The Monster At The Heart Of The Milky Way
Via UCLA Galactic Center Group: An animation of the stellar orbits. Images taken from the years 1995 through 2016 are used to track specific stars orbiting the proposed black hole at the center of the Galaxy. These orbits, and a simple application of Kepler’s Laws, provide the best evidence yet for a supermassive black hole, which has a mass of 4 million times the mass of the Sun. Especially important is the star S0-2 as it has has been observed for more than one full orbital period, which is only 16.17 years. These images/animations were created by Prof. Andrea Ghez and her research team at UCLA and are from data sets obtained with the W. M. Keck Telescopes.
The 2020 Nobel Prize winners have been announced, and among them is UCLA astronomer Andrea Ghez, who split the prize with Roger Penrose and Reinhard Genzel. Ghez, also the fourth woman to ever win the Physics prize, won for her 1998 work that resolved a decades-old debate among astronomers: What lurks at the difficult-to-observe heart of the Milky Way?
After innovating new ways to peer through the obscuring gas and dust, Ghez and her team observed the orbits of stars around the galaxy’s seemingly empty center—and found they fit a pattern explained so far only by a supermassive black hole of at least four million times the mass of our Sun. In the decades since, she and her team have investigated the gravitational forces of the galactic center, and how well they match Einstein’s theory of relativity. (So far, her team has concluded, Einstein seems mostly right, but his theories may not fully explain what’s going on.)
Ira talks to Ghez about how our understanding of the center of the galaxy has evolved, plus the questions that still puzzle her.
Via UCLA Galactic Center Group: This movie shows a 3D visualization of the stellar orbits in the Galactic center based on data obtained by the W. M. Keck Telescopes between 1995 and 2012. Stars with the best determined orbits are shown with full ellipses and trails behind each star span ~15-20 years. These stars are color-coded to represent their spectral type: Early-type (young) stars are shown in teal green, late-type (old) stars are shown in orange, and those with unknown spectral type are shown in magenta. These images/animations were created by Prof. Andrea Ghez and her research team at UCLA and are from data sets obtained with the W. M. Keck Telescopes.
Andrea Ghez is a professor of Astronomy and the director of the Galactic Center Group at the University of California-Los Angeles in Los Angeles, California.
IRA FLATOW: This is Science Friday. I’m Ira Flatow. If we could view it from the outside, the Milky Way would look like many other galaxies, a beautiful spiral of shining stars and glowing dust and gas. But what’s at the very center? If your answer is a supermassive black hole, the mass of 4 million Suns, well, you’ve probably heard something about the work of UCLA Astronomer Andrea Ghez, whose research more than 20 years ago helped pinpoint the existence of an invisible compact object at the heart of our Milky Way.
The astronomers call that likely black hole Sagittarius A-Star. After we learned there’s a hole in the center of the sea of stars, we wondered how it could help us understand the evolution of our galaxy and our universe. Turns out, Sagittarius A-Star can even help us test Einstein’s general theory of relativity. But don’t take my word for it, Dr. Andrea Ghez, who shares the 2020 Nobel Prize in physics for this work, is here with me today.
Congratulations, Dr. Ghez.
ANDREA GHEZ: Thanks so much, Ira.
IRA FLATOW: Take us back to the 1990s. Why was this question about what’s at the center of the Milky Way such a hotly contested one?
ANDREA GHEZ: Because there was a suggestion that incredibly massive, or what we call supermassive black holes, might reside at the center of the galaxies. So this builds on the concept of black holes, much smaller black holes, which theories had long suggested existed as the end state of how massive stars end their lives. And we had plenty of observational evidence. But what was mounting was unusual activity at the center of galaxies. And so while it wasn’t predicted theoretically, there was mounting observational evidence that there might be a much more massive cousin to what we call ordinary mass black holes.
IRA FLATOW: And so you had to go out and find a way of collecting evidence for that then, right?
ANDREA GHEZ: That’s right. And in fact, the idea of these supermassive black holes came from a small set of galaxies in the zoology of galaxies known as active galactic nuclei, and these are galaxies that have very– lot of activity is coming from their centers. But about 50 years ago, it was suggested that maybe all galaxies harbor supermassive black holes and that most galaxies have black holes that you could say are on a diet, because you can’t see a black hole directly.
So what you do see is the light that’s associated with mass falling in towards the event horizon. And if you’re going to ask this bigger question of whether or not all galaxies harbor supermassive black holes, or really, more fundamentally, do these supermassive black holes exist, our galaxy is the best place to look, simply because the center of our galaxy is the closest center of a galaxy we’ll ever have to look at. The next closest galaxy is 100 times further away.
IRA FLATOW: And you thought maybe we can find a way to watch the orbits of stars around where this black hole should be and get some confirmation, correct?
