The team notes that the emission lines could have come from a galaxy—possibly a primitive one that formed just a few hundred million years after the Big Bang—though such an object should have shown up in earlier observations. It may also be an active galactic nucleus, the bright center of a galaxy powered by a black hole, that’s in its dying phase. If that were the case, however, the astronomers should’ve caught other emission lines from the source. Alternatively, the light may have come from a short-lived event, like a star being shredded by a black hole, which would explain why nothing appeared in images taken about a year earlier. But all of these explanations come with their own problems.