After two decades of debate, scientists are getting closer to finding out what the Sun – and thus the entire universe – is made of.
The Sun is composed mostly of hydrogen and helium. There are also heavier elements, such as oxygen and carbon, but how much is controversial. New observations of ghostly subatomic particles known as neutrinos suggest the Sun has a substantial supply of “metals” astronomers use for all elements heavier than hydrogen and helium, researchers May 31 on arXiv Reported on .org.
The results are fully consistent with ” [a] “High metallicity for the Sun,” says physicist Livia Ludhova of the Research Center Jülich in Germany.
Elements heavier than hydrogen and helium are important for creating rock-iron planets like Earth and sustaining 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 do not know exactly how many of these elements are present relative to hydrogen, the most common element in the universe. This is because astronomers commonly use the Sun as a reference point to measure elemental abundances in other stars and galaxies, and the two techniques show very different chemical compositions for our star.
One technique uses vibrations inside the Sun to trace its internal structure and favors a high metal content. The second technique determines the Sun’s composition by how the atoms on its surface absorb certain wavelengths of light. Two decades ago, the use of this second technique suggested that levels of oxygen, carbon, neon and nitrogen in the Sun were 26 to 42 percent lower than previously determined, leading to the current conflict.
Another technique has now emerged that may settle a long-standing debate: using solar neutrinos.
These particles are produced by nuclear reactions in the Sun’s core that convert hydrogen into helium. About 1 percent of the Sun’s energy comes from reactions involving carbon, nitrogen and oxygen, which convert hydrogen to helium but are not used in the process. So the more carbon, nitrogen, and oxygen the Sun actually has, the more neutrinos this CNO cycle should be emitting.
In 2020, scientists announced that Borexino, an underground detector in Italy, had observed these CNO neutrinos (SN: 6/24/20Now Ludova and her colleagues have recorded enough neutrinos to calculate that carbon and nitrogen atoms together are 0.06 percent as abundant as hydrogen atoms in the Sun – the first use of neutrinos to determine the Sun’s makeup. .
And although this number seems small, it is even higher than the one favored by astronomers who favor a high-metal Sun. And this is 70 percent more than the number of low-metal Suns.
“This is a great result,” says Mark Pinsnault, an astronomer at The Ohio State University in Columbus who has long advocated a high-metal sun. “They have been able to demonstrate convincingly that the current low-metal solution is inconsistent with the data.”
Still, because of uncertainties in both the observed and predicted neutrino numbers, Borexino cannot completely rule out a less metallic sun, Ludova says.
The new work is “a significant improvement”, says Gail Bulgen, an astrophysicist at the University of Geneva in Switzerland who favors a sun with less metal. But the estimated number of CNO neutrinos comes from a model of the Sun which he criticizes as too simplistic. Those models ignore the Sun’s spin, which can induce mixing of chemical elements in its life and alter the amount of carbon, nitrogen and oxygen near the Sun’s center, changing the estimated number of CNO neutrinos. , Buldgen says.
Additional neutrino observations are needed for a final verdict, Ludova says. Borexino shuts down in 2021, but future experiments could fill the void.
the stakes are high. “We are arguing about what the universe is made of,” Pinsnault says, because “the Sun is the benchmark for all our studies.”
So if there is a lot more carbon, nitrogen and oxygen in the Sun than it currently is, then so is the whole universe. “It changes our understanding of how chemical elements are made. It changes our understanding of how stars evolve and how they live and die,” Pinsnault says. And, he says, it’s a reminder that even the most well-studied star — our Sun — still holds mysteries.