Even through the largest telescopes, stars appear as mere points of light. Michelson's answer was interferometry: combining light gathered at two widely separated points to detect detail finer than a telescope alone can resolve. Pease mounted a six-metre beam with movable mirrors across the front of the 2.5-metre Hooker telescope. On December 13, 1920, the interference fringes from Betelgeuse faded at a measurable mirror spacing, giving the star's angular size.
The idea of using interference to measure stars went back to Hippolyte Fizeau in 1868, and Michelson had built his reputation on precision optical measurement. Betelgeuse, the bright red star at Orion's shoulder, was a natural first target because astronomers suspected it was enormous. The team, helped by John August Anderson, adjusted the outer mirrors on their steel beam until the fringes disappeared, a point that depends directly on the star's apparent diameter.
The answer was tiny in angle, a few hundredths of an arcsecond, but combined with estimates of the star's distance it showed that Betelgeuse was hundreds of times wider than the Sun. Placed at the centre of the solar system, it would swallow the inner planets.
The measurement held up. A mid-infrared study published in 2000 produced a value entirely consistent with Michelson's findings eighty years earlier, and interferometry, then a delicate one-off experiment, is now the basis of the world's sharpest telescopes.
Key people: Albert A. Michelson, Francis G. Pease, John August Anderson