Ten billion years ago or so, at least according to the current picture, the youthful universe began to produce an abundance of new stars. The very first stars appeared in the young cosmos after about only a few hundred million years, but they were odd. The universe then contained no elements other than hydrogen (and some helium), and so these first stars contained mostly hydrogen and were very massive. By the time a few billion years had passed, however, this first generation of stars had manufactured many other elements that enriched the natal gas; because these elements could cool the gas, they aided gravity as it collapsed material to form a wide range of new stars, many of them similar to the sun. After a few billion years, galaxies had acquired their mature forms and were populated by nearly normal stars -- and then, for reasons that are not understood, they began making stars at unprecedentedly high rates.
Astronomers gazing at distant galaxies find that those whose light has been traveling towards us for between nine and eleven billion years show dramatic increases in star formation, about ten times more active than what we see today, per unit volume. The evidence comes from these galaxies' luminosities and copious ultraviolet radiation, both earmarks of new stars. This general story has been known for about ten years, but there have always been questions about it because distant galaxies are faint and therefore hard to see, and so perhaps only the most unusual bright galaxies in this epoch are undergoing such starbursts.
SAO astronomer Dawn Erb -- part of a team with six other astronomers -- has published an important new paper that describes the most robust study yet of galaxies in this epoch. They considered over 15,000 galaxies in twenty-nine different parts of the sky, and complemented their multiwavelength data with a computer simulation of galaxies that was able to estimate the relative role of unseen, fainter galaxies. They find that the overall picture is in close agreement with the original, but that modest or low luminosity galaxies contribute as much as 80% of the total luminosity in this period. They also report that about one-third of present-day stars in the universe were formed during this time in galaxies of modest brightness. The results help to explain why our Milky Way galaxy looks the way it does, and to account for its evolutionary history.