Throughout most of a galaxy, stars are sprinkled like farmhouses on an open plain. Separated by vast distances, they lead their lives almost independently of one another. Some areas of a galaxy, though, look more like cities than countryside. These are globular clusters--groups of a million or so stars crammed into a volume that elsewhere would accommodate barely a single star. Not only are they congested, they are aged. The roughly 200 globular clusters in our Milky Way galaxy contain some of the oldest known stars in the universe; young inhabitants are nowhere to be seen. Accordingly, astronomers have generally seen globulars as ancient cities, like the historic districts of Rome or Istanbul, formed long ago and little changed--cramped, worn-out municipalities that tell us much about bygone times but little about the way modern galaxies organize themselves.

At least that is what astronomers used to think. Lately they have been busy rewriting the galactic charts (and textbooks). With the penetrating vision of the Hubble Space Telescope, they have seen the lights of new burgs--newfound cities full of bustling activity. Globulars can and do continue to form, apparently when galaxies smash into one another. These discoveries give researchers a handle on key questions in astronomy, such as how and when massive galaxies originate and evolve.

This article was originally published with the title The Unexpected Youth of Globular Clusters.

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