Edmund C. Stoner and the Discovery of the Maximum Mass of White Dwarfs

The existence of a mass limit for white dwarfs is usually attributed solely to the late astrophysicist Subrahmanyan Chandrasekhar (1910–95), and this limit is named after him. But as is often the case, the history of this discovery is more nuanced. In this paper I will show that the existence of a maximum mass was first established by Edmund C. Stoner (1899–1968), a physicist who began experimental research under the supervision of Rutherford at the Cavendish in Cambridge, but later switched to theoretical work. Rutherford recommended Stoner to a position at the Physics department of the University of Leeds where he spent his entire career. According to G. Cantor, he was “probably the leading Cavendish-trained theoretical physicist of the 1920’s”, although he learned theory mostly on his own, and became known for his work on magnetism. Unfortunately, Stoner suffered from diabetes and poor health which restricted his travels, and this may account for the fact that he did not receive wider recognition for his achievements. In 1924 Stoner wrote a paper on the distribution of electrons among atomic levels. In the preface of the fourth edition of his classic book, Atomic structure and spectral lines, Arnold Sommerfeld gave special mention to “einen grossen Fortschritt a great advancement” brought about by Stoner’s analysis, which then came to the attention of Wolfgang Pauli, and played an important role in his formulation of the exclusion principle in quantum physics. Therefore, it is not surprising that Stoner’s interest in white dwarfs was aroused by Ralph H. Fowler’s suggestion that the exclusion principle could be applied to solve a major puzzle, the origin of the extreme high density of white dwarfs, which could not be explained by classical physics. Arthur Eddington expressed this puzzle as follows: “I do not see how a star which has once got into this compressed state is ever going to go out of it.... The star will need energy in order to cool.... It would seem that the star will be in an awkward predicament when its supply of subatomic energy fails. Imagine a body continually losing heat but with insufficient energy to grow cold!” At the time, the conventional wisdom was that the source of internal pressure which maintained all stars in equilibrium against gravitational collapse was the internal pressure of the matter composing the star which had been heated into a gas presumably, according to Eddington, by “subatomic energy”. But when this supply of energy is exhausted and the star cools, Fowler proposed that a new equilibrium would ensue, even at zero temperature, due to the “degeneracy” pressure of the electrons caused by the exclusion principle in quantum mechanics. Fowler, however, did not attempt to determine the equilibrium properties of such a star which he regarded as “strictly JHA, xxxix (2008)