View larger image | Researchers hope to reconstruct the star-formation process during the first billion years of cosmic history by measuring the brightness of 21-centimeter radio waves emitted by hydrogen relative to the cosmic microwave background (chart, bottom). Holes should appear (top, left to right) where radiation emitted by stars ionized hydrogen atoms, stripping electrons. As more stars formed, the holes would grow and merge, leaving the universe completely ionized as it is today. By seeing how quickly ionization proceeded, scientists hope to learn whether the first stars formed singly or in multiples. Credit: Texas Advanced Computing Center; J. Pritchard & A. Loeb/Nature 2010