Ep. 643: Sagittarius A*

All the waiting is over, we’ve finally seen the image of the event horizon from the supermassive black hole at the heart of the Milky Way. Today we’re going to explain the picture, and what’s next for the Event Horizon Telescope.

Download MP3 | Show Notes | Transcript

Show Notes

What Is a Total Lunar Eclipse? (timeanddate)

Milky Way’s Black Hole, Sag A*, revealed in new image (Medium)

Event Horizon Telescope (EHT)

Astronomers Reveal First Image of the Black Hole at the Heart of Our Galaxy (EHT)

Astronomers reveal first image of the black hole at the heart of our galaxy (ESO)

Astronomers Reveal First Image of the Black Hole at the Heart of our Galaxy (Institute for Advanced Study)

Astronomers reveal first image of the black hole at the heart of the Milky Way (MPIfRA)

Astronomers reveal first image of the black hole at the heart of our galaxy (NAOJ)

Astronomers Reveal First Image of the Black Hole at the Heart of Our Galaxy (NRAO)

We got it! Astronomers reveal first image of the black hole at the heart of our galaxy (NSF)

Focus on First Sgr A* Results from the Event Horizon Telescope (The Astrophysical Journal Letters)

Krispy Kreme celebrating black hole with free doughnut (KRON4)

Press Release (April 10, 2019): Astronomers Capture First Image of a Black Hole (EHT)

An Introduction to Lucky Imaging for Astrophotography (Sky & Telescope)

Andrea Ghez – Facts (The Nobel Prize)

The Nobel Prize in Physics 2020 (The Nobel Prize)

VIDEO: Animation of the Stellar Orbits around the Galactic Center (UCLA Galactic Center Group)



What’s Up: Finding Sgr A* and a Total Lunar Eclipse (CosmoQuest)

Very Large Telescope (ESO)

W. M. Keck Observatory

Massive bubbles at center of Milky Way caused by supermassive black hole (University of Wisconsin–Madison)

In a Pair of Merging Supermassive Black Holes, a New Method for Measuring the Void (Columbia University)

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Transcriptions provided by GMR Transcription Services

Fraser:                         Astronomy Cast Episode 643, Sag A*. Welcome to Astronomy Cast, your weekly facts-based journey through the cosmos where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today. With me, as always, is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of CosmoQuest. Hey, Pamela, how are you doing?

Dr. Gay:                      I am doing well. I am continuing to bat 100 percent on having clouds during the eclipses.

Fraser:                         Yeah, we do, too.

Dr. Gay:                      So, I am just so happy to have seen so many photos from so many people last night. It was truly amazing.

Fraser:                         Yeah. Yeah, it’s a good placement. I mean, the best placement obviously, is right over me. But it was pretty good placement for a lot of people to be able to see it. Everyone in South America, everyone – most people in North America could see it. Bits of Europe. So, but yeah, it was, even if, like, I’m glad that – because we had such terrible weather. It was stormy last night. So, that’s fine. I’m glad other people got a chance to see it, and not me. I’m not bitter at all. Not one bit.

Dr. Gay:                      You are such a liar.

Fraser:                         I haven’t mentioned this, I don’t know if I mentioned this in the show. My favorite lunar eclipse experience was when the lunar eclipse was happening early morning, maybe four in the morning, or so. And I was laying in bed, and I could just watch the lunar eclipse go by, just from under the covers. I could sit there, under the covers, look out the window, and watch the eclipse. That is the perfect eclipse experience.

Dr. Gay:                      That’s exactly what you’re supposed to have. Yeah, this one was positioned in one of the two areas of the sky that I can’t see just by looking out an attic window, because of the trees. And I went outside, and I walked, like, four houses down the sidewalk, and it was, clouds everywhere, and going in and out of holes. And I’m like, I’m not gonna go back and get a tripod.

But there was this one amazing image that I linked to on Twitter, where someone caught the eclipse above a thunderstorm. And so, there’s lightning strikes over the ocean, and the eclipse above. And it’s just like, that is the shot to aspire to. It was amazing.

Fraser:                         That’s amazing. Yeah, yeah.

All right. Let’s do our job here. So, all the waiting is over. We finally seen the image of the event horizon, from the supermassive black hole at the heart of the Milky Way. Different shaped blobs, and a black circle in the middle. What are we looking at today? We’re going to explain the picture, and what’s next for the Event Horizon Telescope. All right, so, did you watch the event live?

