RADIO ASTRONOMY — RATCLIFFE 183 



Now let us inquire how these facts can be explained. We first con- 

 sider the radiation from the undisturbed sun, and note that it has all 

 the characteristics of the light which would be radiated in the complete 

 spectrum of electromagnetic waves from a hot body. The radiation 

 from a body at a high temperature would include an appreciable 

 amount of energy in the region of radio wavelengths, and if this radio 

 radiation were really a part of the continuous spectrum it would have 

 the characteristics of noise which are observed in the solar radiation. 

 If we assume that the radiation comes from an area slightly larger 

 than the solar disk (as shown by experiment) we can use the observed 

 intensity of the radiation to calculate the temperature of the region 

 from which it originates. When this calculation is performed, it is 

 found that the equivalent solar temperature is about one million 

 degrees, which is greater than that (6000° K.) deduced from observa- 

 tion of the visible light from the sun. Now the solar corona is supposed 

 to have a temperature of the order of one million degrees and to be 

 completely ionized, so that it consists of free electrons colliding, com- 

 paratively infrequently, with the positive nuclei of atoms. Calcula- 

 tion shows that it would radiate radio waves of the kind observed. 

 If this assumption is made, it is then possible to extend our knowledge 

 of the corona by observations of the radio waves. It is interesting to 

 note that because the corona extends considerably beyond the visible 

 disk of the sun we should expect the "radio sun" to be larger than the 

 visible one, as is, in fact, found. 



After having said that there is a reasonably good theory for the 

 emission of radio waves from the quiet sun, we now have to admit 

 that there is no correspondingly simple explanation of the enhanced 

 radiation from sunspots. 



When we turn to consider the galactic radiation also, it cannot be 

 said that any firm theory has yet been proposed. It is first natural to 

 ask whether a series of bodies like the sun, distributed throughout the 

 galaxy, could produce the observed radiation. In this connection we 

 note that, if all the visible stars emitted radio waves like the quiet 

 sun, the total radiation would only be about 10" 8 of that observed 

 from the galaxy. If they all emitted like the sun when it is most dis- 

 turbed, the total radiation would still fall short by a factor of about 

 10" 2 . But the great constancy of the radiation from each of the 50 or 

 so radio stars so far observed makes it seem unlikely that they emit by 

 any mechanism corresponding to that which occurs on the disturbed 

 sun. If we assume that the strongest observed radio star (that in the 

 constellation of Cassiopeia) is at the distance of the nearest visible 

 star, and that the total number and distribution of radio stars and of 

 visible stars are the same, then we could account for the total radiation 

 from the galaxy. If it is supposed that the whole of the galactic radia- 



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