At the beginning of the twenty-first century, quasi-stellar radio sources (quasars) were the most distant cosmic objects observed by astronomers. Although not visible to the naked eye, quasars were also among the most energetic of cosmic phenomena. Although some quasars may be physically smaller in size than our own solar system some quasars are calculated to be brighter than hundreds of galaxies combined. Quasars and active galaxies appear to be related phenomena each associated with massive rotating black holes in their central region. As a type of active galaxy, the enormous energy output of quasars can be explained using the theory of general relativity.

The great distance of quasars means that the light we observe coming from them was produced when the universe was very young. Because of the finite speed of light, large cosmic distances translate to looking back in time. The observation of quasars at large...