* Astronomy

Astronomers have found the astronomical equivalent of prehistoric life in our intergalactic backyard: a group of small, ancient galaxies that has waited 10 billion years to come together. These "late bloomers" are on their way to building a large elliptical galaxy. Such encounters between dwarf galaxies are normally seen billions of light-years away and therefore occurred billions of years ago. But these galaxies, members of Hickson Compact Group 31, are relatively nearby, only 166 million light-years away. New images of these galaxies by NASA's Hubble Space Telescope offer a window into what commonly happened in the universe's formative years when large galaxies were created from smaller building blocks. The Hubble observations have added important clues to the story of this interacting foursome, allowing astronomers to determine when the encounter began and to predict a future merger. Astronomers know the system has been around for a while, because the oldest stars in a few of its ancient globular clusters are about 10 billion years old. The encounter, though, has been going on for about a few hundred million years, the blink of an eye in cosmic history. Everywhere the astronomers looked in this compact group they found batches of infant star clusters and regions brimming with star birth. Hubble reveals that the brightest clusters, hefty groups each holding at least 100,000 stars, are less than 10 million years old. The entire system is rich in hydrogen gas, the stuff of which stars are made. Astronomers used Hubble's Advanced Camera for Surveys to resolve the youngest and brightest of those clusters, which allowed them to calculate the clusters' ages, trace the star-formation history, and determine that the galaxies are undergoing the final stages of galaxy assembly.

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