Nobel Laureate Andrea Ghez discusses her discovery with 200 WashU community members and visitors (Max Silberg | Contributing Photographer)

Over 200 million years ago, dinosaurs roamed the Earth. Had they been able to look 26,000 light-years towards the center of the Milky Way, they would’ve seen our planet start its current lap around the galaxy, held in place by the mass of four million suns. The only current explanation? A supermassive black hole named Sagittarius A*.

Andrea Ghez, who shared the 2020 Nobel Prize in Physics for her group’s contribution to the discovery of this black hole, spoke at WashU to a lecture hall filled with over 200 students, faculty, and public visitors, on March 26 as part of the 2026 McDonnell Distinguished Lecture Series. 

But, to observe Sagittarius A*, it’s first important to know what a black hole is. At its center there is a generating, impossible “singularity,” where the black hole’s mass is confined to zero volume, making density infinite. Unfortunately, according to Ghez, while “relatively exotic” physics can explain some of the black hole’s effects, this core is left undescribed due to two ideas of physics that we cannot yet unify. 

“We do not have the physics to describe what a black hole is,” Ghez said. “We don’t know how to merge quantum mechanics, which is the field of things that are very small, with general relativity, which is the description of how gravity works. And, black holes are both small and have strong gravity.”

Black holes, strong enough to trap encroaching light, cannot be seen directly, a point Ghez succinctly illustrated to the audience with an entirely black slide. Her team’s solution was to observe the motion of surrounding stars for evidence of something massive holding them in orbit. 

“What we’re after is a way of probing the mass [of the central area], and the most direct way of doing that is to measure the gravitational influence of the black hole on how things move around it,” Ghez said. “Stars are going to orbit the black hole for the same reason planets orbit the sun. For every planet, how long it takes it to go around and how far away it is [from the sun] tells you the mass of the sun. [Similarly,] each star also tells you the mass that’s inside its orbit. So, I want to find stars that are as close to the heart of the galaxy as possible.”

The closer a star is to the center of a galaxy, the tighter and faster its orbit will be about the black hole there. Therefore, more information about its orbit, and thus the black hole, can be gleaned in a (relatively) shorter time 

The Event Horizon Telescope Collaboration captured a picture of Sagittarius A*’s event horizon in 2022, our galaxy’s central black hole. Courtesy of the EHT Collaboration.

However, peering into the center of the galaxy from what Ghez called its “suburbs” is no small feat. Besides the distance, there’s a galaxy’s worth of cosmic dust along the galactic plane we peer into, which is even visible in photos of the Milky Way in the night sky. This requires a very large telescope that can also see in infrared, whose long wavelengths allow propagation of light through the dust. Thus, Ghez’s group used the W. M. Keck Observatory telescopes.

“They’re the largest telescopes in the world that can detect infrared light,” Ghez said. “The way we characterize telescopes is by the diameter of the mirror that collects the light. The diameter of these mirrors is 10 meters, which is like the width of a tennis court … There [are] two campaign promises for why you’d want to build a very large telescope: the first is that these telescopes will allow you to see things that are … faint and far away. The second campaign promise is that you should be able to see things in a lot of detail.”

However, the largest infrared telescopes on Earth suffer from a fundamental problem: they are still on Earth. Ghez noted that the Keck Observatory mitigates some of Earth’s challenges solely by its placement on a 14,000-foot Hawaii mountain peak, which allows for less atmosphere above, smooth airflow, low light pollution, and literally being above the weather. But, the telescopes are still stuck under much of Earth’s atmosphere, which distorts incoming light (causing, for instance, stars to twinkle). 

“This light has been travelling from the center of the galaxy to us for the last 26,000 years, and in that last 30 microseconds it’s getting totally messed up,” Ghez said. “The first cave paintings that were ever made by humans were being drawn when the light was halfway [here].”

The solution came in the form of “adaptive optics” in the 1990s after astronomy research and military declassification. The Keck telescopes shine a laser of a specific wavelength into the upper atmosphere near where the astronomer wants to look to create a usable, reference point source of light. This point source’s emitted light is then distorted by the atmosphere when the telescope collects it. Since the initial conditions of the laser/point source are well-known, a mirror inside the telescope thus changes shape according to said distortion, so that when incoming light from that area of the sky bounces off of the mirror the atmospheric distortion is effectively cancelled out.

“A key part of this technology is you need to look at something you understand, so we create an ‘artificial star’ in the atmosphere by beaming a laser up and making little sodium atoms up in the atmosphere from meteorites shine,” Ghez explained. 

So it was that in 1995, Ghez and her group began studying the movements of stars at the center of the Milky Way using the Keck telescopes. 

“In the beginning, all we were interested in was measuring how they moved in a straight line and seeing if they moved fast when they moved close, which was sort of a crude way of showing how much mass was at the center of the galaxy,” Ghez said. “Now, we have complete orbits. You can probably find my favorite star in the galaxy … It has a rather unassuming name of SO2.”

Astronomers measured orbits of stars near the center of the galaxy. Courtesy of Johan Jarnestad/The Royal Swedish Academy of Sciences.

“[SO2] is the star that told us there is a supermassive black hole at the center of the galaxy,” Ghez said. “With this experiment, we can show that there is 4 million times the mass of the sun inside a volume 10 million times smaller [than previously known]. In other words, we’ve advanced the evidence for the case of a supermassive black hole by a factor of 10 million.”

