Figure 14-21 X-ray Emission from a Cluster of Galaxies (a) An X-ray image of this cluster of galaxies shows emission from hot gas between the galaxies. The gas was heated by collisions between galaxies within the cluster. (b) The galaxies themselves are too dim at X-ray wavelengths to be seen in (a), but are apparent at visible wavelengths. This cluster, one of many cataloged by the UCLA astronomer George O. Abell, is about 1 billion ly from Earth in the constellation Serpens. (a: NASA, CXC, and University of California, Irvine/A. Lewis et al.; b: Palomar Observatory DSS)