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Astronomers have made the first definitive detection of a radioactive molecule in interstellar space: a form, or isotopologue of aluminum monofluoride (26AlF). The new data — made with ALMA and the NOEMA radio telescopes — reveal that this radioactive isotopologue was ejected into space by the collision of two stars, a tremendously rare cosmic event that was witnessed on Earth as a ‘new star,’ or nova, in the year 1670.