Born in Norman, Oklahoma, in 1905, Karl Jansky attended the University of Wisconsin. After graduation in 1928, he joined the Bell Telephone Laboratories in New Jersey. His work there dealt with the problems of the shortwave radio telephone. In 1931 he was assigned the task of tracking down the crackling static noises that plagued overseas telephone reception. At the Holmdel station he constructed a large directional antenna system.
Jansky recorded two well-known kinds of atmospheric static: crashes from local thunderstorms and noise from distant thunderstorms reflected from the ionosphere. From his records he later singled out a weak third kind of static that could hardly be distinguished from the internal receiver noise. Through headphones the weak noise sounded like a steady hissing. At first Jansky thought that the interference came from the Sun, but after a year of careful measurement he concluded that the radio waves came from a specific region on the sky every 23 hours and 56 minutes. Suspecting that the radiation was coming from an astronomical source, he attempted to trace its origin. He knew from his study of astronomy that the period of the Earth's rotation relative to the stars was 4 minutes less than its 24-hour period relative to the Sun, and this was the clue that the radio noise originated in space beyond the Solar System. He found that its direction coincided with the constellation Sagittarius toward the center of the Milky Way. In the publication of his work he made two very perceptive comments: He suggested that the radio emission was somehow connected to the Milky Way and that it originated not from stars but from ionized interstellar gas. (This is not the same thing as 21-cm radiation from atomic hydrogen.)
At the age of 26, Jansky had made a historic discovery--that celestial bodies could emit radio waves as well as light waves. But his results, published first in 1932, received little attention. Not until the end of World War II was the significance of his achievement widely appreciated.
Jansky's serendipitous discovery gave birth to a new branch of astronomy, radio astronomy. In Jansky's honor, astronomers named the unit of radio flux the jansky (= 10-26 W/m2.Hz).