ANDREA GHEZ: That’s right. So the question is, how do you prove that there’s a supermassive black hole at the center of the galaxy? And the challenge is to get direct confirmation. And so what we did is to look at how the black hole might influence things around it. So stars orbit the center of the galaxy for the very same reason that planets orbit the Sun. It’s the gravitational influence of whatever is inside.
So what you do learn from orbits is how much mass is inside the orbit of the star that you’ve measured. So to show that there’s a lot of mass inside a small volume, the key is to find the stars that are as close to the heart of the galaxy as possible. And that’s what led me to want to work at Keck Observatory. So Keck is the largest telescope in the world. It’s co-owned by the University of California and Caltech, and it’s located out in Hawaii.
And if you have a large telescope, in principle, you can get very detailed pictures, which should allow you to see the stars at the heart of the galaxy. But the challenge is the blurring effects of the Earth’s atmosphere, which make images fuzzy, rather than sharp. So I’ve spent the last 20 years of my career, or even more than that, working on techniques that overcome the blurring effects of the Earth’s atmosphere so you can get these very sharp images and then track and measure how these stars move. So these stars’ motions give us the ultimate proof of the supermassive black hole.
IRA FLATOW: Do you feel vindicated after 20 years of looking for all this data?
ANDREA GHEZ: Oh, I do. In fact, I do all the more because, in fact, my very first proposal to use the telescope was turned down because people didn’t think our technology would work, and even if it did that we wouldn’t see stars, and even if we could we wouldn’t see them move. So as is often the case with science, there’s usually a lot of naysayers. So it is amazingly gratifying to have had this work turn out to be so productive and, in fact, in a way that we could not imagine when we first started this project. In fact, when I first proposed it, I thought it would only be a three-year project, and it turned out to be so much more powerful than I imagined. We just kept going with ever more powerful tests of what’s at the heart of our galaxy.
IRA FLATOW: Now, you share this half of the Nobel Prize with Dr. Reinhard Genzel of Germany’s famous Max Planck Institute. What did you two do differently? In fact, were you sort of competitors at this?
ANDREA GHEZ: Absolutely. We’ve been competitors for the last two decades, but it’s been a really productive competition. These measurements are very hard to make, so there’s tremendous value in having two groups that independently demonstrate that what we’re finding is real. It’s also true that there’s nothing like a little competition to keep you motivated and working hard.
And the two groups have constantly looked carefully, scrutinized each other’s work, so the other group is always going to find your mistakes. And I think the true beauty of having independence is that it allows independence of thinking. These measurements are associated with techniques that have never been used before and approaches to these measurements in terms of the methodologies that also had to be developed.
So having the two groups think independently, I think, allowed much more creativity, although, of course, we’re constantly learning about each other’s work. So while we’re always publishing our thinking at various points and going to conferences, so there’s a scaffolding of understanding and borrowing from the other team. But I have to say there’s something that’s been so constructive about two teams going at this from different perspectives.
IRA FLATOW: You know, of course, the Nobel Prize is only given out to a maximum of three individuals. But as you say, there are whole teams of people working on these things.
ANDREA GHEZ: Absolutely. So I’m really fortunate to be working with a team of people. It was a team of three in the beginning, and now I have a collaboration that I lead that has about 30 core people. And that’s simply because the–
IRA FLATOW: Wow. Wow.
ANDREA GHEZ: So it’s like running a little small business, and I’m–
IRA FLATOW: That’s a lot of pizza, isn’t it, late at night?
ANDREA GHEZ: It’s a lot of pizza. And it’s just a pleasure because, of course, everybody brings their own expertise or the students bring their own perspective and own questions. And as this project has gone on, it’s become richer and richer. The technology has advanced in a way that’s allowed us to actually not only answer the original question that we were trying to get at, but has actually presented us with more questions than answers.
For instance, we predict that black holes create this very extreme environment around them. They have strong tidal forces. So that means that if you were to fall into the black hole feet first, your feet would experience much higher gravity than your head.
So the anticipation of that extreme environment is that you shouldn’t have star formation happening near the black hole, because star formation requires a pretty gentle environment that should allow the collapse of clouds, and these fragile clouds would be expected to be torn apart. And yet the dominant part of the population that we can see are young stars. So I like to call this the paradox of youth– how do we get star formation happening near a black hole?
Another example of this is we anticipate that there should be lots of old stars surrounding the black hole. Old stars, by the nature of the fact that they’ve been around for a very long time, interact with the environment and should sink to be towards the most massive object in the system, which means that black holes should be surrounded by a population of old stars. And yet there’s actually a dearth of the old stars, so there’s the question of, where are the old stars?
