This NASA/ESA Hubble Space Telescope image of a dense swarm of stars shows the central region of the globular cluster NGC 2808. Of the 150 known globular clusters in the Milky Way, NGC 2808 is one of the largest.

Astronomers used Hubble’s Advanced Camera for Surveys to measure the brightness and colour of the cluster stars. Its resolution allowed them to sort out the different stellar populations.

The Hubble measurements showed three distinct populations, with each successive generation appearing slightly bluer. This colour difference suggests that successive generations contain different amounts of helium.

All the stars in NGC 2808 were born within 200 million years very early in the life of the 12.5-billion-year-old massive cluster. This is contrary to conventional ideas that propose that all the stars in a globular cluster originate from the same time and from the same material.

The images were taken in May 2005 and in August and November 2006.