Discovering Mount Wilson Chapter 11: The Stellar Interferometer

Image caption: A 20-foot (~6 meters) Michelson interferometer mounted on the frame of the 100-inch (~250 cm) Hooker Telescope, 1920.

We continue ‘Discovering Mount Wilson’ with Chapter 11 and the first successful stellar interferometer on the 100-inch telescope.  This groundbreaking addition to the telescope was built by Albert Michelson and Francis Pease and allowed for great new discoveries in astronomical sciences.        

Nobel Prize winning physicist Albert Michelson (1852-1931) of the University of Chicago came to Mount Wilson in 1919 and frequently returned as a guest astronomer over the next decade.  In historic experiments he made very accurate measurements of the speed of light at Mount Wilson.  He also developed the stellar interferometer, a device for use on a telescope which allowed the measurement of very small distances in the sky.

Betelgeuse Credit: ALMA (ESO/NAOJ/NRAO)/E. O’Gorman/P. Kervella

Michelson, along with Francis Pease (1881-1938) an astronomer and instrument maker at Mount Wilson Observatory, mounted extra mirrors on a 20-foot beam attached to the top end of the 100-inch telescope tube. On December 13, 1920, Michelson and Pease found the angular diameter of the red supergiant star Betelgeuse in the constellation of Orion to be 0.047 seconds of arc, from which they derived a diameter of 240,000,000 miles, based on their assumed distance.

This was the first time the diameter of a star other than the sun had been measured. But we now know that Betelgeuse is very much farther away than they thought, and therefore very much larger, well over 800,000,000 miles across.

Throughout the 1920’s the 100-inch telescope with the 20-foot interferometer was used to make groundbreaking new measurements of the separation between close binary stars, and to make the first ever measurements of the angular diameters of stars.  Mount Wilson Observatory continued opening new doors for new discoveries to come.

‘Discovering Mount Wilson’ returns on August 31, with Chapter 12. We hope you’ll check back for that post here  — and come see us up on the mountain, where ‘We Discovered Our Place in the Universe!’

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