Dr. Gay:                      I did. And I have to admit that their use of doughnut is going to remain one of my favorite things, and the fact that Krispy Kreme got in it and offered free doughnuts to anyone who mentioned the discovery, that is going to be my favorite science tie-in for a long, long time.

Fraser:                         Yeah, yeah. That’s pretty great. Did you take them up on it?

Dr. Gay:                      So, we don’t have a Krispy Kreme that I know of, reasonably nearby, but I was inspired to, along with Tiny Intern, go to – and to be fair, Tiny Intern inspired me to go to a bakery called Heaven Scent, S-C-E-N-T, where we spent far too much money getting the most amazing doughnuts. Cake doughnuts, glazed doughnuts, the whole nine yards. All the doughnuts. And yes, I may have gained weight due to this press release.

Fraser:                         Right. You’ve got some doughnut mass to work off.

So, then, I guess – and we have talked about this, I guess not during the actual show, but we talked about it during the after show. By the way, if you watch us live, we spend another half hour answering questions, and just hanging out. So, if you wanna see more, you should join us live when we record the show, every Monday at 11 a.m. Pacific time.

But we had predicted what is, that we were gonna see. And obviously, it was gonna be the photo of the supermassive black hole. That, or the Milky Way. We were wondering if there might be some kind of quirk, tweak, something else as part of it. Would you say that you – you predicted that, maybe, we’d see something happening, like blobs of gas or something going around it. Do you think, did you call it?

Dr. Gay:                      I think I did. The thing that I had requested of the universe, which the Event Horizon Telescope collaboration provided, was that we’d be able to see the blobby gas that was making it so hard for them to process this image. And what we’re dealing with is, with the M87 image that came out in 2019, that is a massive system. And while it does have blobs of gas and dust going around them, it’s kinda like taking a photo of a cow, and you know it’s being swarmed by flies, but the cow’s so big, you don’t usually see the flies that much in the image.

Fraser:                         Yeah. Six and a half billion times the mass of the Sun.

Dr. Gay:                      Yeah, So, Sag A* is 4 million solar masses.

Fraser:                         That’s adorable.

Dr. Gay:                      So, it’s a lot more like taking a picture of a dung beetle in a swarm of flies. Sure, you can see the dung beetle, but you’re really getting interfered with by those flies that are going all over the place. And one of the images, or sets of images, rather, that they gave us, is one that shows the different distributions of how the gas kept interfering with trying to get a clearer image.

They used a technique, somewhat similar to lucky imaging, where they were looking for images that had the same pattern. So, if they saw two that had the same – try and get my hand symmetric here. If they saw two that had roughly the same pattern, they could overlap them and say, okay, we’re gonna add those two together. If they saw two that had a different pattern, they would overlap those, and they were able to add together the images to get the different ways that the gas and dust line up.

Fraser:                         It sounds like stacking planetary images when you’re recording video.

Dr. Gay:                      Yeah. The technique is called lucky imaging. And this wasn’t precisely lucky imaging. They were working in millimeter length, 1.3-millimeter length radio waves. How you process it is different, and it also gives you different information.

When you’re stalking planetary images, it’s our atmosphere that’s causing the problems. For them, it was the actual stuff at the black hole that was causing the problems. So, it wasn’t noise that needed to be thrown away. It was actual information that could be used to work on modeling the orientation of that black hole, as it hangs out in the center of our galaxy.

Fraser:                         So, we’ve known that we have a 4.1 million mass black hole at the center of the Milky Way. We know that, occasionally, gas and dust fall into it. We know that it’s obscured by the material at the center of the Milky Way. What did we learn from this announcement with Sag A*?

Dr. Gay:                      I think the thing that is going to haunt me the most is, the black hole in the center of our galaxy is knocked over on its side like it’s a black hole cosplaying as Uranus. And disturbingly, its pole is tilted within 30 degrees of being pointed at us. And this was one of those papers that I needed to go read because the way they simplified it, to present it during the press release, or press conference, rather, is they said, the black hole is pointed at us. Directly at us. Face on. And black holes have jets that can be rather high energy, and you don’t want one of those pointed at you.

