Aurion Mission

Thursday, August 11, 2011

A Mysterious Hiss from the Milky Way (Part 1)

August 4, 2011
In the early 1930′s, Bell Labs, the research division of AT&T, wished to use radio “short waves” for transatlantic radio telephone links.  A young engineer, Karl Jansky, was assigned the job of finding sources of radio static that might interfere with radio transmissions.  During his work, he made an accidental discovery that revolutionized astronomy.

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To search for static that might interfere with radio transmission, Jansky built an antenna to detect radio waves at a frequency of 20.5 MHz (a wavelength of 14.5 meters). He mounted the rotatable antenna on four Ford Model-T tires to determine the direction of any radio signal he might find. The antenna was jokingly called “Jansky’s merry-go-round” (see image above).
After recording signals for several months, Jansky found static from nearby and distant thunderstorms.
But he also found a faint steady radio hiss of unknown origin. The intensity of the hiss rose and fell once a day. At first, he thought the unknown static might be radio waves from the Sun.
After a few months of following the signal, the brightest point moved away from the Sun. The signal repeated not every 24 hours, but every 23 hours and 56 minutes, the same period in which the stars rise and set.
Jansky eventually figured out the radiation was strongest in the direction of the center of our Milky Way galaxy, in the constellation of Sagittarius. The discovery of radio waves from the center of the galaxy was widely publicized, appearing in the New York Times on May 5, 1933.
Though he wanted to learn more about the radio waves, Jansky was assigned to another project by Bell Labs managers and did not pursue the subject further.
Many scientists were fascinated by Jansky’s discovery. But no one followed up on it for several years until a modest radio engineer from Chicago became the world’s first true radio astronomer.  We continue that story next week…
Footnote: In honor of Karl Jansky, the unit used by radio astronomers for the strength (or flux density) of radio sources is the Jansky.