Astronomers were surprised when Hubble spied three generations of cluster stars in globular cluster NGC 2808. The discovery is far different from the standard picture of a globular cluster. 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.

Each point in this graph represents one star in NGC 2808. The vertical axis represents the brightness (as measured through Hubble’s near-infrared F814W filter) of the stars (the brightest stars near the top). The horizontal axis represents the colours of the stars, with bluer stars to the left and redder to the right (blue magnitude minus near-infrared magnitude). The three coloured curved lines, red, green and blue, represent the three different stellar generations that are present in the globular cluster.

Credit: European Space Agency, NASA, G. Piotto (University of Padua, Italy), A. Sarajedini (University of Florida, USA) and Martin Kornmesser (ESA/Hubble)