RADIO ASTRONOMY — HAWKINS 287 



back, they find that the ball was a single point in the year 1054, thus 

 confirming the identification. When the Crab nebula is photographed 

 in the red light of hydrogen, as in plate 2, figure 2, we notice a fila- 

 mentary structure and it is clear that the nebula is in a violent state 

 of motion. The expansion of gas again acts as an efficient generator 

 of radio waves, although the exact process is still obscure. Inter- 

 ferometer measurements show that the whole of the visible nebula 

 is transmitting, and the radio image fits almost exactly over the pho- 

 tographic image. Another supernova was observed by the famous 

 astronomer Tycho Brahe in 1572, and this too has been identified as 

 a radio star. The last supernova was recorded by Kepler in 1604, so 

 that if the estimated mean rate of one supernova every 500 years 

 or so is correct, there is a high probability that a supernova will 

 occur in our time. This would present a unique opportunity for 

 studying the entire process with all -the superb equipment available 

 to the modern scientist. 



These three sources in Cassiopeia, Cygnus, and Taurus are among 

 the few radio stars to have been positively identified. Recently a 

 catalog of over 1,900 radio stars was made and the astronomical 

 nature of most of them is still unknown. Much research will obvi- 

 ously be required before this mounting list of mysteries can be solved. 



THE MILKY WAY 



The original observations of Jansky in 1932 were made on the Milky 

 Way, our local galaxy. Radio interference was found which seemed 

 to be coming from the galactic center. Surveys of the sky have since 

 been made in great detail with wavelengths ranging from a few 

 centimeters up to many meters. The radiation comes from a large 

 elliptical area which is aligned with the general direction of the Milky 

 Way. Optically there are dark obscuring clouds or lanes of dust, but 

 these do not appear on the radio maps because the radio waves pass 

 through them. Dark clouds obscure the center of the galaxy, which 

 is probably the most interesting part, but this region is easily visible 

 to the radio astronomer. So far very little research has been done 

 on the galactic center and this remains an exciting field for the 

 future. 



It is not known yet whether the general galactic noise is the com- 

 bined signal from millions of radio stars or whether it originates 

 in the matter between the stars. In a few years time, when large radio 

 telescopes are available, it may be possible to see if myriads of faint 

 stars are producing the noise. Meanwhile much speculation goes 

 on as to the exact origin of the signals. 



There is one component in the radio spectrum, however, that is well 

 understood. Radiation has been detected over a small waveband at 21 



