Walter Sydney Adams, an American astronomer, was born Dec. 20, 1876. In 1862, a tiny companion to the bright star Sirius had been discovered. Somewhat later, a similar dim star had been discovered near the star 40 Eridanus. Both Sirius B and 40 Eri B, as they were called, were puzzling, since they had luminosities that were only about 1/10,000 of their bright companions. A modern diagram comparing 40 Eri B with its companions demonstrates the extreme difference in brightness (second image).
When the spectral type of stars was first worked out near the end of the century, and the letters OBAFGKM were assigned to stars to represent their spectral type (our sun being a G star, for example), it was determined that K and M stars were generally red, cool, and dim, while B and A stars were hot, white, and very luminous. In 1914, Adams announced that 40 Eri B was an “A” star, which meant it was hot and white, but it was not very luminous at all, and then in 1915 he captured a spectrum of Sirius B which revealed that it too was an “A” star. Adams had discovered the first two “white dwarfs”, although the term was not invented until 1922.
In 1925, Adams got an even better spectrum of Sirius B and discovered that the spectral lines were “red-shifted” a small amount. Usually a red shift is caused by a star’s motion away from us, but Albert Einstein had recently predicted, in his theory of general relativity of 1916, that an intense gravitational field, such as one might find around a very dense star, would also cause a red-shift. Adam’s discovery of the gravitational red-shift of Sirius B is one of the three famous confirmations of Einstein’s general relativity that were made between 1919 and 1925.
The two photos show Adams with Einstein (first image), with Adams at far left; and (third image) Adams (left) with cosmologist James Jeans (center) and astronomer Edwin Hubble (right), at Mt. Wilson Observatory.
Dr. William B. Ashworth, Jr., Consultant for the History of Science, Linda Hall Library and Associate Professor, Department of History, University of Missouri-Kansas City. Comments or corrections are welcome; please direct to firstname.lastname@example.org.