Although we are not sure exactly when the first stars began to shine, we know that they must have formed sometime after the era of Recombination, when hydrogen and helium atoms formed (380,000 years after the big bang), and before the oldest-known galaxies existed (400 million years after the big bang). The ultraviolet light emitted by the first stars broke down the neutral hydrogen gas filling the universe into hydrogen ions and free electrons, initiating the era of Reionization and the end of the Dark Ages of the universe.
