After two decades of debate, scientists are getting closer to understanding exactly what the sun is made of – and thus the entire universe.
The sun consists mainly of hydrogen and helium. There are also heavier elements such as oxygen and carbon, but how much is debatable. New observations of ghostly subatomic particles, known as neutrinos, suggest the sun has ample reserves of “metals,” a term astronomers use for all elements heavier than hydrogen and helium, researchers said on May 31 on arXiv.org.
The results are “fully compatible with [a] high metal content ”for the sun, says Livia Ludhova, a physicist at the Julich Research Center in Germany.
Elements heavier than hydrogen and helium are crucial to creating rock iron planets like Earth and sustaining life forms like humans. Undoubtedly the most common of these elements in the universe is oxygen, followed by carbon, neon and nitrogen.
But astronomers do not know exactly how many of these elements exist relative to hydrogen, the most common element in space. This is because astronomers typically use the sun as a starting point to measure elemental abundance in other stars and galaxies, and two techniques suggest very different chemical compositions of our star.
One technique uses the vibrations inside the sun to bring out its internal structure and promotes high metal content. The second technique determines the composition of the sun by how 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 an earlier definition, creating the current conflict.
Now another technique has emerged that could solve the long-standing debate: the use of solar neutrinos.
These particles arise from nuclear reactions in the solar nucleus that convert hydrogen to helium. About 1% of solar energy comes from reactions involving carbon, nitrogen and oxygen, which convert hydrogen to helium but are not consumed in the process. So the more carbon, nitrogen and oxygen the sun actually has, the more neutrinos this CNO cycle must emit.
In 2020, scientists announced that Borexino, an underground detector in Italy, had spotted these CNO neutrinos (SN: 24.06.20). Now Ludhova and her colleagues have registered 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 solar composition.
And although this figure sounds small, it is even higher than that preferred by astronomers who maintain the sun with high metal. And that’s 70 percent more than the number of low metal suns.
“It’s a great result,” said Mark Pinson, an astronomer at Ohio State University in Columbus who has long advocated a high-metal sun. “They have been able to demonstrate that the current decision on low metal content is not in line with the data.
However, due to the uncertainty in both the observed and predicted neutrino numbers, Borexino cannot completely exclude the low-metal sun, Ludhova said.
The new work is a “significant improvement,” said Gael Bulgen, an astrophysicist at the University of Geneva in Switzerland who prefers the low-metal sun. But the projected number of CNO neutrinos comes from solar models, which he criticizes as too simplistic. These models ignore the sun’s rotation, which can cause chemicals to mix throughout its life and change the amount of carbon, nitrogen and oxygen near the sun’s center, thus changing the predicted number of CNO neutrinos, Bulgen said.
Additional neutrino observations are needed for the final verdict, Ludhova said. Borexino stopped in 2021, but future experiments could fill the gap.
The stakes are high. “We argue about what the universe is made of,” says Pinsonneault, “because the sun is the standard for all our research.”
So if the sun has a lot more carbon, nitrogen and oxygen than is currently thought, so does the whole universe. “It changes our understanding of how chemical elements are made. This changes our understanding of how stars evolve and how they live and die, ”says Pinsonneault. And, he added, it’s a reminder that even the best-studied star – our sun – still has secrets.