Meet The ‘Gentle Giant,’ Your Friendly Neighborhood Black Hole
It wasn’t long ago that the idea of capturing an image of a black hole sounded like a joke, or an oxymoron. How do you take a picture of something so dense that it absorbs the very light around it?
But three years ago, we got our first good look with help from the Event Horizon Telescope, which is actually multiple radio telescopes all linked together. That picture was a slightly blurry, red-and-orange doughnut—the best picture to date of the supermassive black hole at the center of a galaxy called Messier 87, which is called Messier 87* or M87*. (Black holes are given an asterisk after the name of their location). Today, it’s possible to buy jewelry and t-shirts with that picture, drink out of a M87*-adorned coffee cup, or just make it your phone background. Now that the first picture of a black hole is practically a pop culture meme, how do you one-up that? In the past weeks, the Event Horizon Telescope team alluded to a new ‘breakthrough’ hiding in the Milky Way.
On Thursday, the team unveiled that breakthrough: the first image of our nearest black hole neighbor in the heart of our galaxy.
This colossal resident of the Milky Way, known as Sagittarius A*, resides 26,000 light years away from Earth. Astrophysicists have long observed plumes of gas and X-ray radiation that suggested a supermassive black hole lingered at the center of the galaxy. Nobel Prize-winning research, looking at the orbits of nearby stars, calculated its mass would be 400 million times that of the sun. But now, we can finally see it. As described in press conferences Thursday morning, the research team used a network of radio telescopes to capture the footprint of Sagittarius A*. What was unveiled in the image matched what the laws of Albert Einstein’s theory of general relativity would predict: a bright ring surrounding a dark shadow—that silhouette being the light-sucking point of no return, the event horizon.
With this picture and data in hand, scientists are now unpacking how our own neighborhood black hole compares to the one already imaged. The Event Horizon Telescope team has already noted some striking differences.
Sagittarius A* is a “gentle giant,” says Feryal Ozel, a member of the global collaboration that created this image. It consumes far less of the gas swirling nearby than M87*, and is far fainter as a result. The Milky Way’s black hole also lacks the galaxy-spanning jets of M87* and, due to its smaller size, the gas around it moves so fast that it took years longer to capture a clear picture.
Ira talks with Ozel about what it takes to obtain such a picture, and what it can tell us about the extreme, high-temperature physics of black holes throughout the universe.
Black holes aren’t just neat to look at. Take a listen to a sonification of the latest image of Sagittarius A* and an audio postcard of two back holes colliding, produced by our friends at World According To Sound.
The audio above is a sonification—translation into sound—of the latest image of Sagittarius A*. Using a radar-like scan, the sonification begins at the 12 o’clock position and sweeps clockwise. Changes in volume represent the differences in brightness the EHT observed around the event horizon of Sgr A*. The material that is closer to the black hole and hence moving faster corresponds to higher frequencies of sound. This sonification was processed in a special way to allow a listener to hear the data in 3D stereo sound, in which the sounds seem to start directly ahead and then move clockwise to one ear then the other as the sweep is made. Credit: NASA/CXC/SAO/K.Arcand, SYSTEM Sounds (M. Russo, A. Santaguida)
Some black holes sound like a wobbling top, while others rumble in low bass tones. Listen to them yourself, thanks to MIT professor Scott Hughes. Produced by The World According to Sound Podcast.
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Feryal Ozel is a professor and chair of Physics at the Georgia Institute of Technology in Atlanta, Georgia.
IRA FLATOW: This is Science Friday. I’m Ira Flatow.
It’s been three years since the first-ever picture of a black hole shook the science world, a giant found in a galaxy called M87, more than 8 billion times the mass of the sun. But hold on, because the world of astrophysics is again abuzz this week, with our first glimpse of the black hole at the center of our own galaxy, the Milky Way. Yes, this time it’s personal, with a stunning doughnut-shaped image of what is called Sagittarius A Star.
If you look at the constellation Sagittarius, you won’t exactly see it, but you’ll know it’s there. And here to share her excitement with us is Dr. Feryal Ozel, Professor of Physics at the University of Arizona, and a member of the Event Verizon Telescope Team.
FERYAL OZEL: Hi, Ira. So nice to be here.
IRA FLATOW: Nice to have you. Now, tell us why you and your colleagues are so excited to get an image of the black hole at the center of our galaxy. I mean, last time we talked to you, what, it was about three years ago– it was about that first black hole picture of M87. So why is this so different?
FERYAL OZEL: It’s true. We talked about M87 first, and that was a few years back. And obviously, we’ve been hard at work at imaging Sagittarius A Star in the meantime. Well, there are two answers to this, really. One is the emotional one, which is it’s our black hole. It’s the one in the center of our galaxy, the Milky Way. So getting to meet it is super exciting.
But the second reason why we are excited is that it is also a doughnut. And you might say, well, isn’t that boring? But no, actually, the fact that two very different black holes, the one M87 and the one in the Milky Way, different in mass, different in activity, different in many different ways, have the same features of the image. When we get all the way near the horizon of the black hole and capture that point of no return, the fact that they look similar is extremely exciting.
