Finding an invisible giant
NOVA: Most people have a hard time wrapping their minds around the concept of a supermassive black hole. As an astronomer, is it easier for you to envision one?
Andrea Ghez: Black holes are very exotic objects. Technically, a black hole puts a huge amount of mass inside of zero volume. So our understanding of the center of black holes doesn't make sense, which is a big clue to physicists that we don't have our physics quite right. In fact, we know that when we can figure out how to make the study of things that are very small—quantum mechanics—work together with the study of things that have huge gravity—general relativity—then we'll understand what's at the very center of a black hole.
But as an astronomer, I get to ignore the details of the things that we don't understand. There's a lot of work that we can do on scales that we do understand, and there is actually a finite size that I can associate with a supermassive black hole.
You helped prove the existence of the black hole at the center of our galaxy, but you weren't the first astronomer to suspect one was there. How far back does the suspicion date?
The idea dates back approximately 30 years, when people first suggested that black holes might exist in some unusual galaxies. Those galaxies are called active galactic nucleis, simply because the centers are very active. And they have very unusual properties, so unusual that people said, "Well, maybe there's a supermassive black hole there that powers this extraordinary activity. Perhaps what we're seeing is matter falling onto these massive beasts at the center of these active galaxies, and it's because these black holes are having a huge meal that we see them lit up." From that, the idea evolved that perhaps there are black holes that aren't being lit up—that are on a diet—that are lurking around within other galaxies, even our own.
So then people started looking for one in the Milky Way?
Yes. People started looking at our galaxy to see if normal galaxies have supermassive black holes. In the '80s, the first clue appeared that a larger concentration of mass existed at the center of the galaxy than we could explain with normal stars and gas. But those observations didn't concentrate that unusual mass into a small enough volume to demonstrate the existence of a black hole.
Did other possibilities exist for what might lie at the center of the galaxy?
Some people suggested it might just be a collection of things that don't shine: dead stars. That was an acceptable possibility, and that was the state of affairs when we began our experiments. There was a hint that it might be possible to demonstrate the existence of a supermassive black hole, but there was not enough evidence yet. And that's a great place to be when you start an experiment.
How can you prove that a supermassive black hole exists?
The key to proving that there's a black hole is showing that there's a tremendous amount of mass in a very small volume. And you can do that with the motions of stars. The way the star moves around the center of the galaxy is very much like the way the planets orbit the sun. And the planets orbiting the sun can actually [help you] measure the mass of the sun, just the way the stars orbiting the center of the galaxy can tell you what the mass is inside their orbit. So you need to measure stars as close to the center to show that there's a lot of mass inside a small volume, which is the key to proving the existence of a supermassive black hole.
Why were you able to pursue this mystery in a way that others hadn't before?
Well, timing is everything, and we were certainly in the right place at the right time. We needed to see the stars that are located at the very, very center of our galaxy. And with ordinary techniques, astronomers can't see that detail. The atmosphere limits us, and to overcome that, you need special techniques and you need a large telescope. The Keck telescope, which is the largest in the world, had opened just before I began my faculty position at UCLA. I was looking around for a new problem to work on, because, of course, you can't get tenure for doing the same old stuff. This just seemed like the perfect problem.
Why did you use a ground-based telescope rather than the Hubble Space Telescope, which avoids the problem of Earth's atmosphere?
Well, to overcome the Earth's atmosphere, you have two solutions. One is to go above it, which is the solution of the Hubble Space Telescope, and the other is to use large, ground-based telescopes with techniques for beating the atmosphere. These techniques have only recently been developed. Ground-based telescopes are much larger than Hubble. Keck is four times larger than Hubble in diameter, which means that you can see four times the detail than you can see with Hubble, if you can figure out how to use these techniques.
And you get to go to Hawaii, too.
That's not a bad perk!
How long did it take to confirm that the black hole was there?
You have to have a lot of patience to do this experiment. We started the measurements in 1995, and we got our first clue that we might be right in 1997. We published our first paper then. But then we did a lot better by watching a star go all the way around, as opposed to just partway around. Those results came out in 2003. So basically a decade of work went into this project.
Black hole on a diet
Given that a supermassive black hole has super-strong gravitational pull, why doesn't it suck in more of the Milky Way, even suck in the Earth?
Fortunately, most things around the supermassive black hole are just going to go around it. They're going to orbit it. They don't actually get sucked in. You have to be incredibly close to the black hole for it to fall into the black hole [roughly closer than the Earth is to the sun]. So the stars that we study will happily orbit the center of the galaxy for their entire lifetimes, just like the sun is going to orbit the center of the galaxy for its entire lifetime. So there's no danger posed by this supermassive black hole for us here on Earth.
