Galaxy cluster Abell 2261, Hubble Space Telescope (HST) image. Combined optical and infrared image of the galaxy cluster Abell 2261. Galaxy clusters are gravitationally bound groupings of galaxies. They can contain hundreds of galaxies and be millions of light years across. Abell 2261 is 3 billion light years from Earth, in the constellation of Hercules. Image data obtained with the HST's Advanced Camera for Surveys (ACS) and Wide Field Camera 3 (WFC3) instruments. Image published in 2012.