Consideration of the phenomenon of nova-outbursts and the study of the nuclei of planetary nebulæ lead to the conclusion that the list of dense objects can be largely extended.
They are all within five parsecs of the sun, but there is no reason to suppose that this is an abnormal region of space. A few other stars besides Sirius B are known in which low luminosity and abnormal blueness are combined with high density these are known as ‘white dwarfs’. The measurement when actually carried out by Adams in 1925 gave a result so near that of Eddington's estimate that the computed small radius and high density of Sirius B may now be accepted with confidence. It was shown by Eddington that this surprising density was not physically improbable, and further, that in the light of Einstein's general theory of relativity, the relative displacement of the lines of the spectrum of Sirius A and Sirius B could be estimated. Adams at Mount Wilson Observatory succeeded in obtaining a photograph of the spectrum of Sirius B, which led to the unexpected conclusion that the density of Sirius B was of the order of one ton to the cubic inch. In 1862 a faint star, Sirius B, was actually seen by Alvan Clark close to the place that had been theoretically assigned to the supposed companion. This led him to suggest that Sirius was in reality double, consisting of a pair of stars, one much fainter than the other. The proper motion of one of Halley's stars, Sirius, was found by Bessel not to be uniform, but to contain a periodic element of about fifty years. He said that the discovery by Halley of the proper motion of some of the fixed stars led to a remarkable succession of researches in pure astronomy, in modern physics, and in cosmogony generally. Milne on May 19 was on the subject of the “White Dwarf Stars”. THE Halley Lecture delivered at the University of Oxford by Prof.
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