Andrea Ghez and the Black Hole at the Center of the Galaxy

On May 12, an imaging team co-led by Caltech, together with their collaborators in the Event Horizon Telescope (EHT) consortium, announced that EHT had generated the first image of Sagittarius A*, the supermassive black hole at the center of the Milky Way galaxy.

The following is an excerpt of a May 2021 article conducted by Lori Dajose (BS ’15) with astronomer Andrea Ghez (MS ’89, PhD ’92), who received the 2020 Nobel Prize in Physics for the black hole’s discovery.

Q What led up to the discovery of this supermassive compact object?

**Andrea Ghez** (PhD ’92) in 1987, her first year of graduate school at Caltech. *Photo courtesy of Andrea Ghez*

A Stellar mass black holes had been theorized for a long time, and now we have strong observational evidence for them from LIGO [the Laser Interferometer Gravitational-wave Observatory]. Roughly 50 years ago, people suggested that all galaxies have supermassive black holes, those that are a million to a billion times the mass of the sun, located at their core.

We set out to prove this. The center of our galaxy was the best place to look because it’s the closest center of a galaxy that we’ll ever have to study.

The center of the galaxy is obscured by a lot of dust, so you need to go to infrared wavelengths. Having worked at Palomar Observatory during my PhD at Caltech, I knew that the Keck Observatory that was just about to come online in 1993 would be a powerful telescope to observe with.

With large telescopes, the blurring effects of the earth’s atmosphere obscure the fine detail. We had to be able to compensate for that, so at first we used a very simple technique called speckle imaging. That allowed us to prove that there were stars in the center of the galaxy and then to track their motions. Then, about 10 years into this work, it became possible to use adaptive optics, which was a much more powerful technique.

We went from measuring just velocities on the plane of the sky to accelerations and then full orbits. From this, we could show that there was 4 million times the mass of the sun contained inside a region that corresponds to the scale of our solar system.

Q What were some of the challenges you encountered?

A We were turned down [on our first request to use Keck to measure these phenomena]. To me, it was just so damn obvious that this was going to work. Then it ended up working out so much better than we could possibly imagine. It had so many phases of excitement: excitement because the first image worked, excitement because we just discovered stars, excitement because the next year we could show that they were moving.

At that point, people started suggesting that there were all sorts of ways that these things could be moving fast without a black hole. So, every step of the project, there was questioning and skepticism. In a sense, that’s how science works. You’re not supposed to just accept, so it drives you on to the next stage of the project.

As the technology has gotten better and better, more and more results have actually come out. We were able to not only answer the question we set out to address, but we’ve also uncovered a host of other questions to answer or to think about.

Q What got you interested in science initially and why astronomy, in particular?

A Growing up, I had no idea what I wanted to do, but the moon landing happened when I was 4. In hindsight, that’s the thing I like to point back to in terms of the first time I was really inspired to think about the universe, and it really took hold with me. Although, at that time, I also wanted to be a ballerina.

I think it became quite clear that I had an aptitude for math and science. I really enjoyed it, and it was a language that came naturally to me. I just thought of it as a giant puzzle. To me, math and science is that world of just figuring out how to put the pieces together in a logical way. So by the time I went to MIT [as an undergraduate], I felt like I had “found my people.”

Q I was excited to interview you today because when I was in high school and applying to college, I wanted to study astrophysics. Looking up women astronomers in Southern California, I came across your name and noticed that you went to Caltech. It feels really cool to finally meet you.

A I love hearing that because one of the things that I really believe is that the best way you can encourage young women into science is by being successful yourself and making that success visible. In other words, really engaging with the public. I remember being an undergrad, at some points I’d look around and think, “Well, there are not a lot of women in this field. Am I on the wrong playground?”

In 1995, I was contacted by a couple [Judith Love Cohen and David Arthur Katz] that writes this series of books: You Can Be a Woman Engineer, and so on. They reached out to me and asked if I would be interested in participating in their project, so we worked on You Can Be a Woman Astronomer. I was delighted to do so because I’ve always been interested in encouraging young women into the sciences. I think it’s great to engage kids at that age.

Q How did your time at Caltech shape your career?

A Tremendously Caltech is such an amazing place in terms of the resources and the facilities. Certainly, in astrophysics, having access to all those telescopes is just remarkable. Then in the community of people there is just deep intelligence and incredible dedication to science. I’m really grateful for the opportunity I had as a student to work with a truly remarkable scientist, my PhD adviser, Gerry Neugebauer (PhD ’60), who was a huge influence.

One piece of advice I remember from Gerry was to have deep respect for the data, paying attention to what the data are telling you without being biased by what you may or may not want to see come out of it.

There are so many pressures today associated with getting the resources that it’s really important as a scientist to respect the process, to be patient, and just get it right. I think I learned that discipline at Caltech.

Q Being a principal investigator and now a Nobel laureate must be a lot of work. What keeps you going?

I think it’s a true privilege to be able to do this kind of work, to be in a field where you get to set the questions that you’re interested in asking.

And then there’s teaching, which is so immediate. I’m very grateful to be able to do these two things because I think they complement each other well. Being able to bring forefront research into the classroom is exciting to the students. Then being able to bring students into the research is also really exciting for me because students ask you those basic questions, and they remind you to think deeply about “Why is it that we do it this way?” and “How do we know what we know?”

Q What was it like getting that 2:45 a.m. phone call?

A I think anyone’s first reaction to the home phone ringing in the middle of the night is “Is everybody OK?” But then, very quickly, you realize this is a very different kind of phone call. It’s breathtaking, kind of unbelievable, and I was just over the moon for days. One of the most delightful things, actually, about getting it during COVID times is it brings such joy to have good news to share.