Yerkes Observatory

Hale and the 40-inch

George Ellery Hale, then a young professor at the University of Chicago, persuaded the Chicago streetcar magnate Charles Tyson Yerkes to fund a 40-inch refractor, and the two founded the observatory in 1892; it was dedicated on 21 October 1897. The lens, made by Alvan Clark and Sons, is the largest refracting telescope lens ever used for research; larger ones cannot be supported only at the edge without sagging.

The site at Williams Bay, on the shore of Lake Geneva in Wisconsin, was chosen for its proximity to Chicago rather than for its sky. Hale built a laboratory beside the telescope for spectroscopy and physics, a new idea in 1897 that gave astrophysics its institution.

Astronomers and classification

Edward Emerson Barnard worked at Yerkes after leaving Lick, photographing the Milky Way and comets. Gerard Kuiper, who arrived in the 1930s, discovered the atmosphere of Titan in 1944 and carbon dioxide in the atmosphere of Mars, and directed the observatory from 1947. In 1943 William Morgan, Philip Keenan and Edith Kellman published An Atlas of Stellar Spectra, which introduced the MK system, the classification of stars by spectral type and luminosity class that astronomers still use.

Subrahmanyan Chandrasekhar, who derived the mass limit for white dwarfs in 1930, joined the Yerkes staff in 1937 and later received the Nobel Prize in Physics.

A second life

The University of Chicago ended operations in 2018. The property was transferred to the Yerkes Future Foundation in 2020, which reopened the site to the public in 2022 and plans to use it for education and science. The 40-inch refractor, still one of the largest in the world, is at the centre of the visitor programme.

A hinge between two eras

Yerkes belongs to both the age of the great refractors and the age of astrophysics. Its refractor was the last of its kind, because glass lenses cannot be supported except at the edge, and the next generation of research telescopes used mirrors. Its laboratories and its staff, though, were the model for what followed: spectroscopy beside the telescope, physicists and astronomers in one building. Its closure in 2018 was a reminder that a historic observatory is also a building to maintain, and its reopening under a foundation shows that a community may carry what a university cannot.

Instruments and technologies

• 40-inch refractor A 102 cm lens with a focal length of about 19 metres, on a massive equatorial mount in a large dome with a floor that rose and fell for the observer.
• Spectrograph and laboratory Spectrographs fed by the telescope, with a laboratory for comparison spectra, set up under Hale.

The people

• George Ellery Hale Founder; astrophysicist who built Yerkes, Mount Wilson and Palomar.
• Edward Emerson Barnard Observer and photographer.
• Gerard Kuiper Planetary astronomer and director from 1947.

Timeline

• 1892 Hale and Yerkes found the observatory.
• 1897 Dedicated on 21 October.
• 1943 The MKK (Morgan–Keenan) spectral classification is introduced.
• 2018 University of Chicago ends operations.
• 2020 Ownership passes to the Yerkes Future Foundation.
• 2022 Reopens to the public.

Key lessons for observation today

• Put the laboratory beside the instrument Hale's spectroscopy laboratory made the observatory a physical science. Modern field sites keep calibration and test facilities next to the sensors.
• A classification becomes a standard when everyone uses it The MK system endured because it was simple and applied to every star. Common vocabularies are what make data comparable.
• Know the limits of scale The 40-inch is the largest refractor ever useful. Every design has a size at which its own weight limits it.

Sources

• Wikipedia, Yerkes Observatory
• Yerkes Observatory, history
• Astronomy, Oct. 21, 1897: Yerkes Observatory is dedicated
• Wikipedia, Gerard Kuiper