IRA FLATOW: And Feryal, I’m looking at this brand new image and I’m seeing the same– as you say– bright doughnut we saw for the last black hole picture around a dark shadow in the middle, the black part. But there are three kinds of extra bright blobs around the rings. Like, if you think of a clock, it’s like 1 o’clock, 5 o’clock, 9 o’clock. Does that sound right? And why are these bright areas there?
FERYAL OZEL: When we look at environments of black holes and we write codes in order to model the physics around it on supercomputers, these types of features do appear in our simulations. They could be, for example, a magnetically enhanced region that looks brighter, or it could be some other thing in the turbulence that is making a particular spot look brighter than another spot. Just like how clouds form, for example, in the sky, sometimes it’s in one place, sometimes it’s in the other.
However, having said that, the particular features that we see in our image are probably artifacts of the missing information and how we fill them in. And in particular, these brighter spots in the image tend to line up with the directions of where we have the most telescopes. So even though it’s natural to expect these from a theoretical point of view, we are not confident that that’s what we are seeing in these pictures.
IRA FLATOW: Let’s talk about what we are actually seeing. We see this bright image of this bright orangey doughnut, but we’re not seeing visible light here, are we? That’s not how the image was collected. It’s not a light telescope that collected it, but a whole network of radio telescopes.
FERYAL OZEL: That’s correct, Ira. The types of black holes that we’re looking, in the vicinity of the Earth, our black hole, Sagittarius A Star, as well as the one, M87, they emit most of their light in the radio wavelengths, and also some light in the X-rays. They are actually pretty dim, even dimmer than in the radio, in the optical wavelengths that our eyes are sensitive to.
So for the Event Verizon Telescope targets, we observe at 230 gigahertz. That’s still a radio frequency. It’s higher than, for example, what we use for our cell phones, but it is a radio frequency, and we collect it with radio telescopes that are sensitive to that frequency. And then we combine the information that we get from, in the part– in the case of Sagittarius A Star today, eight telescopes around the world, looking at the source all at the same time, in order to synthesize this image.
IRA FLATOW: Tell us why there is a black hole at the center of our galaxy. And is that probably the norm for a lot of galaxies?
FERYAL OZEL: That is a great question. That seems to be the norm for pretty much every galaxy that we look at. Anything that is at least sizable, like the Milky Way, like M87, they seem to be a key feature of how early star formation and galaxy formation leads to the formation of these massive giants that then settle to the center of those galaxies and interact with it throughout its life. I mean, throughout cosmic history, pretty early on, black hole seeds seem to form. And then, depending on how much gas is available in their surroundings, either go through periods where they’re growing fast and look like quasars or go through periods where they’re pretty quiescent, like eating very little, like Sagittarius A Star is.
So it seems to be this dynamic give and take between the galaxy and its supermassive black hole in the center.
IRA FLATOW: That’s a great way of describing it. And in terms of describing this black hole, does it confirm theories we have about black holes, or does it upset our ideas about black holes?
FERYAL OZEL: It does some of both. In terms of the gravity around black holes described by Einstein’s Theory of General Relativity– which we keep on trying to crack– we think it can’t be the complete theory; it needs to give in some place– we are still finding that it fits. Our predictions fit our observations perfectly. Or vice versa– our observations confirm the predictions when it comes to these gravitational effects.
But a black hole you might think of as being made up of two parts, its gravity and then this plasma around it– that is swirling around it. And that also has its own challenges and outstanding physics and astrophysical questions. And when it comes to those aspects of black hole environments, these extreme conditions, we found that our theories do well in predicting some of the gross features, but they predict too much variability.
If you think of ocean waves, for example, they have a frequency that’s set by the gravity of the Earth, and then there is an amplitude. It can be one-foot waves or 30-foot waves. And our simulations in the environments of Sagittarius A Star were telling us that things should be moving fast, and they should look more like 30-foot waves. And what we’re finding is that the timescale is correct– they are moving fast– but it’s more like two-foot waves. So we’re probably missing that physics.
IRA FLATOW: Now, when you look at this picture– or now that we’ve got this picture– does it tell us whether the black hole is spinning or not?
FERYAL OZEL: That’s a great question. We are getting the first hints that it might be. And we’re getting that from a couple of different things. One is, when we perform this test of general relativity, by comparing the mass and the predicted size of the horizon that we get from stars orbiting around our galactic center to the image that we get from the Event Horizon Telescope, the only real uncertainty there is the spin of the black hole. It has a small effect on the size of the shadow. And what we’re finding is that models where the black hole is spinning are in better agreement than the models where the black hole is not spinning, in terms just of the shadow size.
It is a very small effect, so I would not claim any sort of confidence in this result. But it’s just a hint that they’re in better agreement when the black hole is spinning.
IRA FLATOW: Speaking of comparing black holes, I mean, one of the images we saw from the old M87 image was that it had a jet, right–
FERYAL OZEL: That’s right.
