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The Galactic Background Radiation

The ever-present sound of our galaxy 

by Dr. Leonard N. Garcia

When making observations of Jupiter you may hear all kinds of radio frequency interference. Most of the interference will likely be of terrestrial origin and may be natural, like distant lightning strikes, or man-made like power line "buzz" or radio stations. There is however a type of interference that will be inescapable and doesn't come from anything on Earth. It is the ever-present galactic background radio noise. It can always be heard but not always at the same strength. The discovery of the origin of this background noise is usually marked as the birth of radio astronomy. 

Karl Jansky, an engineer working for Bell Telephone Labs, was assigned the task of locating sources of interference in long distance radio-telephone communications. He built in 1931 in Holmdel, New Jersey an antenna operating at 20.5 Mega Hertz which rotated horizontally like a merry-go-round. The rotation of the antenna would allow him to determine the direction from which this interference was coming. Jansky heard interference from local lightning storms, interference from distant lightning storms and a third type of interference, "... a steady hiss type static of unknown origin." The direction of this third type of interference gradually changed over the course of a day moving nearly 360 degrees over 24 hours. After months of careful study of these records Jansky concluded as he reports in a paper published in 1933, "the direction of arrival of these waves is fixed in space, i.e., the waves come from some source outside the solar system". His approximate coordinates for the peak in these radio waves was in the constellation of Sagittarius, towards the direction of the center of the Milky Way Galaxy. 

On a clear night away from city lights the Milky Way appears to us as a fuzzy band of light arching across the sky.  The Moon, the Sun and the planets tend to follow a path which intersects this band of light at two points. When Jupiter happens to cross the Milky Way at one of these points especially the point that lies closer to the center of the Galaxy we tend to hear an increase in the galactic background level when we point our antennas towards Jupiter. The figure below shows a plot of the average galactic background levels heard during observations of Jupiter at the University of Florida Radio Observatory. The peaks in the figure in 1972 and 1984 are when Jupiter was in the vicinity of the direction towards the center of the Galaxy. The spacing of 12 years between peaks corresponds to the orbital period of Jupiter. The smaller peaks seen in 1977-78 and 1990 correspond to times when Jupiter was crossing the galactic plane far from the direction of the center. The galactic noise is understood as coming from high speed electrons spiraling around the weak magnetic fields which permeate our galaxy.
Left: A plot of the galactic background antenna temperature (proportional to brightness) at 18, 20, and 22 MHz using Yagi antennas at the University of Florida Radio Observatory.
To learn more
Radio JOVE Multimedia Exhibits - To hear for yourself what the Galactic Background sounds like. Narrated by Richard Flagg. 

Ham radio and radio astronomy- The role amateur radio operators "hams" have played in the development of radio astronomy.

Kraus, John D., Radio Astronomy, Cygnus-Quasar Books, 1988.

Smith, A. G. and T. D. Carr, Radio Exploration of the Planetary System, Van Nostrand Co., 1964.


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