This photo supplied by NASA-ESA on Wednesday, May 2, 2007, shows a Hubble Space Telescope image of a dense swarm of stars in the central region of the globular cluster NGC 2808. Astronomers were surprised when Hubble spied three generations of cluster stars. For decades, astronomers thought that cluster stars formed at the same time, in the same place, and from the same material, and have co-evolved for billions of years. Scientists now believe that they can go through several periods of intense stellar formation rather than the previously accepted single burst. Globular clusters are among the earliest settlers of our Milky Way Galaxy, born during our galaxy's formation. The stars were born within 200 million years very early in the life of the 12.5-billion-year-old massive cluster. Clusters are compact swarms of typically hundreds of thousands of stars held together by gravity. Of the about 150 known globular clusters in our Milky Way Galaxy, NGC 2808 is one of the most massive, containing more than 1 million stars.