For ten straight days at the end of 1995, the Hubble telescope snapped image after image of the same point in the sky--a point that was selected because it contained little in the way of visible stars or galaxies. It might seem strange to spend so much time staring at next to nothing near the handle of the Big Dipper, but in early 1996 astronomers unveiled what they were up to. The 342 separate pictures had been stacked up and added together to create a long-exposure image called the Hubble Deep Field. In that seemingly empty field, it turns out, there are more than 1,500 distant galaxies, most billions of times fainter than the naked eye can see.
At first glance, the whole thing is a jumble of swirls and splashes. But astronomers are working to determine the age and distance of all the galaxies, which will allow them to put the galaxies in a chronological order. The hope is that they will be able to use these snapshots from the lives of a thousand galaxies to piece together the evolution of one typical galaxy. According to Harry Ferguson of the Space Telescope Science Institute in Baltimore, one of the biggest surprises so far is the relative dearth of fully formed galaxies in the earliest universe. That suggests to some researchers that a large spiral galaxy like our Milky Way may have come together only 9 billion years or so ago, after many of its stars had already formed. That’s later than most people would have thought, Ferguson says. Nine billion years doesn’t leave a whole lot of time to build something like the Milky Way.