STORMY WEATHER ON THE SUN — W. 0. ROBERTS 165 



The sun is an enormously large sphere of gas. Nowhere within 

 it do we know of solids or liquids. Its diameter is approximately 

 864,000 miles. The visible surface or "photosphere" glows with an 

 intense yellow- white light. The temperature of the photosphere, ac- 

 cording to several independent methods of estimation, is about 6,000° 

 K. (centigrade absolute). But as you go down toward the center of 

 the enormous sphere of gas, the temperature mounts rapidly ; probably 

 near the center it reaches values as high as 25,000,000° K. There near 

 the nucleus the pressures reach perhaps to tens of millions of tons per 

 square inch, and the gaseous material is compressed till its density is 

 probably more than 10 times that of a bar of iron. Our knowledge 

 of the interior conditions depends upon theoretical researches of the 

 very highest quality carried out by such people as Lane, Eddington, 

 Jeans, Russell, and Chandrasekhar, just to mention a few of the many 

 who have worked profitably in this field of analysis. It is here in the 

 depths that the atomic energy derived from nuclear fusion supplies 

 the sun with its heat. It is here in the depths that the fuel hydrogen 

 is "burned" by atomic fusion, leaving the ash, helium, as the end 

 product. 



Lying directly on the surface, and cooled by processes as yet not 

 fully understood, are the giant sunspots. These huge, dark, irregular 

 spots fluctuate in size and number with the also-mysterious 11-year 

 solar cycle. They are the focus of extensive and intense magnetic 

 fields from which are emitted potent corpuscular streams that fly 

 out into space. Perhaps even cosmic rays are born in or near these 

 giant blemishes on the sun's smooth face. 



The transition layer between the spotted surface of the sun and the 

 solar atmosphere has many interesting properties. It may seem queer 

 that a body like the sun, which is entirely gaseous, can have a sharply 

 defined edge, so that it shows up in a telescope just as clearly as though 

 it were the edge of the moon. Yet this is the way the edge of the sun 

 looks. The theory of this behavior and of the rapid transitions of 

 the opacity of the sun's gases near the surface layers has been studied 

 by many investigators, including E. A. Milne of England, who recently 

 died after a brilliant and useful scientific career. At the edge of the 

 sun, a mere 20 miles or so carries us from regions where the gases are 

 nearly opaque out to the tenuous and transparent gases that charac- 

 terize the solar atmosphere. This short distance carries us from the 

 dense regions, where the spectrum of the sun consists of a continuum of 

 light of all colors, out to the region where the gases show the bright line 

 spectrum that is characteristic of tenuous gases. Much of the success of 

 modern solar astrophysics depends upon observations of the spectrum 

 lines of the nearly transparent atmosphere, and also of the absorption 

 spectrum formed just above the opaque surface of the sun by the rela- 



