182 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1952 



In addition to the radio stars, there is also a general background of 

 radiation which cannot be resolved into discrete sources. The distri- 

 bution of intensity in this unresolved background is found to follow 

 closely the contours of the galaxy as determined from star mapa 

 It appears that the disklike assembly of visible stars which forms our 

 galaxy has associated with it sources of radio radiation which are 

 also distributed throughout this disk, so that we receive most radiations 

 from its plane, just as we see the Milky Way as a concentration of 

 stars in this plane. 



The question now arises whether the galactic radiation is, in fact, 

 smoothly distributed over the galaxy or whether it is the sum of 

 radiations from innumerable radio stars which have not yet been 

 resolved. The fact that the discrete sources so far discovered are 

 distributed roughly evenly in all directions is not significant, because 

 it is most likely that they represent the strongest, and therefore the 

 nearest, sources and they may all be nearer than the shortest dimension 

 of the galaxy. It has so far proved impossible to measure the parallax 

 of any radio star, because of the small resolving power of the tele- 

 scopes ; the most that can be said is that they are farther away than 

 1/20 parsec, which is considerably less than the smallest galactic di- 

 mension. The present evidence appears to be neither for nor against 

 the supposition that all the galactic radiation comes from discrete 

 radio stars which are at present unresolved. 



The fact that the radiation comes from the galactic plane, and 

 hence probably from sources throughout the galaxy, suggests that 

 our galaxy is probably also radiating outward into space. If this 

 were so, we might expect other galaxies to send some measurable 

 radiation to us. It has, in fact, been found that four very distant 

 galaxies, presumably similar to our own, fall within the somewhat 

 inaccurate directions found for four of the radio stars. 



We now turn to consider the radio radiation received from the sun. 

 It is found that when the sun is not appreciably disturbed by the 

 presence of sunspots, the intensity varies with the wavelength of the 

 radiation. This fact has an important theoretical significance which 

 will be discussed later. The distribution of emission across the 

 quiet solar disk has also been determined on several wavelengths, 

 and it is found that the "radio" sun is somewhat larger than the 

 "optical" sun and that, unlike the optical radiation, the radiation falls 

 off gradually near the edge. 



When the sun is known, from optical observation, to be disturbed, 

 the radio radiation is often considerably enhanced. The enhanced 

 radiation is very variable, and sometimes sudden bursts of radiation 

 are associated with solar flares. Experiments with special aerials have 

 shown that the sources of enhanced radiation coincide fairly accurately 

 with visible sunspots. 