This data and evidence, decades in the making, is what ultimately earned Ghez a share of the Nobel Prize. At the same time, a separate team led by Reinhard Genzel was similarly tracking the galaxy’s centermost stars, and their results converged: both groups’ data pointed to the same invisible, central mass. So, in 2020, the prize was shared between Roger Penrose — the theoretical physicist whose earlier work established the existence of black holes within Einstein’s theory of general relativity — Ghez, and Genzel: an arrangement Ghez was satisfied with.

“Half the prize was awarded to a theorist, and the other half was awarded to two observers,” Ghez said. “I love that, because it’s the balance between theory and experiment or observation … And then the observational side was split into two competing teams, which I thought also was really elegant in terms of messaging: how important it is to have differing schools of thought, that rather than thinking about it as ‘may one win,’ but that science actually wins when you have two competing teams who think differently.” 

For junior undergraduate student Luc Bourgeade, a physics researcher in the audience, this was a particularly inspiring sentiment.

“It was … unique, because I know about a lot of competition in the research field and I’m not sure how to think about it, but to hear her say that ‘Ultimately, science wins,’ is pretty inspiring,” Bourgeade said. “It was so inspiring, it made me want to shoot for the stars.”

However, even with this award-winning discovery, questions about the center of our galaxy still remain, such as the chicken-and-egg problem of which formed first: the supermassive black hole, or the larger galaxy? Ghez explained that one did not cause the other, but rather both must have formed together, evidenced by the correlation between the black hole and the galaxy’s central “bulge”.

“In other words, whatever formed one, formed the other,” Ghez said. “There has to be some feedback which keeps these two things, and we have no other environment in which we can really probe that feedback in such great detail.”

The data also produced results to which theory has not yet caught up with, and even stars they “didn’t even think to predict.” For instance, according to Ghez, there was an unusual amount of young stars near the black hole, which was unexpected as the strong tidal forces of the black hole should lead to young stars’ spaghettification while they are still forming.

“This aspect of work I also really love because almost every single prediction about what we should see around a supermassive black hole has been inconsistent with the observations,” Ghez said. “It could be that we’re seeing a lot of unexpected things because we’re only seeing the largest, brightest things that are there. So, it’s really important for us to continue to push forward how we observe the center of the galaxy.”

Of course, one way to increase a telescope’s resolution is to simply make the telescopes bigger. Another way is to better account for atmospheric distortions. According to Ghez, astronomers’ solution to this (which they are already building), is to add more lasers. 

“We’re in the commissioning stage. We’re launching multiple lasers to do effectively a CAT scan on the atmosphere, so you really know all the layers,” Ghez said. “There are three telescope projects around the world that are 30 meters in diameter … the dome would be the largest moving structure ever built on the planet.”

Ghez’s path to the Nobel Prize was not a totally smooth one. Initially joining a high-energy astrophysics group in graduate school, she then switched to another group, which ultimately failed to see the central supermassive black hole. But, the technology that group used was useful for studying star formation (her eventual PhD), which Ghez used in another group. Even after her new group for looking at the central supermassive black hole was formed and her Nobel Prize-winning plan generated, they were initially turned down and denied telescope time.

“I think it’s really important for the students to hear the bumps in the road that can happen along the way,” Ghez said. 

Later, during the Q&A, she answered how she overcame their proposal’s rejection. 

“You’re sad for a minute, and then if you’re convinced the thing will work, you figure out what went wrong … I was a new kid on the block, people didn’t know me,” Ghez said. “I think it is kind of an important message in terms of: you’ve got to talk to your colleagues. Writing these proposals in isolation isn’t enough. I had amazing collaborators; it’s part of your community support system.”

The Nobel laureate’s astrophysics journey, which was not always straightforward, was compelling for audience members like Aavik Wadivkar, a sophomore astrophysics student.

“I think the humanity of it was really compelling,” Wadivkar said. “It could’ve been a dime-a-dozen science story that ended up being one of the most important contributions to our understanding of our galaxy and the greater universe that we’ve had. I think it’s just a testament to how you really don’t know what the results might be before the fact.” 

Ghez’s presentation had frequent moments of humor, such as putting up a pitch-black slide to show the problem with observing a black hole. Down-to-earth moments like these, along with Ghez’s graduate student story, helped Michael Abreu, a physics graduate student who attended the event, think about his own research journey.

“She had to make some decisions on, ‘Do I go this way or do I go this way?’ and I feel like I’m sorta at that crossroad now with the research I’m doing,” Michael said. “They just enjoy what they do. They’re not that different from you or I, and that really brings it into perspective that anyone can do this if they want to and are really passionate about it.”

Ultimately, Ghez’s Nobel Prize was anything but normal, receiving it in her own backyard during the height of the COVID-19 pandemic. Despite this, she and the other 2020-2021 winners got to attend the celebration in Sweden for the 2022 awards. They were also able to contribute their personal or professional “artifacts” to the Nobel Prize Museum.

“I decided to bring the first data tape that we used to record our data in 1995, because it really captures that moment: it’s not a form of data presentation we use today,” Ghez said. “And, together, I also brought out first log sheet … and this is what it said, this is the first observation: ‘Holy shit!’”