RADIO ASTRONOMY — RATCLIFFE 181 



that an astronomical source of radiation which subtends a very small 

 angle would be received with periodically increasing and decreasing 

 intensity. If the source subtends an angle comparable with the angle 

 between two maxima, then it will always overlap one or other of them 

 and the response will not vary much. If there is a small concentrated 

 source, superposed on a general background of diffuse radiation, it 

 will produce an oscillating trace, whereas the diffuse radiation will 

 produce a steady trace. Figure 2 shows the type of record obtained 

 in practice. It indicates the presence of a general background of 

 diffuse radiation the intensity of which varies with time as the dif- 

 ferent parts of the sky come under observation, with two strong 

 isolated sources which produce the two oscillating traces at times 

 when the receptivity diagram is swept past them by the movement 

 of the earth. 



1600 1700 1800 1900 2000 



UT 



Figure 2. — Record of two isolated sources of radiation against a background of diffuse 

 radiation. From Ryle, M., Reports on Progress in Physics, vol. 13, 1950. 



We now turn to examine some of the results which have been 

 obtained by the observation of astronomical sources. Records of 

 the type shown in figure 2 have indicated the existence of a large 

 number of discrete sources of radiation distributed throughout the sky 

 and have enabled their positions to be fixed within the angular 

 accuracy appropriate to the apparatus used. 



The discrete sources of radiation which have been detected in this 

 way have been called radio stars. Their intensities cover a wide 

 range and the strongest are so intense that at a wavelength of 5 m. 

 two are stronger than the sun. It is interesting to inquire whether 

 the radio stars are, in fact, also visible stars, but in asking this question 

 we must remember that, on account of the limited angular accuracy 

 of the radio telescopes, it is not possible to locate the radio stars as 

 accurately as the visible one. Within the limits of ±14 minute of 

 arc, to which any radio star can be located, there are always a large 

 number of visible stars. It seems clear, however, that the most intense 

 radio stars do not coincide with the most intense visible stars, and that 

 no visible star of magnitude greater than 12 lies in the region ascribed 

 to any one of the four strongest radio stars. It appears that the in- 

 tensity of radio stars is not related simply to the intensity of the 

 light which they emit. 



