Neutrinos prove the Sun is doing a second kind of fusion in its core - Universe Today

Like all stars, our Sun is powered by the fusion of hydrogen into heavier elements.

Nuclear fusion is not only what makes stars shine, it is also a primary source of the chemical elements that make the world around us.

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

Whenever atomic nuclei undergo fusion, they produce not only high energy gamma rays but also neutrinos.

Solar neutrinos were first detected in the 1960s, but it was difficult to learn much about them other 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.

For larger stars with hotter and more dense cores, a more powerful reaction known as the CNO-cycle is the dominant source of energy.

The CNO cycle is the reason why these three elements are among the most abundant in the universe (other than hydrogen and helium).

We now know that the solar neutrinos detected from early experiments come not from the common pp-chain neutrinos, but from secondary reactions such as boron decay, which create higher energy neutrinos that are easier to detect.

Then in 2014, a team detected low-energy neutrinos directly produced by the pp-chain.

While the pp-chain dominates fusion in the Sun, our star is large enough that the CNO cycle should occur at a low level.

It should be what accounts for that extra 1% of the energy produced by the Sun.

“Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun.” Nature 587 (2020): 577.