And so, the combination of the physics involved in, how do you knock a black hole over on its side? The concept that it was pointed directly at us, which seems to imply that we’re in a special place, it was all a bit much. But we know from studying other radio galaxies, that black holes do have a variety of different orientations, where when you look at the disk of the galaxy, we will often see these radio jets coming out at a variety of different angles that aren’t perfectly perpendicular to the plane to the disk. Seeing one that is knocked entirely over sideways, we can’t, because the disk of the galaxy is gonna absorb that energy.

So, to me, it was, really, a surprise, just how much it is knocked over, and I’m much more comfortable knowing that it is pointed within 30 degrees of us rather than directly at us, plus or minus three degrees or something.

Fraser:                         Sure. But even at that kinda distance, it’s not an issue. But the – I know this was one of the big questions, and I, actually, had asked a couple of people this question. Is the supermassive black hole at the heart of the Milky Way lined up with the rotation of the Milky Way? And the question that I had gotten from researchers working on this very question was, we don’t know. We haven’t been able to figure this out yet. There isn’t, you know, it’s not feeding, there’s not enough information to give us the answer to that question.

And so, now, we do have that answer. It’s not pointed directly, but it is within 30 degrees of us, which is pretty exciting, and gave a very unique view down into the black hole.

And I guess it’s like, it’s the axis of rotation. It’s hard to really describe a black hole, what direction a featureless black sphere that absorbs all radiation, is pointed at. But it is.

So, we learned that the supermassive black hole is roughly face-on to us, from our perspective.

Dr. Gay:                      Yeah.

Fraser:                         That’s cool. What a coincidence. What else did we learn?

Dr. Gay:                      So, it’s rotating counterclockwise, which means that if you’re using the right-hand rule, we’re essentially looking at its north pole. So, I just find that interesting. Its north pole is pointed vaguely at us, and we see counterclockwise rotation.

Beyond that, it’s starving. It has absolutely no evidence of any jet activity, absolutely at all. And its size is kind of remarkable.

We already knew it was going to be physically tiny, because we’ve seen stars going around it. This is what last year’s Nobel Prize in physics was given for, Andrea Ghez and Reinhard Genzel, Gerhardt, rather, each received Nobel Prizes for their work in the ‘90s, figuring out how to image in the infrared stars orbiting in the center of our galaxy. And whatever was in there with its 4.1 million solar masses of material had to fit within a solar system sized volume, or these stars orbits would make no sense. And just how tiny it is, really, is something that I find amazing.

Fraser:                         They give us an analogy, right?

Dr. Gay:                      Yeah. So, if it’s a doughnut –

Fraser:                         Another doughnut analogy, yeah.

Dr. Gay:                      I get so many doughnut analogies. So, I’m gonna use two different doughnut analogies here. The first one is, if you – I should’ve carried down a doughnut, darn it. I have doughnuts upstairs. If you have a doughnut, and you set it in the center of a large soccer stadium, that soccer stadium’s ring of seats is the glowy doughnut from M87, and that normal doughnut, like you’d get at Krispy Kreme, that you sit in the center of that soccer field, that’s the size of our galaxy, its central supermassive black hole.

Now, on the sky is 52 milliarc – microarc seconds in size. I’m not even used to thinking in micro, so I have to keep stopping myself from saying milli. It’s the same size as if you took that exact same doughnut, set it on the surface of the moon, and tried to look at it.

And another way to put this is, 52 microarc seconds is, if you pull out a piece of hair, hold it at arm’s length, divided into a million pieces, put 52 of those pieces next to each other, the width of 52 millionths of a strand of hair held at arm’s length is the size on the sky of this object.

Fraser:                         Yeah. It’s funny because people are like oh, well, then, can James Webb – is James Webb gonna be able to take a picture?

Dr. Gay:                      No.

Fraser:                         And no. James Webb can take a picture of the region, and it’s actually perfectly adapted to be able to see through the gas and dust, and it will be able to see those stars zipping around the black hole. But no, it’s just a single telescope. They’d use a telescope the size of planet Earth to be able to resolve this doughnut, this moon doughnut, in space, which is just absolutely incredible.

Dr. Gay:                      Yeah. And it all comes down to what wavelength are you looking at. We have these tremendous images coming out from Meerkat of the center of our galaxy, where we see these unresolved blobs of gas and dust and features called snakes. If we go out and look at it with our own eyeballs, all we see is glowiness. In fact, right now, I’m kind of amazed this press conference occurred when you can go outside and see Sagittarius in the sky, you just need to go out after midnight. Go out, find the center triangle, follow it down towards the horizon along that glowiness, and where the Milky Way gets bigger, that’s the gas and dust and optical that we can’t look through.

