288 ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 7 



cm. This emission line is produced by the neutral hydrogen atom. 

 If the spins of the proton and electron are aligned in the same direc- 

 tion there is a tendency for one of the spins to change. The proba- 

 bility of the change is very low so that a hydrogen atom waits several 

 million years before changing. At this time it emits 9.4 X10~ 25 

 joules at a frequency of 1,420 mc. Although this seems an insignifi- 

 cant power output, the number of atoms in the direction of the an- 

 tenna beam is usually sufficient to give a detectable signal. The signal 

 strength gives a measure of the temperature and space density of 

 the hydrogen, but, what is more important, the exact frequency of 

 the emission gives the velocity in the line of sight. As in the case of 

 sound waves and light waves, the observed frequency of a source is 

 higher when it is approaching and lower when it is receding so that 

 the velocity of the source can be found. By measuring the velocity 

 of the hydrogen with respect to the sun the astronomer is able to go 

 one step farther. The galaxy is rotating about its center and each 

 star follows an orbit which is nearly circular. Stars on the edge of 

 the galaxy travel more slowly than stars near the center. Hence a 

 measure of velocity gives a measure of the distance of a hydrogen 

 cloud from the galactic center and the position of hydrogen in space 

 can be deduced. 



Extensive surveys at 1,420 mc. have been made. It is found that the 

 neutral hydrogen is concentrated within the spiral arms of our galaxy. 

 By means of the hydrogen emission these arms may be traced out far 

 beyond the optical limit which is set by interstellar absorption. For 

 the first time we can picture the sun as it is set in one arm of a great 

 spiral system as shown in figure 4. 



The hydrogen line has been detected in other galaxies besides our 

 own. Recently emission was received from the great cluster of galax- 

 ies in Coma Berenices at a frequency of 1,387 mc. Thus the radio 

 signal is at a lower frequency, or reddened, by the velocity of re- 

 cession of the cluster in the same way that the visible spectrum is 

 shifted. Absorption by hydrogen has also been noted in the noise 

 from the colliding galaxies in Cygnus. Again there is a shift of 

 the radio line which corresponds to the red shift observed optically. 



JUPITER 



It is scarcely a year since the radio signals from Jupiter were dis- 

 covered. Many tape recordings have already been made which illus- 

 trate the effects that this planet can produce. There are components 

 of the hissing sound which are usually associated with the random 

 motion of thermal electrons. It is unlikely that the noise is really 

 thermal in origin because it is difficult to visualize how high tem- 

 peratures could be produced on Jupiter. The atmosphere is com- 



