Neutrinos are very light particles with no electric charge. They are emitted when unstable atomic nuclei decay. Such radioactive decay happens all around us. Minerals in rocks and even potassium in bananas decay to produce neutrinos. But far more neutrinos come from the nuclear reactions in the sun than from any other source.
No matter what their source, neutrinos are completely harmless because they do not stick together or interact with much of anything. They stream from the sun and outer space and pass right through ordinary matter—even our own bodies and Earth itself—without us ever knowing. In that way, they seem a bit like ghosts—there, and yet invisible. But because neutrinos carry information about what is happening inside the hearts of stars and galaxies, scientists want to know a lot more about them. Neutrinos may even help us understand what was going on when our universe first formed nearly 14 billion years ago!

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