And then the last, and perhaps my most favorite, is a population of objects that are being tidily distorted as they make their closest approach to the black hole. So this is rather amazing, given how far away we’re looking, that we can actually see the evolution of these objects as they come towards the black hole, they get stretched apart, and then they become much more compact as they move away. And these objects have to be about a factor of 100 times bigger than anything we had anticipated at the center of the galaxy. So what in the world are these objects that are being tidily torn apart? So just three examples.
IRA FLATOW: Yeah, it seems like we have so many unanswered questions about black holes. And I want to ask you another– I’m glad I have you here, because I’ve been reading recently about one of the theories is that black holes left over from the Big Bang could have been the source of the dark matter we have in galaxies now that we don’t know what it’s made out of. What is your take on that?
ANDREA GHEZ: Well, so there is an interesting question of dark matter, and there’s a couple of things you can ask about dark matter leftover from the original process of galaxy formation. Today, we think that– the dominant thinking in the field of astrophysics is that dark matter is probably in the form of elementary particles, rather than the big black holes. But there’s still controversy surrounding it.
But the dominant thinking is around the particle idea. But it is true that the black hole, the central black hole, should be surrounded by a sea of these dark particles. And in fact, that’s one of the other things that we’re attempting to investigate is to discover if we can detect the presence of dark matter surrounding the black hole, because those particles, like the old stars, should be populous around the black hole.
IRA FLATOW: That’s fascinating. Let’s talk about relativity. Your research looking at the movement of one star around this black hole concluded that Einstein’s general theory of relativity explains it better than Newton. Is that similar to Mercury going around the Sun?
ANDREA GHEZ: It’s related in the sense that Einstein’s ideas predict this mixing of space and time. And there are various imprints of that on things that pass through this environment. What was actually being tested at first was the impact of that mixing on how photons, or light particles, travel from the object or SO-2, my favorite star, to us. And those photons actually have to lose energy in order to crawl out of the strong gravitational field of the black hole.
So that was what was tested initially. And now what we’re all going after is what’s known as the procession, which is what we saw with Mercury going around the Sun. And of course, what we’re looking for is how does gravity behave around a supermassive black hole?
That’s particularly intriguing, because black holes, in a sense, represent the breakdown of our understanding of physics, the fact that we don’t know how to make the field of general relativity, which is what Einstein was so famous for, describing gravity, work together with the field of quantum mechanics, the field of things that are very small. And of course, black holes are both. So what we’re looking for as we probe how gravity operates around these objects is really a clue for that breakdown, because that’s really the way– we’re assuming that this is the path forward to understanding, ultimately, how gravity can come together with quantum mechanics.
IRA FLATOW: I’m Ira Flatow, and this is Science Friday from WNYC Studios. You mentioned your work at Hawaii’s Keck Observatory in Mauna Kea, which was interrupted by native Hawaiians protesting the construction of the 30-meter telescope. At the time, you acknowledged that the lost time was irreplaceable to your research, but also that the mountain’s culture and spiritual significance needed to be respected. We have talked in the past to native Hawaiians who’d like to have more sovereignty in resolving this conflict. Do you see a way forward with this?
ANDREA GHEZ: I do. And I think it’s incredibly important that we have these hard conversations. In a sense, it’s almost like the discussion that we had a moment ago about competition. Having two disparate points of view is ultimately the way of finding a more creative solution. So I guess coming from this position of 20 years of competition and seeing that arriving at better science, what I hope ultimately is having a better collaboration between the scientific community and the native Hawaiians will help us arrive at a solution that’s right for all of humanity.
IRA FLATOW: Do you think the public gets this? Do you think they understand what’s going on with black holes and your work?
ANDREA GHEZ: I think black holes are fascinating to the public. For some reason, unlike so much of physics, black holes capture people’s imagination. I mean, of course, I think it’s helped by science fiction, where people play with all sorts of concept of space travel. But I mean, it’s one of the things I really like about working in this field is that you can capture that hook that people have or that curiosity about black holes.
IRA FLATOW: So where do you go from here? You’ve won the Nobel Prize, what’s next on your agenda?
ANDREA GHEZ: Oh, gosh, it’s doing science. For me, it’s never doing this– it’s not about prize winning, but rather about scientific exploration. So there’s so much more for us to do in terms of understanding gravity and understanding the astrophysical role that black holes play. And then, quite frankly, working through these issues associated with the 30-meter telescope, which they’re complicated, they’re thorny, and in some sense you need people with the, I don’t know, the scientific stamp of approval that can be viewed as leaders.
IRA FLATOW: Well, that’s a good place to end it. I hope that all comes to pass, Dr. Ghez. Thank you for taking time to be with us today, and congratulations again to you and all your staff.
ANDREA GHEZ: Thanks so much. Thank you, Ira.
IRA FLATOW: Dr. Andrea Ghez is a Professor of Astronomy at the University of California in Los Angeles, where she also directs their Galactic Center Research Group.