In 1937, an astronomically inclined electrical engineer named Grote Reber built a homemade radio dish out of lumber and sheet metal in his backyard in Wheaton, Illinois. The following year, he confirmed Karl Jansky’s 1931 discovery of radio waves emanating from the center of the Milky Way.
Over the next few years, Reber extended his research of what he called “cosmic static,” eventually publishing sky maps showing prominent radio sources in the constellations Cassiopeia and Cygnus. The first, called Cassiopeia A, is the brightest extrasolar radio source in the sky, now known to be a young supernova remnant.
The other source, Cygnus A, is a galaxy more than 750 million light-years away. Reber had made the first observation of a black hole, one more than 2 billion times the mass of the Sun. Astronomers now think such enormous “supermassive” black holes reside in the centers of most big galaxies, including our own. Recently, observations of Cygnus A using the Very Large Array in New Mexico revealed another bright radio source — possibly another giant black hole — near the galaxy’s center.
Our growing understanding of black holes has unfolded along three distinct but related tracks: first through efforts to understand gravity itself, then explanations of how so-called active galaxies like Cygnus A could emit such vast amounts of energy, and finally the discovery of small stellar-mass black holes in our home galaxy and beyond.
Although the term black hole wasn’t coined until 1967, when American physicist John A. Wheeler first mentioned it in a talk, the quest for these enigmatic objects began much earlier.
In May 1783, the English scientist-turned-clergyman John Michell envisioned a star so large that its escape velocity equaled the speed of light. He wrote that “all light emitted from such a body would be made to return towards it, by its own proper gravity,” rendering the star invisible to astronomers. Although undetectable by its own light, he noted, the object could be found by observing irregularities in any “luminous bodies” that happened to revolve around it. Next, France’s Pierre-Simon Laplace independently postulated the existence of black holes in a book published in 1796.