302 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1963 



showed that the stars are made out of the same familiar elements of 

 matter that we have here — hydrogen, sodimn, calcium, iron, and even 

 compounds such as cyanogen and titanium oxide. Li the 20th century, 

 modem atomic physics has provided us with two basic innovations 

 in our thinking about the nature of stars both of which were first 

 noted by Sir Arthur Eddhigton : the first of these is the fact that the 

 material throughout a star, even at its center, is virtually a perfect 

 gas although the material in the central regions of the Sun is 100 times 

 as dense as water. This is because the temperature is so high — at 

 about 15 million degrees — that the atoms lose almost all their electrons 

 and become ionized ; the stripped nuclei and free electrons that remain 

 take up very much less space than do ordinary atoms, and so they 

 can be compressed to very considerable densities without departing 

 appreciably from the ordinary behavior of ideal gases as summarized 

 in Boyle's and Charles' laws. A second important consequence of 

 modern atomic physics has been the realization of the mechanism which 

 enables the stars to shine: they derive most of their energy from 

 thermonuclear reactions that are very similar to the controlled fusion 

 processes that people are now trying to reproduce in the laboratory. 



These nuclear fusion reactions most commonly involve hydrogen, 

 which is the most abimdant element in the universe, and the energy 

 that comes out of the reaction is the nuclear bmding energy that is 

 released when four hydrogen nuclei, or protons, are fused together 

 into one nucleus of helium, or a-particle. The weight of the helium 

 nucleus is slightly less than four times the weight of the proton, and 

 the difi^erence is available as energy in accordance with the well-known 

 equation of Einstein, E—mc^. It is quite amusing to note that helium, 

 which is the second most abundant element in the universe after hydro- 

 gen, was first discovered in the spectrum of the Sun by Sir Norman 

 Lockyer at the total eclipse of 1868, a quarter of a century before the 

 gas was isolated in the laboratory by Sir William Eamsay; and in 

 a more general way we can say that the nuclear transformation of 

 hydrogen into heliiun that is going on in the deep mteriors of stars is 

 just one example of the fact that astronomy can provide the physicist, 

 and nowadays even the technologist, with an extension of his labora- 

 tory facilities to a vast range of temperatures, pressures, and, of course, 

 sheer size, if only we have the wit to understand what is actually 

 happening. In fact, one astronomer of my acquaintance recently 

 remarked that the whole of physics and chemistry, as studied in 

 laboratories, is just a special case of astrophysics. 



Apart from the application of astronomy as an extension of our 

 terrestrial laboratories, astrophysics provides us with an opportunity 

 of investigating the answers to questions of a more or less historical 

 character, such as : {a) How did the Sun, the planets, and in particular 

 the Earth come into existence? and (b) Wliy do the chemical elements 



