In 1670, a Carthusian monk named Père Dom Anthelme discovered a “new star,” or nova, near the constellation Cygnus, pointing out to his fellow monks a star that did not appear on maps of the sky. Now, as astronomers report online today in Nature, this nova, CK Vulpeculae, had an unusual cause: The explosion probably occurred when two stars orbiting each other spiraled together and merged into one. New observations reveal molecules in the gas surrounding the merged star (white and yellow show the brightest glow at visible wavelengths; green contours indicate carbon monoxide gas). The molecules contain lots of isotopes that arise during nuclear reactions, so they likely spilled out of the stellar interiors when the stars joined together. Astronomers have recently discovered that rare “red novae”—named for their color—result when stars merge; now the aftermath of the 17th century nova indicates what such stellar mergers look like centuries later.