RADIO ASTRONOMY — HAWKINS 285 



spot. The cloud is invisible optically, but on radio wavelengths it 

 shows temperatures of billions of degrees. The enhancement of radio 

 emission may continue for several days, and during periods of sun- 

 spot activity noise storms occur once every five days on the average. 

 If the sun were to behave in the visible spectrum as it does at radio 

 wavelengths the world would have been burnt to a cinder long ago. 

 One of the most spectacular phenomena is the noise outburst which 

 occurs after a solar flare. The flare is usually accompanied by an up- 

 ward surge of hot gas which leaves the chromosphere with a velocity 

 of about 100 km. per second and then falls back again into the sun. 

 An intense radio source, associated with the surge, moves outward 

 with a velocity of the order of 2,000 km. per second. This movement 

 has been followed in a number of surges with the rapid scanning in- 

 terferometer and there is evidence that the radio source does not fall 

 back again but leaves the sun completely as a corpuscular stream of 

 electrons and positive ions. As the stream forces its way through the 

 ionized layers in the corona it is able to emit radiation of increasing 

 wavelength. Three receivers would therefore detect the noise one 

 after the other as shown in the records of figure 3. After a time lapse 

 of about 24 hours the corpuscular stream reaches the earth and excites 

 the atmosphere to make it glow with the beautiful colors and forms of 

 the aurora. A portion of the sun has been presented with majestic 

 pomp to the earth. 



RADIO STARS 



For many years the astronomer, with modest pride, has felt that he 

 could count with certainty the number of bright stars in the sky. 

 There are many, however, that he would have overlooked because 

 they are invisible optically. Provisionally, these objects are called 

 radio stars but it is certain that most of them are quite different from 

 the stars of optical astronomy. The brightest radio star is in the con- 

 stellation of Cassiopeia. It corresponds in position with one of the 

 faintest nebulae that can be detected with the 200-inch telescope on 

 Mt. Palomar. The nebula was found only after repeated searching 

 near the radio position and it would probably have remained unde- 

 tected if the radio data had not been available. So far the nature of 

 the object is a mystery. Spectroscopic evidence shows that it is an 

 irregular cloud of gas with violent internal motions and high excita- 

 tions. The object is known to be within our local galaxy but opinion 

 is divided as to whether the gas is dispersing or condensing, possibly to 

 form a new star. 



Cygnus A is the second brightest radio star. It corresponds to an 

 object at a distance of 2 X 10 21 km., a distance so great that its light 

 and radio waves take 200 million years to reach us. Homo sapiens was 



