This past June, James Gunn, Eugene Higgins Professor of Astronomy Emeritus, was awarded the Kyoto Prize honoring his breakthrough achievements in the astrophysical sciences.
The Kyoto Prize, which is presented by the Inamori Foundation of Japan, recognizes lifetime achievements in the three categories of basic science, advanced technology, and arts and philosophy. Gunn was honored primarily for his work on the Sloan Digital Sky Survey (SDSS), a project that aimed to produce a comprehensive, three-dimensional map of the cosmos.
“It was his brainchild,” said Michael Strauss, Chair and Professor of Astrophysical Sciences.
Data from the Survey has enabled astronomers to measure the distance to almost 4 million galaxies, characterize the distribution of objects in the universe, and improve our understanding of dark energy and matter. Nearly 10,000 scientific papers have been published as a result of the SDSS, and the Survey laid the groundwork for ongoing research that will expand our knowledge of how the universe was formed.
In a ceremony on Nov. 10–11 in Kyoto, Gunn delivered a Commemorative Lecture entitled “Understanding the Universe and the Things That Live in It Through Astronomical Surveys,” in which he discussed his work developing the instruments to study the population of objects in space — something like an astronomical census.
Gunn conceived of and designed the SDSS, which launched around the year 2000. As stated in the award citation, the outcomes of the Survey included clarifying cosmic history, uncovering properties of celestial objects, and finding the parameters of the expanding universe.
Gunn was born in 1938 in Livingston, Texas. At an early age, he fell in love with a children’s book about astronomy and soon moved on to his father’s undergraduate textbooks.
“I never really had much doubt about what I wanted to do. I knew what I wanted to do was study the stars,” Gunn said.
In 1965, he received a Ph.D. from the California Institute of Technology and worked at the NASA Jet Propulsion Laboratory from 1966–1968. He became an assistant professor at the University in 1968 before leaving for CalTech in 1970. He returned to Princeton in 1980, where he served as a Professor of Astrophysical Sciences before transferring to emeritus status in 2011.
While at CalTech in 1976, he proposed and won a competition to design a camera aboard what was then known as the Space Telescope and is now known as the Hubble Space Telescope. The camera was what is known as a charge-coupled device (CCD), which had been invented a few years earlier and detects movements of electrical charge to capture images. This technology would be central to the development of cameras on the Sloan Survey.
Gunn began planning for the SDSS in the early 1990s. Standard telescopes only imaged a tiny piece of the sky at any one time. One of the main challenges of astronomy was to use these images and deduce which objects are close, and which are far away.
Previous attempts, dating back to the 1950s, were not sensitive or quantitative. The standard at the time was the Palomar Observatory Sky Survey (POSS), which was purely photographic. Gunn saw the need for an electronic survey that would generate accurate data.
One way to remedy this was by introducing a new way of mapping the skies, taking advantage of something called redshift to measure distances. The universe is constantly expanding, meaning that objects in space are moving away from Earth. Depending on how far away the objects are, the wavelengths they appear to emit are slightly different — with some appearing to have a lengthened, or “redshifted,” wavelength, referring to the longer wavelength of red light.
The Sloan Survey also introduced the use of spectroscopy alongside imaging. While photographic imaging uses wide swaths of the visible spectrum, much like in normal cameras, spectroscopy captures data in much finer detail, allowing scientists to gather information about how fast objects are moving, their distances from Earth, and the chemical composition of objects.
The 700-lb camera for the SDSS telescope was constructed by Gunn and his team over 6 years in the basement of Peyton Hall. It is now mounted on the telescope at the Apache Point Observatory in New Mexico.
Although the Sloan Survey was initially motivated by an understanding of the distribution of objects in galaxies, it also unearthed more unexpected findings.
For example, the Baryon Acoustic Oscillation phenomenon, which is related to the sound waves emitted by the tiny bump of our galaxy in the very early universe. Or the thousands of asteroids, which could now be sorted not only into orbital groups, but by chemical composition. Or the discovery of “brown dwarves,” which are of too low a mass to ignite the nuclear reactions that make our sun and other stars shine.
The data from the survey were shared publicly, with no restrictions on use or collaboration. This openness was unusual at the time, especially for astronomers who were not used to such wide-scale cooperation. This also made it unnecessary for astronomers to work for years on smaller telescopes to study the galaxies they were interested in. For many scientific projects, the data were already there.
Jenny Greene, Professor of Astrophysical Sciences, did her graduate thesis on the size of black holes using data from the Sloan Survey.
“I experienced firsthand what it meant for data like this to be made public and user-friendly to the entire astronomy community,” Greene said. “It was truly revolutionary and changed the way we think about astronomical data.”
The discoveries of the SDSS have been essential to improving our understanding of something that scientists still don’t fully understand: dark matter. Current estimates state that around four percent of density in the universe is standard matter, while six times that is dark matter, Gunn explained.
Looking at how physical matter behaves can tell us about what dark matter does. However, much about dark matter and energy remain unknown.
“The data from the survey have given us some of the best constraints on just how much dark energy and dark matter there is in our universe,” said Strauss.
“It’s a little discouraging, to live in a universe in which the stuff you know about is only 4 percent of the total,” Gunn said. “But that’s the way it is.”
The SDSS has laid the groundwork for a current project, the Subaru Telescope, a sort-of next-generation version that is much larger — with a primary mirror diameter of 8.2 meters as opposed to 2.5 — and aims to understand the universe in cosmological history. The Subaru Telescope is run by the National Astronomical Observatory of Japan.
The Subaru Telescope will “explore some of the same questions and ask questions about how the universe evolves and changes over time,” Straus said. It will do what Sloan did for a time in the universe when the stars first formed, Greene explained.
Currently, the Subaru Telescope is capturing images with the Hyper Suprime-Cam, with photographic data collection nearly halfway completed.
Since the end of the Sloan Survey in 2012, Gunn has been playing a key role in the construction of the Prime Focus Spectrograph, which will capture the spectroscopic data. It is to be mounted on the Subaru Telescope in 2021, with data collection to begin in 2022.
Astronomers often divide themselves into three groups: theorists, observers, and instrumentalists — the last of which are those who build the telescopes and equipment that enable observers to view the skies and theorists to formulate models.
“Jim is a superstar in all three,” Strauss said. “In the world of astronomers, he is sort of well-known as a triple-threat, as someone who has done incredibly important work in all three of those major areas.”
“But I think instrumentation is his first love,” Strauss added.
Gunn is a bit more modest. He dabbled in all three, he said: theory when he was younger, observation later on, and mainly instrumentation since the start of the Sloan Survey.
About Sloan, Gunn said, “I was the father of it.” But he also sees himself as a sort of father for the young scientists who were working on the project.
“And I’m very proud of that,” he said. “What I did to make this thing happen really enabled an enormous amount of very, very good science by really very hardworking and excellent young scientists.”