Figure 14-16 Galaxies Forming by Combining Smaller Units (a) This image indicates how astronomers visualize the burst of star formation that occurred within a few hundred million years after the Big Bang. The arcs and irregular circles represent interstellar gas illuminated by supernovae. (b) Using the Hubble and Keck telescopes, astronomers discovered two groups of stars (arrows) 13.4 billion light-years away that are believed to be protogalaxies, from which full-grown galaxies form. These protogalaxies were discovered because they were enlarged by the gravitational lensing of an intervening cluster of galaxies. (c) The Chandra X-ray Observatory imaged gravitationally bound gas around the distant galaxy 3C 294. The X-ray emission from this gas is the signature of an extremely massive cluster of galaxies, in this case, at a distance of about 11.2 billion light-years from us.