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Our Sun, like all stars, works by fusing hydrogen into heavier elements. Nuclear fusion not only makes the stars shine, it is also a fundamental source of the chemical elements that make up the world around us.
Much of our understanding of stellar fusion comes from theoretical models of the atomic nucleus, but for our nearest star there is another source: the neutrinos created in the Sun’s nucleus.
And when atomic nuclei merge, they produce not only high-energy gamma rays, but neutrinos as well. As gamma rays heat the sun’s interior for thousands of years, neutrinos leave the sun at almost the speed of light.
Solar neutrinos were first discovered in the 1960s, but it was difficult to know much about them other than the fact that they were emitted by the sun.
According to theory, the predominant form of fusion in the Sun should be the fusion of protons, which produce helium from hydrogen. It is known as the pp chain, and it is the easiest reaction that stars create.
For larger stars with hotter, denser nuclei, the more powerful interaction known as the CNO cycle is the dominant energy source. This reaction uses hydrogen in a cycle of reactions with carbon, nitrogen and oxygen to produce helium.
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The CNO cycle is part of the reason why these three elements are among the most abundant in the universe (with the exception of hydrogen and helium).
And over the past decade, neutrino detectors have become more efficient. Modern detectors are also able to detect not only the energy of the neutrinos, but also its base.
We now know that the solar neutrinos detected from the first experiments do not come from common neutrinos in the PP chain, but from side reactions like the decay of boron, which create easily detectable high-energy neutrinos.
Then in 2014, my research team discovered low energy neutrinos directly produced by the pp chain. Their observations confirmed that 99% of solar energy is generated by proton-proton fusion.
And while the pp chain dominates the fusion in the Sun, our star is large enough that the CNO cycle must occur at a low level. This should be what represents the additional 1% of the energy produced by the sun.
But since CNO neutrinos are so rare, they are difficult to detect. But recently, researchers have noticed it successfully.
One of the biggest challenges in discovering CNO neutrinos is that their signals tend to be buried in the noise of ground neutrinos. Nuclear fusion does not occur naturally on Earth, but lower levels of radioactive decay of Earth’s rocks can lead to events in the neutrino detector similar to detections of CNO neutrinos.
Thus, the team created a complex analysis process that filters the neutrino signal from false positives. Their study confirms that CNO fusion takes place inside our sun at expected levels.
The CNO cycle plays a minor role in our sun, but it is fundamental for the life and development of more massive stars.
This work should help understand the cycle of large stars, and it could help us better understand the origin of the heavier elements that make life on Earth possible.
Source: ScienceAlert
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