Free Astronomy Magazine September-October 2020

6 SEPTEMBER-OCTOBER 2020 STELLAR EVOLUTION mine their physical diame- ters. Thanks both to the instrument and technique, developed primarily by Michelson, the two scien- tists obtained for the an- gular diameter of Betel- geuse the value of 47 mas (milliarcseconds) at the yellow-light wavelength of 5750 Å. This angular di- ameter measurement was valid only for a uniformly illuminated stellar disk. Since it was reasonable to assume that Betelgeuse, like the Sun, was also bet- ter characterized as dark- ening at the edge, Michel- son and Pease adjusted their value by bringing it up to 55 mas. At that time, the parallax angle of Betelgeuse was estimated to be 0.018”, but the two scientists did not know that this value was about four times larger than the actual one, resulting in their underestimation of the distance to the star: about 180 light- years. This, in turn, produced an underes- timation of the star’s size, which, none- theless, turned out to be colossal: 450 mil- lion km in diameter. Placed at the center of our solar system, Betelgeuse would have A lbert A. Michelson (left) and Francis G. Pease were the first researchers to calculate the angular diameter of Betelgeuse. To do this, they in- stalled an inter- ferometer on top of the Hooker Telescope at Mount Wilson Observatory, visi- ble on the side.