Three generations of stars in NGC 2808
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 thousands of millions of years.
Each point in this colour-brightness 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, with the brightest stars near the top. The horizontal axis represents the colours of the stars (blue magnitude minus near-infrared magnitude), with bluer stars to the left and redder to the right.
The three coloured curved lines, red, green and blue, represent the three different stellar generations that were discovered to be present in the globular cluster.