216 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



before it escapes from the surface and begins its long journey out into 

 the depths of interstellar space. 



But if the journey ahead of the escaping radiation is long, its past 

 history is by no means negligible. The radiation takes some 50 million 

 years to worm its way slowly upward from its birthplace close to the 

 sun's center to the outer edge. A snail's pace, indeed, for light, which 

 could traverse that distance in two and a half seconds if an enormous 

 number of atoms did not bar its progress. 



Inside the sun, atoms are so closely packed that a speeding packet 

 of light waves can travel only 50-millionths of an inch or so, before it 

 runs head-on into one of the waiting atoms, to be deflected into a new 

 track and run into still another atom a quadrillionth of a second later. 

 In the face of so dense a traffic jam, one wonders how the radiation 

 could ever escape. But the outer layers of the sun are less dense than 

 the inner layers. The radiation inevitably, if slowly, finds a path 

 through the atomic maze, eventually to escape completely. And so 

 the sunlight that warms us today was born in atomic explosions that 

 took place when dinosaurs and other prehistoric animals roamed the 

 earth. These facts enable us to draw one basic conclusion : the sun is 

 highly opaque to its radiation. 



Some scientists have speculated that the sun's well-known 11-year 

 cycle, most clearly depicted by the variation in numbers of sunspots, 

 may in some way be due to a periodic fluctuation of heat production 

 deep in the center. A pulsation of the entire sun every 11 years, for 

 example, could change the output of energy. Compression would heat 

 the sun, increase the rate of atomic fusion, and result in the generation 

 of more energy. However, the radiation takes 50,000,000 years to es- 

 cape and if only 11 years separate each pulse we must find almost 

 5,000,000 pulses still underneath the surface, separated by only 200 

 yards or so on the average. A structure something like an onion with 

 alternate layers of warmer and cooler gases might conceivably ensue. 



Such a structure could persist, however, only if the radiation flowed 

 directly outward. But we have already noted the roundabout path 

 that the energy must pursue in order to reach the surface. The radia- 

 tion mills around and around, as if performing a dance whose rules 

 require one to take 999,999 billion steps backward for each million 

 billion steps forward. With such slow progress, the hypothetical 

 onionlike layers completely disappear. The sun's interior must be 

 drably uniform, except for the progressive decrease in temperature 

 and density from core to edge. And if this argument applies to the 

 sun, it also applies to stars in general. Stellar variability, except for 

 the rare extreme when a star suffers complete destruction in a super- 

 explosion, can scarcely be more than skin deep. 



We know that energy flows from one place to another by one of 

 three fundamental processes: Conduction through a solid, as when 



