134 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



Now we can see more or less what is happening at 10 million degrees 

 in the interior of the sim. Crowded together within a cubic centi- 

 meter there are more than a quadrillion atoms, about twice as many- 

 free electrons and 20,600 trillion X-rays (British reckoning). The 

 X-rays are traveling with the speed of light, and the electrons at 

 10,000 miles a second. Most of the atoms are hydrogen atoms or 

 rather, since they have lost their satellite electrons, simply protons 

 traveling at 300 miles a second. Here and there there will be heavier 

 atoms such as iron lumbering along at 40 miles a second. I have told 

 you the speeds and the state of congestion of the road ; and I will leave 

 you to imagine the collisions. Small wonder if the atoms are found 

 with their garb of electrons badly torn or even stripped naked. 



The stripped atoms are continually capturing free electrons and, so 

 to speak, repairing their dress; but scarcely has the captured electron 

 settled when an X-ray bears down on it and explodes it away. This is 

 not a fanciful picture. These are phenomena which have been found 

 happening in the laboratory when we use X-rays of the same wave- 

 length and electrons of the same speed as in the sun. There is no need 

 to go beyond the limits of terrestrial experijnent to discover what is 

 happening to the population, and all the calculations have an experi- 

 mental basis. 



The atoms and electrons are rushing violently hither and thither; but 

 on the whole they do not get any forwarder; gravitation pulls them 

 back and keeps the material of the star in equilibrium. But the 

 X-rays gradually leak outward. They are subject to gravitation, it is 

 true; but their velocity of 186,000 miles a second is sufficient for escape 

 from any star. It is just the same as in the theory of planetary atmos- 

 pheres, where gravitation is sufficient to retain the heavier constituents, 

 but the lightest atoms have sufficient velocity to escape. The planet 

 thus loses the lightest gases ; and in the same way the star loses (or, as 

 we say, radiates) photons of radiation. I should explain that, although 

 these photons are X-rays in the interior of the star, they are trans- 

 formed down to longer wave length in passing through the last few 

 thousand kilometers of comparatively cool matter; so that it is in the 

 form of light and heat waves that they finally escape. 



So you may picture a photon of radiation, barging first one way, 

 then another, like a man in a rioting mob — absorbed by an atom and 

 flung out again in a new direction. In this way a photon in a star 

 will wander aimlessly round in the interior for a million years or more 

 until, just by accident, it finds itself at the exit of the maze — ^shoots 

 through — and makes a bee-line across space to the Oakridge reflector, 

 where Professor Shapley photographs it. 



Having first ascertained the particulars about the population that 

 I have been describing, we can apply the laws (based on laboratory 

 experiment) which determine the amount of obstruction offered by 



