An astronomer by the name of Cecilia Payne-Gaposchkin figured out that stars everywhere are all made up of only hydrogen with small amounts of helium and other chemical elements. We have been trying to figure out how the centers of stars work given this basic set of ingredients ever since.
A star shines by converting its hydrogen into other elements by nuclear fusion. For nearly all stars, four hydrogen atoms come close enough together in the hot, pressurized centers of a star to make one helium atom plus energy plus a tiny particle called a neutrino. The energy that is released explains why stars are so hot. In addition to this energy which we can feel through our skin, each hydrogen fusion reaction also produces a neutrino which is so small that we cannot feel it. Neutrinos actually pass straight through the Earth and even through us without their presence even being noticed. For a while it was thought that neutrinos were massless. With great effort one can detect neutrinos, which is done by setting large tanks of water deep underground away from the direct sunlight and waiting for a rare direct collision of a neutrino with a molecule of water. But how many neutrinos should there be? Scientists work out the problem backwards by asking what is the rate of nuclear reactions every second in the Sun’s core needed to produce the energy emitted from the Sun’s surface every second? According to the best computations, there should be 700 million tons of hydrogen being converted into helium every second in the center of the Sun. When the water tank detectors were first set up, somewhat surprisingly only 1/3 the expected number of neutrinos were found. Hmm - there can be only one of two explanations possible: either Payne-Gaposchkin’s idea is not correct, or we are just not very good at finding the elusive neutrino particles. The engineers set out to double check their sophisticated detectors, and the scientists investigated the possible subtleties of the problem. The nuclear physicists realized that one reason we may be detecting exactly 1/3 of the predicted number of neutrinos is if the remaining 2/3 of the population could disguise themselves en route to Earth. What if neutrinos interacted with each other, exchanged energies with each other, such that after a while they would be unrecognizable? The scientists estimated the properties of the two other possible types or flavors of neutrinos in detail and passed along this information to the engineers. To make a 30-year story short, in the end all three neutrino flavors were detected! We had unmasked the mysterious 2/3 of the neutrinos that were not found originally because they were of different flavors. Now we know not only what a star is made of, and also exactly what happens in a stellar core each second of its life. A Nobel Prize in Physics was awarded for solving the Neutrino Problem.