Neutrinos reveal final secret of Sun’s nuclear fusion

The theories for why the Sun shines date back to the 1930s, but one crucial bit of evidence was missing. Now, physicists have found direct evidence for a type of reaction, involving carbon and nitrogen nuclei, in which four protons fuse into a helium nucleus. The smoking gun? Neutrinos, elementary particles the reaction releases. These zip straight out from the Sun’s core, reaching Earth just 8 minutes later. The carbon–nitrogen pathway is not the only type of fusion in the Sun, and it’s not even the main one — neutrinos from all the other reactions had been detected before — but it is thought to be the dominant energy source for larger stars.