What accounts for our black hole's relatively quiet state? Am I right that it's not quite as hungry or powerful as other black holes?
Well, there are black holes that are very much the drama queens of our universe. They shout out loud, "I'm here!" They're very identifiable because matter is falling onto them, and we see the matter that's being dumped onto the black holes radiate light. So our black hole is quiet simply because there's not a lot of matter falling onto it. You could say that our black hole is on a diet.
Generally, there's a lot more matter in a galaxy when it's young. So you expect a black hole at the center to become more quiet as a galaxy ages. However, we know that galaxies are likely to merge with other galaxies, and when a merger happens, it will provide a new source of fuel or matter to the center of the new galaxy that comes out of the merger. So, in fact, our black hole could become rejuvenated if we merge with a nearby galaxy. Which is quite likely to happen.
Not in our lifetimes, I hope!
There's certainly no imminent merger. More like about three billion years from now.
Phew. So, is our galaxy unique, or do all galaxies have similar black holes?
Our galaxy's pretty ordinary, garden-variety. So if we believe our galaxy has a supermassive black hole, that tells us that most, if not all, galaxies host such a black hole at their centers.
Do black holes have to exist for galaxies to exist?
There seems to be a strong relationship between the existence of a supermassive black hole at the center of a galaxy and the existence of the galaxy itself. In fact, a correlation exists between the mass of a black hole and the mass of the central part of a galaxy. For a long time people asked, "Well, is this a chicken-or-egg problem? Which came first?" But today we know that these things probably had to form together. Whatever's responsible for the formation of the galaxy had to, as a by-product, produce the supermassive black hole at its center.
Twin stars and rare planets
You study more than black holes. Tell us about your work on star formation.
I've studied how stars and planets form, and the key here is the same technology used in the experiment looking for a supermassive black hole, the technology that allows us to beat the atmosphere. If you look at sites of star births—stellar nurseries—the atmosphere limits your studies to a scale that corresponds to the size of the solar system. So you can't really learn about the characteristics of young suns.
Because we had this technology, we learned that almost every single star starts its life with a sibling, with another star. They are separated by scales that are much smaller than the size of our solar system. This was tremendously surprising.
Is our sun an oddball for not having a sibling?
The key question here is, Is our sun unique? Did it form as a single star, or did it become a single star? I think today the question is still out there. It probably didn't have a star that was very close to it, because these companion stars are likely very disruptive to the planet-formation process. The fact that we have a nicely formed, very hospitable planetary system around our own sun suggests that it formed as a single star. So that suggests that our sun is a bit unusual compared to the majority of stars in our galaxy.
Does this finding have implications for the number of planets that might exist in the universe?
Absolutely. Having a companion star makes it less likely for a planetary system to form. And, of course, what we really want to know is whether or not an Earth-like planet that's hospitable to life is out there. The most likely kind of system that's going to host an Earth-like planet is a star that doesn't have a companion. And that is not the norm.
"The most important thing you can do to encourage girls and young women into science is to show them that it's possible."
Has there been any single moment that really stands out as the thrill of your career?
There have been some great moments in my career. The one that probably stands out the most is the second time we went to the telescope to do this experiment, to study the center of the galaxy. We'd already been in 1995. We had one picture, which already was gratifying because, in fact, people didn't even believe that we could apply this technology to get a really clean picture. So we already had our first clean picture.
We went back a year later. The question was, Were we going to see anything move? And how much were these stars going to move? And if there was a black hole, these things should move quite a lot. And that first year, we could see very easily that stars had moved, and we were just thrilled.
I think the thrill was heightened by instrument failure at the beginning of our night. It's very difficult to get Keck time—you might have a few nights a year. I think this run we had two nights, and the first night was lost because of weather, and the second night our instrument failed at the beginning. And just before the center of the galaxy set and we wouldn't be able to see it anymore, things came together and we got the picture.
So I think it was a sense of relief and the thrill of discovery that we actually had been able to capture this image, and it really did look like things were going to show the existence of a supermassive black hole.
A girl who loved math
What inspired you as a kid?
I'm of the generation that saw the Apollo moon landings when I was very young, and I was very struck by this. I think I announced to my mother when I was four that I was going to be the first woman to the moon! That probably tells you more about my mom than it tells you about me.
Was she encouraging?
Absolutely. I was very encouraged by my parents to pursue whatever I was interested in. My parents bought me a telescope after these initial expressions of interest. But, of course, I was growing up in the middle of a city—I grew up in Chicago, where you couldn't see too much. So I think I quickly lost interest. And at that time, I think I also wanted to be a ballet dancer. So, you know, these were the days when you could be first this, then that, when you grew up. You could be a lot of different things.
There's a stereotype that girls are math-phobic. Were you?
