The massive galaxy cluster Abell 2744, nicknamed Pandora's Cluster, takes on a ghostly look in this Hubble Space Telescope view where the total starlight from the cluster has been artificially colored blue. This plot reveals that not all the starlight is contained within the cities of stars – the galaxies – which appear as bright blue-white blobs. A fraction of the starlight is dispersed throughout the cluster, as seen in the darker blue regions. This light comes from "dead" galaxies. The galaxies were torn apart long ago by the cluster's gravitational forces, and their stars were scattered into "intracluster" space – the space between the galaxies. These orphaned stars roam the cluster, without being gravitationally tethered to any single galaxy. Because these extremely faint stars are brightest at near-infrared wavelengths of light, this type of observation could only be accomplished with Hubble's infrared sensitivity to extraordinarily dim light.
The galaxies that are not colored blue lay either in the foreground or background and are not part of the cluster.
The intracluster light was detected as part of the Frontier Fields program, an ambitious three-year effort, begun in 2013, that teams Hubble with NASA's other Great Observatories – the Spitzer Space Telescope and the Chandra X-ray Observatory – to probe the early universe by studying large galaxy clusters. Abell 2744 resides about 3.5 billion light-years away.