1931

Radio Astronomy

Karl Guthe Jansky (1905–1950)

Young Karl Guthe Jansky grew up in Oklahoma surrounded by physics and radio science: his father was a professor of electrical engineering and dean of the engineering college at the university in Norman, and his older brother was a radio engineer. It was no wonder that he went to college, majored in physics, and in 1928 took a job with the fledgling Bell Telephone Laboratories, the relatively new research arm of Alexander Graham Bell’s original American Telephone & Telegraph Company (AT&T).

At Bell Labs, Jansky studied the problem of how noise and static might interfere with possible long-distance transatlantic radio telephone service. He needed a way to monitor the intensity and direction of noise sources, so he built a radio telescope, 100 feet (30 meters) long, steerable on a set of four Ford Model T tires. The telescope could detect radio signals with a wavelength half its length, or a frequency around 20.5 megahertz (MHz).

In the summer of 1931 Jansky began “observing” with his radio telescope. He was successful in finding sources of background static, detecting radio signals from nearby and distant thunderstorms as well as a faint and relatively steady hiss that he couldn’t initially identify. Over time he found that the strength of the signal varied with a period of 23 hours and 56 minutes—the exact duration of the sidereal day (the time it takes the Earth to spin on its axis relative to the fixed stars). He found that the hiss was most intense when looking toward the constellation Sagittarius, and specifically toward the region that astronomers had identified as the center of the Milky Way galaxy.

Karl Jansky had basically just invented radio astronomy. He had discovered what 40 years later was identified as the intense radio (along with X-ray and infrared) emission from Sagittarius A* (pronounced “A star”), a region thought to contain a 40-million-solar-mass black hole at the center of our galaxy. Jansky was not an astronomer, but his initiative, skill, and creativity with the world’s first radio telescope inspired a new kind of astronomy, and a new way of seeing and studying the cosmos.

The large radio antenna built in New Jersey in 1931 by Bell Labs radio engineer Karl Jansky to search for radio frequency noise. The antenna was 100 feet (30 meters) long and 20 feet (6 meters) high, and could rotate a full 360 degrees, earning it the nickname “Jansky’s merry-go-round.”