The scientific legacy of Yerkes Observatory

Located at low altitude in Wisconsin and fitted with telescopes of modest aperture, Yerkes Observatory is not exactly a modern professional astronomer’s dream facility. Yet where the location and technology fall short, the people and ideas have driven the observatory to an outsize role in the evolution of astrophysics. Over the past century and a quarter, astronomers including George Ellery Hale, Otto Struve, and Subrahmanyan Chandrasekhar have created a research environment in which generations of scientists have flourished before going on to lead other major facilities in the US and abroad.

As the University of Chicago prepares to cease operations at Yerkes in October (see the related news story ), it’s worth exploring the considerable historical impact of the observatory.

Jumping out of the gate

The vision for the Yerkes Observatory came from Hale. Shortly after its founding in 1890, the University of Chicago hired Hale as a professor and moved to establish itself in astronomy and astrophysics. Hale and university president William Rainey Harper persuaded financial tycoon Charles Tyson Yerkes to fund an observatory and its centerpiece telescope, a 1-meter-aperture refractor that to this day remains the largest of its kind. Finding a site was complicated, though: The facility needed to be far from Chicago’s industrial soot to optimize observing but close enough to the city so that Yerkes could tout his contribution to Chicagoans. (Never afraid to resort to bribery and blackmail, Yerkes had a deserved reputation in the city as a shady businessman.) Those restrictions led to the selection of Williams Bay, Wisconsin, a small village on Geneva Lake about 110 km as the crow flies from the Windy City.

As soon as it was dedicated, in 1897, Yerkes Observatory became a center for US science—in effect, it was the Big Science of its day. The early visibility was entirely due to Hale’s influence on the wider stage. Yerkes became the home base for the Astrophysical Journal, which Hale had begun publishing with James Keeler in 1895, and hosted the inaugural meeting of the American Astronomical Society, which Hale founded in 1899. Hale also arranged for the National Academy of Sciences to hold a meeting there in 1903.

Hale may have given Yerkes an early leg up, but the observatory’s astronomers quickly justified the attention with a series of important advances. During the first several years of the 20th century, Frank Schlesinger pioneered the use of photographic plates to obtain reliable trigonometric distances to nearby stars, a technique that astronomers at other observatories quickly adopted. At the same time, Edward Emerson Barnard conducted surveys of dark nebulae, dense regions associated with the formation of stars. Those areas of study—characterizing stars and analyzing the interstellar medium—would prove to be hallmarks of research at Yerkes for decades to come.

Stellar and interstellar advances

Even decades after it was installed, the refracting telescope at Yerkes proved useful for determining distance and other properties of stars, thanks to its long focal length, relatively wide field, and stability. Around 1970 William van Altena extended Schlesinger’s stellar distance measurements. He developed new techniques that achieved significantly higher precision and worked on fainter stars.

Kyle Cudworth took advantage of the work of his predecessors to obtain important insights about the stars within clusters. On modern plates, he recorded data on the same globular clusters that Yerkes astronomers had studied more than 80 years before. By using the same telescope his predecessors had used, Cudworth was able to detect the very small internal motions of stars within the clusters. Combined with Doppler line-of-sight motions, those data constrained dynamical mass models for the clusters and provided an independent way to measure their distances and velocities. Globular clusters are generally old, and their stars have low abundances of heavy elements, so they are useful for performing important tests of stellar evolution models via the Hertzsprung–Russell (stellar luminosity versus stellar temperature) diagram.

Yerkes astronomers also compiled catalogs of stellar properties based on the systematic analysis of spectra. In 1950 Nancy Grace Roman used the spectroscopic classification system of Yerkes colleague William W. Morgan to determine distances and velocities for samples of bright stars. She noted that stars with velocities higher than 70 km/s with respect to the Sun are always classified as weak-lined, whereas stars moving more slowly can have spectra that are either weak-lined or strong-lined. The picture that emerged is that the weak-lined stars have a lower abundance of heavy elements and formed earlier in the history of the Milky Way. Roman’s work established a relationship between the galactic orbit of a star and its age, which provided strong constraints on the origin and evolution of the stars that surround us.

Other Yerkes researchers focused on the space between the stars. In the mid 1920s, Struve pointed out certain absorption lines detected in stellar spectra, identified as calcium, whose likely origin was within interstellar gas clouds. His observation was among the first to demonstrate that diffuse gas can be detected and studied via absorption spectra. Decades later, Yerkes astronomer Lewis Hobbs used UV spectrographs from the Hubble Space Telescope to conduct detailed studies of the chemical composition, velocity, structure, and physical properties of those absorbing systems.

