Every second of every day, trillions of neutrinos enter your body. These ghostly particles almost never interact with other matter, though, so the odds are that in your entire lifetime, every single one will pass through you without a trace. Yet astronomers work hard to catch neutrinos because they’re the only direct probes of what’s happening in the center of the Sun. The Sun’s core is a giant nuclear reactor. Through a series of steps, four protons combine to form the nucleus of a helium atom. Two of the protons are converted to neutrons. Each conversion produces a positron, a gamma ray, and a neutrino — a particle with no electric charge and almost no mass. The positrons never make it out of the Sun’s core. And neither do the gamma rays — at least not directly. Instead, they’re converted to other forms of energy before they shine out into space — a process than can take hundreds of thousands of years. Since neutrinos almost never interact with other matter, though, they zip straight
