Stars and Stuff (part 1): The White Dwarf

Impression of the view from beyond Sirius looking towards Earth.
Reproduced by kind permission of Sky & Telescope.

Sirius is one of the brightest stars. At this time of the year (March) it is visible high in the northern sky just after dark: about 20? from the zenith in Auckland and 30? in Dunedin. Sirius is actually not the most luminous star but it is, in astronomical terms, very close: only 8.7 light years, or 82 million million km, away. Its distance was determined from parallax from opposite points of Earth’s orbital diameter.

The parallax motion of Sirius, however, is not regular; it exhibits a cyclic deviation consistent with it having a binary companion orbiting it. In the 1840s Bessel painstakingly measured the parallax and deviation and concluded that the companion had nearly the same mass as the Sun and that the orbital period was 50 years. Because Sirius is so bright and the orbital separation is minimal, this companion, Sirius B, was not directly observed until a new large 18?-inch telescope with improved optics was used by Clark in 1862.

But Sirius B proved to be faint, and the settled science at the time was that its density (water = 1) would be close to 1.5 ? equivalent to that of the Sun ? and therefore it had to be cool and red to be so faint. However, a spectral analysis in 1915 by Adams, using the Mount Wilson 60-inch reflector telescope, showed that Sirius B wasn’t red at all; it was spectral class B (now reclassified as DA2), blue-white hot at 25,000?C. Since apparent luminosity is proportional to both temperature and size, this implied that its diameter was only about two thirds that of the Earth and therefore the density of this almost solar mass was enormous ? in fact it’s 125,000 ? the equivalent of a 1 cm3 cube of sugar weighing 125 kg or as much as a prop-forward. “This, of course, is entirely out of the question,” opined Gore (Irish astronomer J. Ellard, not Al).

A new class of star entered the celestial catalogue ? the white dwarf. Most stars, including the Sun, will end up as a white dwarf, usually after swelling up to a red giant and then collapsing. This process takes billions of years, so right now there is no cause for alarm. The early years of the 20th century saw many such new classes of discovery: Einstein’s theory of relativity and gravitational waves (of which, more later), Pauli’s exclusion principle that no two electrons can occupy identical states, Fermi-Dirac quantum mechanics statistics of electron degeneracy ? it was a fertile time for physics.

Notwithstanding these developments, the scientific consensus in celestial mechanics was that white dwarf (and other) stars could be modelled as a perfect gas obeying Boyle’s law (1662) and the ideal gas law derived therefrom in 1834, which states that pressure times volume divided by (absolute) temperature, for any given mass, is a constant. As the mass of a white dwarf was increased, the force of gravity would crush it into a smaller and smaller volume and the pressure and temperature would increase to maintain equilibrium. There was no limit to this, regardless of the mass considered.

The leading proponent of this model was Sir Arthur Eddington, doyen of astronomers and leading light in the Royal Astronomical Society (RAS). Nobody questioned Eddington; his word was law. Until, that is, in 1929, Professor Edward Milne, a distinguished astrophysicist and mathematician, proposed that every star had a core which actually did not obey Boyle’s law and instead exhibited electron degeneracy ? that is, the enormous temperature and pressure would force the electrons from lower energy orbits into higher energy orbits.

Eddington’s revenge for this heresy was swift.

I have not read Professor Milne’s paper, but I hardly think it is necessary, for it would be absurd for me to pretend that Professor Milne has the remotest chance of being right. End quote.

Sir Arthur Eddington

And… Quote.

It is difficult to discuss this paper. Professor Milne did not enter into detail as to why he arrives at results so widely different from my own; and my interest in the rest of the paper is dimmed because it would be absurd to pretend that I think there is the remotest chance of his being right. End quote.

Sir Arthur Eddington

At which point the RAS, the scientific community, the press and social media shut down the argument, deplatformed Milne ? calling him a ‘denier’ ? and got him fired?well, no; this was 1929, not 2019. Milne had his say: Quote.

By the current theory I mean that of which the researches of Sir A. S. Eddington are the basis. It would give this paper a controversial air wholly foreign to its purpose if I were to mention Eddington?s name in every place where the underlying ideas of the present investigation differ from his or where the results differ from his?The whole of Eddington?s criticisms may be dismissed as beside the point. End of quote.

Milne, E.A. (1930). The analysis of stellar structure. Monthly Notices of the Royal Astronomical Society 91(1), 4-55.

Heated arguments subsequently raged between Eddington and Milne both in private and during meetings of the RAS. Meanwhile a gifted young physics and mathematics graduate embarked on a voyage to England having been awarded a colonial Government of India scholarship to read for a doctorate at Cambridge…

(to be continued)