CONSTITUTION OF THE STARS' 



By Sir Arthur Stanley Eddington 

 Professor of Astronomy, University of Cambridge 



When we turn a telescope on the sun, we look at it through its 

 tenuous envelopes — the corona and chromosphere^ — then down through 

 a few hundred Idlometers of its outermost atmosphere, to a level 

 where it becomes too opaque for us to see further; just as, in looking 

 down on the ocean, we can see down a few feet but no further. At 

 the vaguely defined level which is the limit of our vision, the tempera- 

 ture is about 6,000°. What lies below that level? What is it like 

 deep down in the interior of the sun — and the other stars? 



The exploration of the deep interior of the stars began in 1869 

 with a paper by Homer Lane of Washington, which he entitled, "On 

 the Theoretical Temperature of the Sun, Under the Hypothesis of a 

 Gaseous Mass, Maintaining Its Volume by Its Internal Heat and 

 Depending on the Laws of Physics as Known to Terrestrial Experi- 

 ment." Evidently he didn't believe in snappy headUnes. This paper 

 has been the foundation of developments by Ritter, Emden, and 

 others, which are being continued at the present day. 



There is a phrase in the title of Lane's paper which I would under- 

 line: ^'Depending on the laws of physics as known to terrestrial experi- 

 ment.'^ That expresses the principle of which 1 profess myself a 

 devotee. We want to find out how far the phenomena which we 

 observe in the sky agree with, and are a consequence of, the laws that 

 have been assigned to matter as the result of terrestrial experiment. 

 Take ordinary matter — some mixture of the elements that we know — 

 and apply on a large scale the properties of matter and radiation that 

 have been found by experiments on a small scale. Treat it as though 

 you were designing a large dam instead of a large star — with just the 

 same land of calculations and exercising so far as possible the same 

 kind of foresight. The conditions in the star are very extreme; but 

 the ultimate things to be dealt with — electrons, atomic nuclei, X-rays — 

 are the same in the star as in the laboratory, and we can ap ply our 

 laboratory knowledge of them. Calculate in this way what will be 



' Address given at the Harvard Tercentenary Conference of Arts and Sciences. Publislied by permission 

 of the Director of the Harvard Tercentenary. 



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