I wasn't math-phobic at all. I loved math. And I was highly encouraged to consider math. When I went to college, I wanted to be a math major. Math I think I viewed as a game, and certainly the influences around me were very positive in terms of pursuing this and not discouraging me.
In high school I started to become aware of the fact that perhaps not the entire world thought that girls should be doing math, and I took this on as a challenge. I remember learning about some study that had just demonstrated that boys were smarter at math than girls. So I think I challenged the boys in my math class to a contest to see who could do better on the tests.
"Now my work has just become one big puzzle, and that's what keeps me going."
Were you encouraged by your teachers? Did you have mentors?
I had some wonderful mentors—my math teachers in high school, and a wonderful chemistry teacher. She was one of the only female science teachers I ever had, and she was incredibly encouraging. I think it was important for me to see, very early on, a woman in this role.
Do you think that in the era in which you were growing up, there was more of an ethos that "girls can do anything?" Have things changed?
This is a very difficult question to answer. Things have changed, I think, both in positive and negative ways. I don't think you see as many people today saying "girls can do anything." (I think when I was growing up I said "girls can do anything better"!) You don't see so much of that today. But you also have many more role models today. There are many more women scientists who are actually showing young girls that they can do this.
What can be done to encourage more girls and young women to pursue science as a career?
I think the most important thing you can do to encourage girls and young women into science is to show them that it's possible, that it's an allowed path. And the best way to do that is to put role models in front of them, to show them that there are women in these fields. I think kids very early on make observations about who does what and then make decisions about that. Who is a scientist, and what does a scientist look like, and do they look like a scientist? So putting women [scientists] in front of them and showing them that this is possible is probably the most powerful thing that can be done.
Do you like being a role model?
I love it! It's exciting. It makes me feel like I can do something of real value. And I love teaching.
How do you convince freshmen at UCLA who don't set out to be science majors that physics is fun?
I have to say my favorite thing to teach is freshmen: freshman physics, freshman astronomy. And that's because they're still trying to figure out what they want to do. They haven't declared majors for the most part, and I think the best way to convince them that physics is fun is to have fun doing it. To show them that you love what you do, to bring the research into the classroom, to encourage them into research. And just to show that this is something that can be enjoyed.
Last year, the erstwhile president of Harvard, Lawrence Summers, caused quite a stir in the academic world when he suggested it might be worthwhile to look into the question of whether intrinsic aptitude, rather than social or cultural forces, was behind the fact that there aren't more women in high ranks of science and engineering. Do you think it was wrong of him to make that speculation?
Well, it's hard to say whether or not it was right or wrong to make the statement. But let me tackle instead whether it was helpful or not helpful for him to make this comment. While I think my initial reaction was absolute outrage, in hindsight I've realized that it was quite a useful remark for him to make.
I think in our culture tremendous biases exist against women in the physical sciences and engineering, and in the sciences in general. And these are unspoken biases, so they're things that women have to fight against without them being explicit. What Summers did was make the issue explicit and open discussion, which I think was very helpful. I, of course, don't think there are any intrinsic aptitude differences, but I do think that there are strong cultural biases against women going into these fields.
So you think it was actually helpful that Summers's comments made headline news?
I think it was wonderful that they were aired. I don't agree with his implication, but I do think that there has been a positive outcome. Maybe this is just looking at the glass half full as opposed to half empty.
There are certainly a number of very accomplished women in astronomy and astrophysics. Do you think that it's a relatively friendly field toward women?
One of the things that distinguishes astronomy and astrophysics is that there've been women in the field for a long time. A long time ago, some famous women worked at Harvard Observatory, ironically, who were there not necessarily for their brainpower but to catalogue and categorize things. Yet many of these women made very important discoveries. As a result, I think it made this field more open to women. Now, mind you, there's still a lot of progress to be made in encouraging more women into this field.
You still have to have a strong sense of yourself and confidence in your abilities to make a career out of it.
Yes, but a lot of women are doing it, and I think that's just great.
In addition to a very busy professional life, you have a busy family life—you've got a couple of young kids. Would you want your own kids to become scientists, would you encourage them?
I want them to explore what interests them. I don't want them to feel inhibited by any biases that our culture might have about what they should or should not be, and that, of course, includes my own biases. If they're interested in science and they want to become scientists, that's great. But I certainly don't want to pressure them.
What keeps you excited about your work?
Research is a wonderful career, because once you've started to do work on one question, what you find is not only the answer to the first question but often new puzzles. And I think that's what keeps me going, is that there are always open questions, open puzzles. I think from very early on I loved puzzles. I loved the crossword puzzle. I love any kind of puzzle. And now my work has just become one big puzzle, and that's what keeps me going.