Posted on: 3rd August 2005

“A star is drawing on some vast reservoir of energy by means unknown to us. This reservoir can scarcely be other than the sub-atomic energy which, it is known, exists abundantly in all matter; we sometimes dream that man will one day learn how to release it and use it for his service.” F.W.Aston’s experiments seem to leave no room for doubt that all the elements are constituted out of hydrogen atoms bound together with negative electrons. The nucleus of the helium atom, for example, consists of 4 hydrogen atoms bound with 2 electrons. But Aston has further shown conclusively that the mass of the helium atom is less than the sum of the masses of the 4 hydrogen atoms which enter into it – and in this, at any rate, the chemists agree with him. (…) Now mass cannot be annihilated, and the deficit can only represent the mass of the electrical energy set free in the transmutation. We can therefore at once calculate the quantity of energy liberated when helium is made out of hydrogen. 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.”

From “The Internal Constitution of the Stars”, Presidential Address of Professor A.S. Eddington to Section A of the British Association at Cardiff, on 24th August 1920, published in The Observatory Vol. XLIII No. 557, October 1920

picture of Sir Arthur Stanley Eddington

Sir Arthur Stanley Eddington

Sir Arthur Stanley Edidngton was born in Kendal, England in 1882 and died in Cambridge, England in 1944. For most of his career he worked in the Cambridge Observatory. He was knighted in 1930 and received the Order of Merit in 1938. In his landmark lecture from 1920, Arthur S. Eddington for the first time ever realised that fusion powers our Sun as well as all other stars, thus solving a major mystery of contemporary science. He could do so only thanks to his exceptionally prompt support to the Einstein’s Theory of relativity, including the relationship between energy and mass E=mc2. The above excerpt from Eddington’s lecture shows his bold and ingenious application of the formula on the brand new results of precise measurements of atomic weights made by Francis William Aston (Nobel Prize in Chemistry, 1922) in the Cambridge Cavendish laboratory – a birthplace of many other important results, including the discovery of D-D fusion. Notice that the knowledge of subatomic structure was very poor in 1920 so the lecture had to have a very vague frameset.

Actually, helium atoms do not consist of “4 hydrogen atoms bound by 2 electrons”, but of a light cloud of two electrons and a 100.000 times smaller nucleus in its centre with two protons and two neutrons bound by the so called strong force. For this reason, we would not say today that the energy released in fusion is “electrical”. No wonder that it took a long time to evaluate how fusion reactions exactly work in the Sun. First calculations were published in 1929 by Robert E. Atkinson and Fritz G. Houtermans – five years before fusion reactions were actually observed in a laboratory. However, a reliable theory, complete with the results of several cycles of fusion reactions, was published only in 1939 by Hans Bethe (Nobel prize in Physics, 1968). For more details see Bethe’s Nobel lecture “Energy production in stars“.

Picture of the Month

picture of sun with huge eruption

This is a picture of the surface of the Sun in the helium spectral line showing a huge eruption. The image was taken by the ESA / NASA satellite SOHO (SOlar and Heliospheric Observatory) and its EIT diagnostic (Extreme ultraviolet Imaging Telescope). The wavelength of the observed He line was 30.4 nm, corresponding to ultraviolet (invisible) light, i.e. the image is not in true colour. Courtesy of SOHO/EIT consortium. SOHO is a project of international cooperation between ESA and NASA.