Seeing the invisible: How Nobel laureate Andrea Ghez found the supermassive black hole in the Milky Way's center

Astrophysicist Andrea Ghez has spent her scientific career peering at the center of the Milky Way, trying to see the invisible.

In 2020, she won a Nobel Prize for that work, which dramatically strengthened the evidence that a supermassive black hole, invisible to our instruments, is lurking at the heart of the galaxy. She also became just the fourth woman to win the Nobel Prize in Physics in the award's more than a century of history, a statistic she told Space.com she views as a responsibility.

Meanwhile, the same observing campaign that informed the prize-winning research continues today, as she and her colleagues tackle ever-deeper questions about the universe. "The goal is never to get a prize, but rather to do science," Ghez, who researches and teaches at the University of California, Los Angeles, and whose research is funded in part by the National Science Foundation, told Space.com. 

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When she began her research, Ghez combined her previous work on imaging technology with a longstanding interest in black holes to tackle an intriguing mystery: What was creating strange radio light at the center of the Milky Way?

"There had been certainly the suggestion that there might be a black hole there, but there was a tremendous amount of controversy surrounding that possibility," Ghez said. "Its emission looked like what people were seeing elsewhere that was being connected with black holes. But it was really wimpy, it could have been so many other things."

So Ghez set about studying the center of the galaxy. Her approach was to track the stars closest to the center of the galaxy, figuring that they were under the influence of whatever was creating the strange emission. Ghez recognized that technology she'd previously worked with called adaptive optics, which accounts for the distortion to astronomical data caused by Earth's atmosphere, could sharpen astronomers' view of those stars enough to follow their motion.

Others weren't as convinced; she noted that the first proposal for observing time for the project was rejected. And even once she had time on the telescope, there were hurdles to overcome. "First you have to prove that your imaging works, and that you can take a picture, and that there's something to see — that there are actually stars in this region and that you can track their motion."

But eventually, each of those steps fell into place and Ghez and her colleagues began tracking a host of individual stars near the center of the galaxy.

One of those stars went from being an unknown speck of light to becoming Ghez's favorite star, now known as SO-2. "We discovered it and it was moving fast, it was clearly of interest. It was one of the stars of interest and it definitely became the star of the show," she said. "It turned out it was the one with the shortest [orbital] period, but you couldn't figure that out in the beginning."

Tracking SO-2 and its companions with a hefty dose of patience meant that Ghez could begin to estimate just how much mass was packed into that mysterious object at the center of the galaxy. As observations accumulated, Ghez and her colleagues became convinced that at the center of the galaxy was a huge amount of matter in a tiny amount of space. Those features match a black hole, but the evidence wasn't strong enough yet.

"But still, there were alternative explanations, and so what that tells you is you have to keep going," Ghez said. Fortunately, continuing the research was easier than starting it had been. "At that point, it was clear that the technique would work, it was clear that something interesting was going on so we could get more telescope time."

Massive implications

As time passed and the data piled up, Ghez's research, along with unrelated research conducted by Reinhard Genzel, confirmed that it was a supermassive black hole that lurked at the center of the Milky Way; in 2020, Ghez and Genzel split one half of the Nobel Prize.

But the discovery was not just about marking monsters on our map of the galaxy. Because scientists don't have any evidence that our galaxy is particularly special, understanding how our own supermassive black hole and the rest of the galaxy interact has broader implications.

"It's the closest example of a center of a galaxy we'll ever have to study; the next closest center of a galaxy is 100 times further away," Ghez said. "So we now have this wonderful laboratory for understanding both the physics of black holes and understanding really how gravity works in this region around an object where the laws will ultimately break down."

Ghez's work has also supported what is perhaps the most heralded advance in studying supermassive black holes at galactic hearts since her discovery, the Event Horizon Telescope. That project is a global network of telescopes collaborating to create a visual representation of the shadows of the supermassive black holes at the centers of the Milky Way and of the galaxy called M87, which is located 54 million light-years away in the constellation Virgo, according to NASA.

The collaboration is still analyzing observations of the Milky Way, but in April 2019 released what is often called the first image of the shadow of the black hole at the heart of M87. (Scientists affiliated with the project have said that the Milky Way's black hole is trickier to study than M87's because it is more active.)

"You're looking at a ring that's created by the gravitational influence on the photons," or particles of light, Ghez said of the Event Horizon Telescope's work. "It's the same way we see the black hole: It's the gravitational influence on the things around it."

For Ghez, that makes the Event Horizon Telescope's work an exciting scientific milestone. "It's so powerful when you can bring multiple approaches to studying the same phenomena," she said. "It really gives you much deeper insight when you can bring multiple techniques together."

Even as scientists are joining forces to tackle the mysteries of black holes, Ghez and her colleagues are continuing their careful monitoring of the stars at the heart of the Milky Way. And they have no intention of stopping any time soon, since every new observation increases the power of the full set of data.

The research has also pointed to new puzzles that science has yet to explain, like how new stars can form so close to a supermassive black hole. "There's a number of really interesting paradoxes that have arisen from the ability to see the center of the galaxy in these new and different ways," Ghez said.

And then there's a favorite pastime of physicists everywhere: checking up on the work of Albert Einstein to see if it still holds up.

First, Ghez's research confirmed that photons close to the supermassive black but escaping its gravity hole lost energy in a specific way that matched Einstein's predictions. Now, the team is working on a second test, this time analyzing how objects move through spacetime. The scientists should have enough data within a few years to evaluate his predictions there, too.

"I always love that stage in an experiment where you're not quite sure what's going to happen, but it's clear that a signal is clearly emerging," Ghez said.

Related: 20 trailblazing women in astronomy and astrophysics

Pushing for diversity

Parallel to her science work, Ghez has also wrestled with the challenges of working in a field that can be hostile to women. She sees a stark improvement in the rates of women earning Ph.D.s in physics today compared to when she began her own doctoral but says there's still more work to do on including all minoritized groups in science.

For Ghez, the Nobel Prize is a tool for that. "I've always been interested in this question of how do you encourage girls into the sciences, and I think the best way you can do that is by succeeding, visibly succeeding," she said.

And because the award means hosts of speaking invitations, it can be a tool for reaching a broad audience, she said, with some careful selection. "Winning the prize comes with lots of opportunities and responsibility," Ghez said. "It forces you to ask yourself, which activities will help you advance what you think is most important?"

Meanwhile, Ghez said she encourages those struggling to see themselves in science to find support systems. "Especially when you're a minority, finding those communities that you can be part of where you're not a minority, I've always found this critical to my own professional involvement," she said.

In the end, that's about weathering the hurdles of the field. "My favorite expression is every challenge is an opportunity," Ghez said. "So how do you surround yourself with people that can help you translate challenges — not to make little of those challenges — into opportunities?"

Email Meghan Bartels at mbartels@space.com or follow her on Twitter @meghanbartels. Follow us on Twitter @Spacedotcom and on Facebook.