Neutrinos have proven that our Sun is undergoing another type of fusion in its core

Like all stars, our Sun is driven by the fusion of hydrogen into heavier elements. Nuclear fusion not only makes stars shiny, but is also a primary source of the chemical elements that make up the world around us.

Much of our understanding of stellar fusion comes from theoretical models of nuclear nuclei, but for our closest star, we also have another source: neutrinos built into the Sun’s core.

Whenever atomic nuclei undergo fusion, they produce not only high energy gamma rays but also neutrinos. While gamma rays heat the interior of the sun over thousands of years, neutrinos exit the sun at the speed of light.

Solar neutrinos were first discovered in the 1960s, but it was difficult to learn much more than the fact that they were emitted from the Sun. This proved that nuclear fusion occurs in the Sun, but not the type of fusion.

According to the theory, the predominant form of fusion in the Sun should be the fusion of protons forming helium from hydrogen. Known as PP-chain, it is the easiest reaction for stars.

For larger stars with hotter and more dense cores, a more powerful reaction called the CNO-cycle is the major source of energy. This reaction uses hydrogen in a cycle of reactions with carbon, nitrogen and oxygen to produce helium.

The CNO cycle is part of the reason that these three elements are the most abundant in the universe (apart from hydrogen and helium).

The CNO cycle kicks at high temperatures.  (RJ Hall)The CNO cycle kicks at high temperatures. (RJ Hall)

In the last decade, neutrino detectors have become very efficient. Modern detectors are also able to detect not only the energy of a neutrino, but its taste as well.

We now know that early experiments detect solar neutrinos not from common PP-chain neutrinos, but from secondary reactions such as boron decay, which produce high-energy neutrinas that are easy to detect.

Then in 2014, a team detected low-energy neutrinos produced directly by PP-chains. His observations confirmed that 99 percent of the Sun’s energy is generated by proton-proton fusion.

Energy levels of various solar neutrinos. (Heron / Brown University)

While PP-chains dominate fusion in the Sun, our star is large enough that the CNO cycle must be at a low level. It should be what accounts for that extra 1 percent of the energy produced by the Sun.

But because CNO neutrinos are rare, they are difficult to detect. But recently a team successfully celebrated him.

One of the biggest challenges with detecting CNO neutrinos is that their signal is buried within terrestrial neutrino noise. Nuclear fusion does not occur naturally on Earth, but low levels of radioactive decay from terrestrial rocks can trigger events in a neutrino detector that are similar to CNO neutrino detection.

So the team created a sophisticated analysis process that filters the neutrino signal from false positives. Their study confirms that CNO fusion occurs within our Sun at the predicted level.

The CNO cycle plays a small role in our Sun, but is central to the life and development of more massive stars.

This work should help us understand the circle of large stars, and can help us better understand the origin of the heavy elements that make life on Earth possible.

This article was originally published by Universe Today. Read the original article.


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