One hundred years ago today, on 24 August 1920, with over 1000 people gathered in Cardiff for the annual meeting of the British Association, Arthur Eddington gave his address as the incoming president of the physical and mathematical sciences section. He elected to speak on the subject of the “Internal Constitution of the Stars”. When I first came across the text of the address last year (published in Nature in 1920), I was amazed to find as early as this such an insightful proposal that stars are powered by the synthesis of helium from hydrogen. But what really brought me up short was this sentence:
If, indeed, the sub-atomic energy in the stars is being freely used to maintain their great furnaces, it seems to bring a little nearer to fulfilment our dream of controlling this latent power for the well-being of the human race – or for its suicide.
Twelve years before Chadwick discovered the neutron, twenty-five years before Hiroshima, thirty-seven years before the nations of the world agreed to cooperate to make peaceful fusion power a reality, Eddington basically saw the whole package.
What Eddington had learned in 1920 — and it was enough to open the whole vista to him — was the rest-mass deficit:
The nucleus of the helium atom, for example, consists of four hydrogen atoms bound with two electrons. But Aston has further shown conclusively that the mass of the helium atom is less than the sum of the masses of the four hydrogen atoms which enter into it; and in this, at any rate, the chemists agree with him. There is a loss of mass in the synthesis amounting to about 1 part in 120, the atomic weight of hydrogen being and that of 1·008 and that of helium just 4.
Francis W. Aston in fact presented this (Nobel-Prize winning) result at the Cardiff meeting the following day, as recorded in the meeting’s archives. So Eddington the relativist had what he needed: energy would necessarily be released if four hydrogen atoms could be persuaded to combine to form a helium nucleus. How they might decide to do this, and then how the helium nucleus might contrive to stay together against the electrostatic repulsion that the four protons exerted on one another, offset by only two electrons: these were not things that Eddington even talked about. Eddington knew that answering these questions wasn’t necessary at this point. All that mattered was that the mass deficit, converted into energy, was ample to power the stars:
If 5 per cent. of a star’s mass consists initially of hydrogen atoms, which are gradually being combined to form more complex elements, the total heat liberated will more than suffice for our demands, and we need look no further for the source of a star’s energy.
Notice that Eddington is not stopping just with helium. He expects this process will synthesise even heavier elements, although he observes that the energy payoff is not so dramatic as for the conversion of hydrogen into helium.
Apparently most people in 1920 still seemed to believe in Kelvin’s hypothesis that stellar contraction and the liberation of gravitational energy should power the stars. To prepare for his nuclear fusion hypothesis, Eddington earlier in the article ruthlessly destroys this idea, particularly pointing out that the short lifetime of stars implied by Kelvin would already have produced observable changes in the pulsation periods of some Cephied variable stars. Anyway, he says, most scientists no longer take Kelvin’s idea seriously, even though they have nothing to replace it with:
Lord Kelvin’s date of the creation of the sun is treated with no more respect than Archbishop Ussher’s.
Mindful that, even so, his audience may not be prepared to switch to the nuclear hypothesis so readily, he acknowledges
I should not be surprised if it is whispered that this address has at times verged on being a little bit speculative …
and then he goes on to defend the role of speculation in theoretical physics, including pointing out that even wrong speculations can help advance a field if they motivate experiments that clarify a subject.
Eddington’s address covers much more than just how stars shine. He starts by trying to establish the track in the Hertzsprung-Russel diagram that stars follow as they evolve. This is not what we understand today, although it was a hugely important step in his time: he thinks giant stars are young stars that evolve into a Sun-like stage and then become dwarfs. It would take a good number of years before the theory of nucleosynthesis in stars would lead to the more complicated evolutionary tracks we are familiar with today. And speaking of dwarfs, the story is well-known of how, fifteen years later, Eddington arrogantly rejected the perfectly sound calculations of a young Chandrasekhar, who had had the temerity to suggest that dwarfs had a maximum mass, so that evolutionary tracks of very massive stars had to lead elsewhere — Eddington was not having anything to do with what we now call black holes. Maybe speculation had lost its lustre in those fifteen years!
Eddington of course published a book with the same name as the lecture, the first edition appearing in 1926 and covering much the same material, but more quantitatively and extensively. So his Cardiff presidential address was just the taster. But what a taster!
Eddington’s address was printed as a news item in Nature a week later: vol 106, p. 14. It is freely available from Nature here. The book of the same name was published by Cambridge University Press, which now has an online edition available, with an introduction by none other than Chandrasekhar. Eddington in his book also credits J. Perrin for independently coming to the same conclusion about the fusion of hydrogen to helium, also in 1920: Revue du Mois 21, 113.