If on a summer's night you look low in the southern sky, you will see the constellation Sagittarius, a centaur pulling back a bow and arrow. But to many, it looks more like a teapot, and if you look toward its spout, you will be staring straight into the center of our galaxy, the Milky Way.
But what you can't see at the center, which is 26,000 light years away, is the supermassive black hole that's four million times as massive as the sun. Because of the black hole's powerful gravity, any orbiting stars zip around at up to three million miles per hour—more than 10 times faster than the stars in the sun's neighborhood. While scientists long suspected that the galaxy's center harbored a black hole, no one knew for sure until astronomers led by Andrea Ghez (PhD '93) of UCLA observed these speeding stars in 1998.
Ghez pioneered techniques to remove the blurring effects of Earth's atmosphere, allowing her and her colleagues to follow for the first time the dramatic action of individual stars hurtling around the galactic center. They found that the stars traveled on elliptical paths around a small, dark area. Because those stars orbit at such high speeds, there must be an extremely massive object at the center—only such an object would have the gravity to hold onto stars that whip around so fast. And the only thing that could be that massive and also occupy such a compact space is a black hole. Ghez's observations constitute the most direct evidence yet that there's a supermassive black hole in the heart of the Milky Way.
Since that landmark discovery, Ghez has continued to develop methods to get increasingly high-resolution images of the galactic center. "Galactic-center work, which is the nearest and dearest to my heart at the moment, has expanded in so many exciting directions," she says. Most galaxies harbor supermassive black holes at their centers, so understanding the action around the Milky Way's black hole will help astronomers learn how other supermassive black holes affect the formation and evolution of their host galaxies.
In particular, Ghez and her colleagues are studying the type of stars that populate the Milky Way's center. One surprise, she says, is that the stars appear to be young. Previously, researchers thought that the black hole would have produced conditions not conducive to making stars, and therefore any stars in the area would have to be old—perhaps born elsewhere and dragged in to the galactic center, over countless eons. One possible explanation for the stars' youth, Ghez explains, is that they formed from an existing disk of gas that surrounded the black hole.
These stars are the nearest to a black hole that have ever been directly observed. Studying their motions, Ghez says, provides a rare opportunity to precisely test Einstein's theory of general relativity in a unique and extreme environment. Scientists can compare the stellar trajectories with what Einstein predicted—either confirming his theory or revealing profound new insights into physics.
For her observations of the galactic center, Ghez won this year's Crafoord Prize in Astronomy, which she shared with Reinhard Genzel of the Max Planck Institute in Germany. Most recently, she earned Caltech's Distinguished Alumni Award, the highest honor Caltech bestows upon its graduates.
"It was a wonderful place to do my PhD," she says of her time at Caltech, where she worked closely with many faculty members, including her advisor, Gerry Neugebauer, the Robert Andrews Millikan Professor of Physics, Emeritus, and Keith Matthews, chief instrument scientist of the infrared astronomy group. She was able to use the Hale 200-inch Telescope at Palomar Observatory—for decades the largest telescope in the world. "Nothing can beat that—staying up all night and working with Gerry and Keith," she says. "Keith would always encourage me to go one step further and always kept me honest in terms of my research."
Now, Ghez uses the Keck Observatory's two 10-meter telescopes, the largest telescopes of their kind in the world. Technology drives her research, she says, and she tries to ask the most compelling scientific questions that can be answered with the tools at hand. Learning to ask the right questions, she adds, was one of the most valuable lessons she learned at Caltech.
While a graduate student, Ghez was a member of the Caltech Masters Swim Club. "I did that to keep my head screwed on straight," she says. The club was where she met her husband, a graduate student in geology at the time. She also helped establish Women in Science and Engineering at Caltech, a group that aimed to foster interaction between graduates and undergraduates. "It gave us this network of women that was a wonderful support system across all fields," she says.
Born in New York and raised in Chicago, Ghez went to MIT for her BS in physics before heading to California, where she's remained ever since. Her time at Caltech was a memorable one, she says: "It was an amazing experience."