Struve also pioneered measurements of diffuse emission from the interstellar medium. In 1938, to detect the very faint emission, he and other Yerkes astronomers built a spectrograph that occupied 75 feet of hillside at McDonald Observatory in Texas. They were able to map the intensities of the H-alpha spectral line as well as those of oxygen and nitrogen within the disk of the Milky Way. Their work proved that the emitting gas is pervasive, is mostly hydrogen, and accounts for a significant fraction of the mass of the disk.

While at Yerkes, William Hiltner made the discovery that starlight is polarized, which provided evidence for galactic magnetic fields and shed light on the nature of interstellar dust grains. (John Hall at Lowell Observatory made the same discovery independently.) On the basis of Hiltner’s research, Jesse Greenstein devised a picture for the magnetic-driven alignment of nonspherical grains shortly after he left Yerkes. Meanwhile, Chandrasekhar and Guido Münch tackled the problem of what could be learned about the statistical properties of absorbing material from the variation in brightness of diffuse light from the Milky Way. Their study enabled quantification of the shape and extent of the galaxy based on counts of stars, as well as the clustering of matter on very large scales based on counts of galaxies.

People with big ideas

Throughout the 20th century, Yerkes’s telescopes became increasingly outclassed by instruments at other observatories. Yet the quality of the scientists and the ideas they devised, both at Yerkes and after they moved on to other institutions, maintained the Wisconsin observatory’s status as a leading center for astrophysics research. In fact, Yerkes astronomers played important roles in the creation or operation of observatories far from Wisconsin whose collective data have driven progress in ground-based and airborne astronomy.

Under the leadership of Struve, who was Yerkes director from 1932 to 1947, the observatory brought on researchers including Chandrasekhar, Morgan, Gerard Kuiper, and Gerhard Herzberg. Chandrasekhar’s influence was especially great, due to his publications of definitive monographs in broad fields of astrophysics (among them stellar structure, stellar dynamics, radiative transfer, and hydrodynamic stability) and his recruitment of numerous scientists, both young and established, to Yerkes. Louis Henyey, Bengt Strömgren, and Margaret and Geoffrey Burbidge rounded out the intellectual scene.

Struve also recognized the importance of pooling resources from multiple institutions. He led the University of Chicago’s 1932 construction and subsequent operation of the McDonald Observatory, giving Yerkes astronomers access to larger-aperture telescopes at a good site. The experience of operating McDonald led Struve to propose the multi-institutional concept that eventually became the Kitt Peak National Observatory in Arizona. In the early 1960s Kuiper, Hiltner, and Aden Meinel were involved in the founding of a Southern-Hemisphere resource for US astronomers at what is now the Cerro Tololo Inter-American Observatory in Chile.

Yerkes also proved central to the emergence of IR astronomical observations. Kuiper, who worked at Williams Bay from 1937 to 1960, was among the first to develop and use IR detectors for astronomy; he collaborated first with Harold Johnson at Yerkes and then recruited Frank Low. Kuiper moved on and founded the Lunar and Planetary Laboratory at the University of Arizona, where he flew spectrometers with IR sensors on a NASA Convair 990 jet. Yerkes researchers led by Doyal Harper, a student of Low’s, pioneered airborne far-IR astronomy—first with a 0.3 m telescope in a Learjet, then a 0.9 m telescope in a C-141 (named the Kuiper Airborne Observatory), and most recently a 2.7 m telescope in a Boeing 747 (the Stratospheric Observatory for Infrared Astronomy, or SOFIA).

In the early 1990s, Harper persuaded the NSF Office of Polar Programs to establish an astronomical observatory at the US Amundsen–Scott South Pole Station, with experiments at near-IR, submillimeter, and millimeter wavelengths. Establishing the South Pole as a practical site to undertake astronomy required advances in communications. Yerkes’s Robert Loewenstein brought internet to the South Pole by using satellites that had drifted away from equatorial orbits and had thus become visible from the South Pole for a few hours each day.

Yerkes has also collaborated on other major projects, including the Sloan Digital Sky Survey. In 1994 the Fermilab group that was leading the software development held a workshop at Yerkes, an event that turned out to be the first meeting of the survey collaboration.

In recent years, research efforts at Yerkes shifted to the University of Chicago campus, and the Yerkes telescopes were adapted for educational purposes. Yerkes Observatory has evolved in the past to remain a hub for astrophysics and its history. If whatever happens in the near term fosters a similar spirit, Yerkes can continue to make contributions for many years to come.

Richard Kron holds positions at the University of Chicago and Fermilab and is former director of the Yerkes Observatory, 1989–2001.