But it’s only when you go and you combine an entire planet’s worth of telescopes on one hemisphere that you can start to get this kind of a resolution, and it’s only when you’re doing it in this millimeter sized wavelength. We don’t have the capacity to combine the light from multiple telescopes and infrared optic x-ray, or any of these other shorter wavelengths. We just can’t do it.

Fraser:                         In the same way, I mean, the Very Large Telescope does it.

Dr. Gay:                      Well –

Fraser:                         But not in the same way.

Dr. Gay:                      Yes. But it’s doing it mechanically, with fiber optics.

Fraser:                         Yes, yes. Not recording the data separately, and then doing it through time.

Dr. Gay:                      Yeah, and we don’t exactly have fiber optic connected telescopes spanning continents. I wouldn’t wanna try and do the maths for that.

Fraser:                         So – and I’m gonna be complete rebel. But isn’t it kind of interesting to think that if you had, say, the Keck Observatory, and something, and then, the Very Large Telescope, and they took a picture of something at the same time, and they recorded their data, and you knew that they recorded at the same time, in theory, the data would be in there, but there’s no way that we could actually merge it together? It’s beyond our reach, it’s beyond our computational ability.

Dr. Gay:                      It’s a matter of, when we’re combining the light from different telescopes, we need to combine wavelengths that were released from the object at the same time. And with radio, we can actually tune in to the wavelengths. This particular set of telescopes is 230 GHz on the dial, if you could tune to that part of the dial.

Now, the resolution that we’re able to see something is related to two different factors: what wavelength of light are you looking at, and what is the largest diameter, from edge to edge, of observing dishes? So, by having our entire planet, essentially, being the dish, and using, essentially, the smallest possible radio wavelengths, they got the absolute highest resolution we can get. I’ve heard some folks saying, won’t this be amazing when SKA comes online? No, because SKA is a completely different suite of wavelengths.

Fraser:                         Yeah. Yeah, the only way to make this better is to send a spacecraft that can do the same wavelengths out into space, and then increase the baseline to, say, the moon, or something like that. But until then, this is the best that we can do, that we can possibly do. What else did we learn?

Dr. Gay:                      So, the thing that I think was the greatest tease, that we learned, is they did this particular image with just eight telescopes, eight really good telescopes, perfectly spaced around the world. But they have, since then, done additional observing where they have increased the number of telescopes in their system. So, while this is the best we have today, it’s not the best we’re gonna get. We can still improve it a little bit, and the kinds of improvements that we can look forward to is – if you think about how those telescopes were laid out, it was a triangle of telescopes that does a pretty good job of giving you good resolution in all the different directions. But the more you can spread out the telescopes, the more spatial resolution you’re going to get.

So, there is hope for some increased sensitivity, and increased sensitivity over time, and allowing us to build even better movies of what’s going on, and it’s those better movies that give us the most information.

In trying to understand the rotation and trying to understand the orientation, they developed computer models with 11 different variations that they could deal with: mass, the rotation, the magnetic field strength, all these different physical characteristics of the black hole. And they tried to match the changing suite of images that they were seeing to what they saw in the movies they created. And their best fit models could only match 10 of the 11 parameters. And that’s still good, but it’s not 11 out of 11 match. And so, it’s going to take more data, more modeling, to figure out exactly what’s going on. But this tells us we can get there.

Fraser:                         So, if you had 11 out of 11, then Einstein would be 100 percent right?

Dr. Gay:                      Well, we’re pretty much sure Einstein was 100 percent right anyways. He didn’t delve quite so much into how the magnetic field was affecting everything, and those kinds of details.

Fraser:                         Right, yeah. So, a lot of the follow-on work has been done to make these additional predictions.

Dr. Gay:                      Right.

Fraser:                         Yeah. Yeah. So, you were kinda starting to move into my next set of questions, which is, what comes next? We’re still waiting on the polarization data.

Dr. Gay:                      Yes.

Fraser:                         We saw that for M87. So, we’re waiting on that. The Event Horizon Telescope has been gathering more data, but not in the same way, right? They’re not doing observing run every year, are they, in the same way that they did the first time?

