Eddington in Cardiff 100 years ago today: the first proposal that stars are powered by fusion

“… fulfilment [of] our dream of controlling this latent power for the well-being of the human race – or for its suicide.”

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.

Black Lives Matter to Physics

A racist attack on a black person is an attack on the core of America. White Americans need to understand that it is an attack on them too.

Today I am joining the Strike for Black Lives at the urging of the particle physics movement Particles for Justice. Here I want not just to show my support publicly, but to share my concern that this issue, which I thought way back in 1965 we were well on the way to eliminating, continues to poison American society. Let me summarise where I am going by saying that thinking about this struggle as just an issue of failed justice, as the particle physicists’ webpage name suggests, is not going to be enough to save black lives in the future.

We all need to understand that what happens to black people happens to America. Killing black people harms the whole society. I am afraid that the demonstrations of the past days won’t accomplish their aim if the struggle is framed as one of protecting and bringing justice to a minority. The frame must be bigger. Americans need to begin to see black people as a cross section of America. Simply, black people embody America.

Before explaining my point of view, I want to start with the situation in physics. I started my PhD in 1967, and there were no black Americans in my cohort. There was also only one woman. Today there are significant numbers of women physicists (although in the US it is far short of 50%), but many fewer black Americans. There are also plenty of immigrants, many with dark skin from South Asia. Now, physics may be esoteric to many but it carries high prestige. When physicists are interviewed on TV or give advice to government, what they say is important, but so is the metadata: these white men, these women, these immigrants are making important contributions to society. The paucity of black Americans in these forums certainly does nothing to change the minds of people who think that black lives don’t much matter. Physics is failing here. And this doesn’t even mention how we are failing the young blacks who – just as we ourselves were – are turned on by the subject, but who – unlike us – won’t get the chance to go for it.

The disparity in the progress of women and black Americans in physics between 1967 and today contains a bitter irony. The women’s movement for equal treatment took huge inspiration from the black civil rights movement of the 1960s. The Civil Rights Act of 1968 prohibited discrimination on the basis of race, religion or national origin. It wasn’t until 1974 that it was amended to include sex in that list. Protection against discrimination for people with disabilities and for gay people followed even later.

This takes us to the core of my concern. In the words of Pulitzer Prize winner Nikole Hannah-Jones in last year’s podcast series 1619 (which I highly recommend) on American slavery, “It is black people who have been the perfectors of democracy in this country.” They haven’t taken democracy for granted they way most of us whites have (and now realize that we shouldn’t have). If there were two things that America has, at least until recently, been admired abroad for, they were its dedication to democracy and its creative, ebullient musical culture. America owes both to its black population.

Black Americans have roots in their country that go much, much further back in time than do those of most white Americans. The ancestors of any black American, taken together, probably did more to build America than my ancestors did, more than the ancestors of almost any other American who claims German or Irish or Scandinavian or Italian or Polish heritage. If any group could be said to own America, black Americans have the second-best claim, after native Americans. But of course neither group runs the place.

And then there is the question of what we mean when we talk about black Americans. There is no well-defined distinction between black Americans and the rest of us. Their ancestors’ heritage in Africa was just as culturally diverse as that of the later European immigrants. And as geneticists have learned, their ancestors in Africa were genetically more diverse from one another than any European is from any other non-African, let alone from other Europeans. And then of course most black Americans have European genes as well, from a long history of rape. Uncomfortable as that may be to whites, what it means is that black Americans are the best example America has of the melting pot. Slaves were brought to America and thrown together with greater differences than our European ancestors had in languages, customs, skin colors, stature, lip shapes, nose shapes — you name it.

From all this variety, black Americans built much of America, gave it its music, led the defence of its democracy. We white Americans must understand, no – we must have respect for, how central to the core of America our fellow black citizens are. There is a political battle going on in America right now over who defines the country, and very clearly democracy and the rule of law are under threat. Science is also clearly under attack by the same forces, already partly forced to toe the political line. No people in America has a greater interest in preserving America’s core values than the black population. It is not enough for us whites or us physicists just to want to help people who are beleaguered. A racist attack on a black person is an attack on the core of America. White Americans need to understand that it is an attack on them too.

Black people embody America. Unless we whites understand that, I fear that there won’t be enough momentum to change once and for all the deeply racist institutions that over the years have kept their power, that have kept denying the important fact that black lives matter. BLM matters to democracy, to science, to everyone.

(Image of Washington DC and the White House courtesy of Planet.)