IRA FLATOW: –tweaking out of it. But this one does not have a jet. It doesn’t appear to have one, as they said in the news conference. Why would M87 have that and this one not have that jet?
FERYAL OZEL: Well, that’s a hard question. Something about how magnetic fields organize in the flow around the black hole must be responsible for launching powerful jets in some cases, like M87. We have tried to get hints of whether there might be a jet, even a small one, hiding somewhere, and we haven’t detected one to date. So from that point of view, M87 and Sagittarius A Star are as different as black holes get.
We don’t know exactly what launches these jets. But one of our excitement today in sharing the picture of Sag A Star was that the doughnut is there at the heart, even when on large scales two black holes could behave very differently.
IRA FLATOW: The Event Verizon Telescope was gathering this data at the same time as it was looking at that first black hole, but why did it take then three years longer to create this image?
FERYAL OZEL: This took longer for a couple of reasons. One is COVID. I mean, it just slowed things down a little bit. The second reason is that Sag A Star actually turned out to be a more challenging environment. We are looking through the disk of our galaxy, through the gas clouds in that disk, the arms of the galaxy, towards our galactic center. And that scatters the light that we obtain in our telescopes. I mean, it scatters the light that the source emits on its way to our telescopes. So there are some corrections that we need there.
And also, because it’s a smaller mass black hole. You’re going to say, how could a 4 million solar mass object be small? But M87 is 6 billion solar masses. So things happen more slowly around M87 compared to Sag A Star. The material close to the horizon can go around it in a matter of minutes. So our techniques needed to be improved, and we needed to understand the additional blurring that that motion causes, and really perform extra tests, both because of the scattering in the galaxy and because of the motion of the gas around the black hole, to make sure that there are robust features, like that doughnut, that we trust.
IRA FLATOW: This is Science Friday, from WNYC Studios.
In case you’re just joining us, we’re talking to astrophysicist Feryal Ozel about the stunning new picture of our very own black hole at the center of our Milky Way.
And there was a sonification, turning the image the EHT collected into sound. I want to play a selection of that and have you tell us what we’re hearing.
What is that? It almost sounds like waves on the shore.
FERYAL OZEL: It does, doesn’t it? Waves, whether they are sound waves or waves of light that our telescopes are sensitive to, have quite similar properties. So we are able to get some of the properties of the image, as we see it in waves changing over time, and turn it into amplitude of sound waves. And hopefully, in the future, when we observe in different frequencies, maybe we will even be able to combine different frequencies of sound to really get a fuller picture of what we hear in that environment, if we could hear it.
IRA FLATOW: This announcement makes me think of the LIGO discoveries of merging black holes and gravitational waves. When we had that first announcement, it was much anticipated. Then we had a second one, where we weren’t sure what to expect, but it kind of helped verify the first. And now we get results fairly often, but we don’t talk about them anymore. They’re not unique. Do you think that’s going to happen with the Event Verizon Telescope– we’ll be just taking pictures of black holes left and right and it’ll be a common occurrence?
FERYAL OZEL: I hope so. I mean, the fact that LIGO releases its new data sets– and of course, scientists talk about it, but we don’t talk about it as broadly as we’re talking about their first two announcements or the Event Verizon Telescope announcements– means that they’ve really transformed into an astrophysical observatory, where we’re doing our daily things– oh, yes, there is another source. What is its mass? What are its properties?
So one can only hope that, with future observations and with all the algorithms we now have in the bank, that we are going to be able to do these observations more frequently. Maybe the first movie is going to be– if we ever get that, and the sources behave in the way that we hope–
IRA FLATOW: Yeah. Yeah.
FERYAL OZEL: –maybe that’s going to be big news. But other than that, we would love to just churn out results that people are like, OK, OK, thank you. Another one.
IRA FLATOW: You say you’ve been studying black holes for 20 years. When you look up now at the Milky Way on a clear summer’s night, now that you’ve seen our black hole at the center of the Milky Way, is it going to make you feel a little bit different?
FERYAL OZEL: I think it will. Being part of that galaxy and being able to see it when the sky is clear is already a wonderful feeling. But now we’re like, you know what? There is a black hole that is 26,000 light years away from us and we took a picture of it, and I know where you are. I hope that our listeners, too, can feel that when they look up at the night sky.
IRA FLATOW: I hope they will, too. And I want to thank you. And congratulations to you on the announcement, and to all of your colleagues and people who helped out. I know there were a lot of them.
FERYAL OZEL: Thank you, indeed. I just want to emphasize again, I was blessed to be the person sharing it, but it is a large team, a huge effort, over a long period of time. And we really value everybody’s contributions. So thank you.
IRA FLATOW: Dr. Feryal Ozel, Professor of Astronomy and Physics at the University of Arizona, and member of the Event Verizon Telescope Science Council.
And if you somehow haven’t seen the picture of Sagittarius A Star yet, you’re in luck. Head to our website, Sciencefriday.com/MilkyWay, to see the new picture. And take a listen to the sound of two black holes colliding while you’re there. That’s Sciencefriday.com/MilkyWay.