carbon, neon and nitrogen.
But astronomers don’t know exactly how many of these elements exist compared to hydrogen, the most common element in the cosmos. This is because astronomers typically use the sun as a reference point for measuring elemental abundances in other stars and galaxies, and two techniques involve very different chemical compositions for our star.
One technique exploits the vibrations inside the sun to deduce its internal structure and favors a high metal content. The second technique determines the composition of the sun by the way in which the atoms on its 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 a previous determination found, creating the current conflict.
Now another technique has emerged that could decide the long-standing debate: using solar neutrinos.
These particles result from nuclear reactions in the sun’s core that transform hydrogen into helium. About 1% of solar energy comes from reactions involving carbon, nitrogen and oxygen, which convert hydrogen to helium but are not depleted in the process. So the more carbon, nitrogen and oxygen the sun actually has, the more neutrinos this CNO cycle should emit.
In 2020, scientists announced that Borexino, an underground detector in Italy, had spotted these CNO neutrinos (SN: 06/24/20). Now Ludhova and his colleagues have recorded enough neutrinos to calculate that carbon and nitrogen atoms together are about 0.06 percent more abundant than hydrogen atoms in the sun, the first use of neutrinos to determine the composition of the sun.
And while that number sounds small, it’s even higher than the one favored by astronomers who support a high-metal sun. And that’s 70 percent greater than the number a low-metal sun should have.
“This is a very good result,” says Marc Pinsonneault, an astronomer at Ohio State University in Columbus who has long supported a high-metal sun. “They were able to solidly demonstrate that the current low metallicity solution is inconsistent with the data.”
However, due to uncertainties in both observed and predicted neutrino numbers, Borexino cannot completely rule out a low-metal sun, Ludhova says.
The new work is “a significant improvement,” says Gaël Buldgen, an astrophysicist at the University of Geneva in Switzerland, who prefers a low-metal sun. But the predicted numbers of CNO neutrinos come from models of the sun that he criticizes as oversimplified. These models neglect the sun’s rotation, which could induce mixing of chemical elements over its lifetime and change the amount of carbon, nitrogen and oxygen near the center of the sun, thereby changing the predicted number of CNO neutrinos, Buldgen says.
Further observations on neutrinos are needed for a final verdict, Ludhova says. Borexino closed in 2021, but future experiments could fill the void.
The stakes are high. “We are discussing what the universe is made of,” says Pinsonneault, because “the sun is the reference point for all our studies.”
So if the sun has a lot more carbon, nitrogen, and oxygen than is currently thought, so does the entire universe. “This changes our understanding of how chemicals are made. It changes our understanding of how stars evolve and how they live and die, “says Pinsonneault. And, he adds, it reminds us that even the most studied star, our sun, still has secrets.