Dr. Gay:                      Not that I know of. They didn’t release a lot of details, though. So, that, I’m not entirely comfortable speculating on, I just know they have more partners, more data, and more images coming.

I think, for me, the thing that is most exciting about this is, we’re actually starting to be able to understand the spins of black holes better. And we knew the sucker spinned, we thought, but it wasn’t 100 percent certain. And now. We’re getting to the point where we can say, yes, it’s spinning. Yes, here’s the magnetic field.

And we also have this great enigma of how is it that there is, as near as we can observe, absolutely no jet associated with our black hole? We’ve seen bubbles blown in x-ray light, where we see the shockwaves in the center of our galaxy. We know there has been activity at some point in the past. Why there’s zero jets, or if we’re just not finding it, because we’re not looking right, those are some of the things I’m really looking forward to.

Fraser:                         What else can they take a picture of? Are they done? Is it just these two – because it’s astonishing, right? When you look, when you think about the Sag A* versus M87, they are 1,000 times different in size, and 1,000 times different in distance, and they line up to be, roughly, the same size in the sky, which is a wonderful coincidence, kinda similar to the moon and the sun. But there’s factor 400. So, are there any other black holes that they could look at?

Dr. Gay:                      Not with this kind of detail, because M87’s, really, the big one. But what’s interesting is, sometimes, research results seem to come out as though the entire science community’s feeding off of each other along different ideas. And there was a paper that came out Friday – it may have come out a different day. There was a paper that came out recently, that I read on Friday, that talks about how you can take our new understanding of black holes having the shadow of darkness and this ring of light and look at systems that have binary black holes. And as the one passes in front of the other, it gravitationally, first, magnifies that ring of light. So, you’re seeing the two rings of light, and you get a spike of brightness.

But then, it’s the shadows that magnify. So, you end up with a greater void. And then, as it moves, you get spike again. And so, you can start to get to measure the interplay of doughnut and void shadow by looking at these light curves’ binary systems. And for this, all that matters is, you’re looking at an eclipsing system, and you can accurately measure the light curve.

So, while there’s not a lot of binary supermassive black holes out there, they do exist. And while they’re going to be randomly oriented in the sky so that they have every possible angle, some of them will work just like some planetary systems work out to be eclipsing systems.

Fraser:                         Right. An eclipsing binary, but black hole.

Dr. Gay:                      And so, someday, this is what we’re gonna be to somebody, when we and Andromeda come together. And now, we’re in a position where we can take this knowledge that supermassive black holes have a doughnut of light – ours is the size of Mercury, I failed to bring that up earlier.

Fraser:                         Like, the orbit of Mercury?

Dr. Gay:                      Yeah, so, about that 52 microarc second on the sky is, physically, we’re seeing something the size of Mercury’s orbit. And this knowledge allows us to understand other systems, and that’s kind of awesome.

Fraser:                         Well, it feels great. We got some closure to something that has been haunting us for five years, now, and now, we’ve finally seen the pictures from both black holes, we’ve answered all of these questions, and I’m looking forward to what comes next. Thank you, Pamela.

Dr. Gay:                      Thank you, Fraser. And thank you to all of our audience members out there who have taken the time to join our Patreon community and support all of the people that make the show possible. Fraser and I, join Universe Today on Patreon, join Star Strider on Patreon, we do our own things. And this show, you are funding Richard Beth, Nancy, Chad, occasionally, I think in the past, we are a crowd of humans that make this possible.

And this week, I would like to thank Allan Mohn, Kenneth Ryan, Eran Segev, Omar Del Rivero, Benjamin Müller, Micheal Regan, Don Mundis, Dean McDaniel, NinjaNick, Frode Tennebø, Scott Briggs, Janelle Duncan, Moose and Deer, Jim McGihon, Matt Rucker, Benjamin Carryer, Michelle Cullen, Mark H Widick, Bruce Amazeen, Mark Steven Rasnake, J. Alex Anderson, Philip Grand, Abraham Cottrill, Dwight Illk, Brent Kreinop, Anitusar, Kimberly Rieck, schercm, Jen Greenwald, Father Prax, Dustin A Ruoff,  Gfour184, Gabriel Gauffin, and The Mysterious Mark. Thank you all so much. And if you would like to join our Patreon community, go to patreon.com/astronomycast.

Fraser:                         Thanks, everyone, and we’ll see you next week.

Dr. Gay:                      Bye-bye.

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