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Scientists have long focused on uncovering the secrets of the sun, moon, planets, and stars, and according to their latest discoveries, the sun works, like all stars, by fusing hydrogen into heavier elements. Not only does nuclear fusion make 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 closest star, there is another source: the neutrinos created in the core of the Sun.
And when atomic nuclei fuse, they produce not only high-energy gamma rays, but neutrinos as well. As gamma rays heat the interior of the sun for thousands of years, neutrinos leave the sun almost at 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 the theory, the predominant form of fusion in the Sun should be the fusion of protons, which produce helium from hydrogen. It’s known as the pp chain and it’s the easiest reaction stars create.
For larger stars with hotter and denser cores, the most 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.
The CNO cycle is part of why these three elements are among the most abundant in the universe (besides hydrogen and helium).
And in the last decade, neutrino detectors have become more efficient. Modern detectors can also detect not only the energy of the neutrino, but also its base.
We now know that the solar neutrinos detected in the early experiments do not come from common PP chain neutrinos, but from side reactions such as boron decay, which create high-energy and easily detectable neutrinos.
Then in 2014 my research team discovered low energy neutrinos produced directly by the pp chain. Their observations confirmed that 99% of the Sun’s energy is generated by proton-proton fusion.
And although the pp chain dominates the fusion in the Sun, our star is large enough that the CNO cycle must occur at a low level. It should be what represents the additional 1% of the energy produced by the sun.
But because CNO neutrinos are so rare, they are difficult to detect. But recently researchers have noticed it with success.
One of the biggest challenges in discovering CNO neutrinos is that their signals tend to get buried within the noise of terrestrial neutrinos. Nuclear fusion does not occur naturally on Earth, but low levels of radioactive decay of Earth’s rocks can cause events in the neutrino detector similar to CNO detections.
So the team created a complex analysis process that filters the neutrino signal from false positives. Their study confirms that CNO fusion is occurring within our sun at expected levels.
The CNO cycle plays a minor role in our Sun, but it is essential for the life and development of more massive stars.
This work should help us understand the cycle of large stars and could help us better understand the origin of the heavier elements that make life on Earth possible, says ScienceAlert.
