Galaxy cluster Abell 2125, Chandra X-ray image. Situated 3 billion light years away in the constellation Ursa Minor, Abell 2125 is in the early stages of building a massive galaxy cluster. The large bright gas cloud at the top left is the core of the cluster, enveloping hundreds of galaxies. It has a temperature of millions of degrees Celsius and is enriched with heavy elements such as iron, which have been stripped from the galaxies as they fall through the gas. It will eventually merge with the gas clouds to the bottom right, to form a massive cluster.