How Do Galaxies Get Into Formation?

23:10 minutes

an image of the milky way with bits of red highlighted on one of the arms
The wave of “stellar nurseries” are highlighted in red. Credit: Harvard University

The Milky Way and distant galaxies are a mix of gas, dust, and stars. And while all of this is swirling in space, there is a structure to a galaxy that holds all of this cosmic dust in order. A group of researchers discovered a nearly 9,000 light year-long wave of “stellar nurseries”—star forming regions filled with gas and dust—running through the Milky Way, and could form part of the galaxy’s arm. 

The study was published in the journal Nature. Astronomers Alyssa Goodman and Catherine Zucker, who are authors on that study, tell us what this star structure can tell us about the formation of our galaxy. 

Plus, astrophysicist Sangeeta Malhotra talks about one of the oldest galaxies formed 680 million years after the big bang, and the difference between these ancient galaxies and our own. 

Further Reading

Segment Guests

Alyssa Goodman

Alyssa Goodman is a professor of Astronomy at Harvard University in Boston, Massachusetts.

Catherine Zucker

Catherine Zucker is a Ph.D student in Astronomy at Harvard University in Boston, Massachusetts.

Sangeeta Malhotra

Sangeeta Malhotra is a research astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

Segment Transcript

IRA FLATOW: A long time ago in a galaxy far, far away, there was a cosmic battle between hydrogen and its electrons. By removing the electrons, the atoms were being stripped of their negative charge. This event would end a period that astronomers called the Cosmic Dark Ages. And closer to home in the Milky Way, stellar nurseries, regions of dust and gas that give rise to stars, these nurseries aligned to form a 9,000-light-year-long wave, spanning a part of our home galaxy, the Milky Way. This would come to be called the Radcliffe Wave.

No, this is not the plot of the next Star Wars movie, but is the synopsis of two presentations from this week’s American Astronomical Society meeting. That presentation on the Radcliffe Wave was also published this week in the journal Nature, and I want to bring on two authors from that study to talk about their findings and ideas about how galaxies form and the structures inside of them. It’s all kind of really interesting stuff.

Alyssa Goodman, professor of astronomy at Harvard, Catherine Zucker, an astronomy PhD student also at Harvard. Welcome to Science Friday.

ALYSSA GOODMAN: Thanks, Ira. It’s nice to be with you.

CATHERINE ZUCKER: Thank you for having us.

IRA FLATOW: Don’t be afraid. Just speak right up there. Well, let me get to you first, Catherine. The Radcliffe Wave that you all describe connects stellar nurseries. What is a stellar nursery? What’s going on inside there?

CATHERINE ZUCKER: Yes, so a stellar nursery, it is a cloud of gas and dust, and parts of this cloud will become so cold and so dense that it will collapse under its own gravity and form new stars.

IRA FLATOW: And the Radcliffe Wave is in the Milky Way. It’s close to our solar system?

CATHERINE ZUCKER: Not only is it close, but it’s right in front of our noses. It’s only 500 light years at its closest point. And if you compare that to the diameter of the Milky Way, 100,000 light years, it’s closer than we could have ever imagined.

IRA FLATOW: And let me ask you, Alyssa, we’ve had to change our ideas about how nurseries are connected and things that surround our sun to understand this new idea?

ALYSSA GOODMAN: Yes, absolutely. So Catherine and her other PhD advisors, Doug Finkbeiner and his group, have been working on ways to measure distances in a galaxy much more accurately to these clouds of dust. And so before, we kind of knew that these stellar nurseries or star-forming regions were around us kind of nearby, but they were scrambled because there was big uncertainty in their distance.

Now, what happened is we put these accurate distances into a 3D picture, and we saw this gigantic wave that we certainly didn’t expect was there. We knew that there was roughly some kind of spiral arm, and maybe there was this Gould’s Belt thing of expanding regions, a region expanding around sun. That didn’t seem to really be right.

Anyway, so instead there’s this gigantic wave that apparently is our section of the local arm of the Milky Way.

IRA FLATOW: I’m Ira Flatow. This is Science Friday from WNYC Studios. So let me see if I can understand the picture. We have our Milky Way. It’s got a nice spiral going around it. But if you look closer, there’s this giant wave going through it. Is that correct, Alyssa?

ALYSSA GOODMAN: Yes. Importantly, the wave is perpendicular to the plane of the galaxy, so up and down out of the disk. And what else is super weird is that if you did look from the top down and you expected that spiral arm, our local section of that spiral arm seems unusually straight, very, very straight. If you look at pictures of what’s called spiral galaxies outside of our own galaxy, if you look very carefully, you’ll notice there are large, straight sections of the spiral arms. It’s more like a dashed line of straight things making what almost looks like a spiral.

And so from the top down, the Radcliffe Wave looks like one of these very long dashes, and it’s from the side, which is our vantage point from in the disk at the sun from the side that it looks like a giant sinusoidal wave.

IRA FLATOW: And so no one thought that this was here. I mean, we were just tooling around and thinking that, well, suddenly there’s a wave here. Somebody is doing the wave here in our galaxy.

CATHERINE ZUCKER: Yes, we had no idea. And so as Alyssa mentioned, there is this paradigm called the Gould’s Belt where we thought all of these stellar nurseries were arranged in a ring around the sun, and that’s existed since the 1870s. And so by mapping these clouds to very high accuracy, we’ve overturned that theory and we found that there is this sine wave instead, which is a shape, as Alyssa said, that we have never seen before in our galaxy or in other galaxies exactly.

IRA FLATOW: How did it get here?

ALYSSA GOODMAN: Ooh, that’s a really good question. So part of the reason it’s called the Radcliffe Wave is because a great deal of this work was done by Joao Alves, who went actually can’t join us today, the first author of Nature Paper and Catherine and some, me at Radcliffe. But in honor of that, we’re having a very small conference later this year called the Radcliffe Wave at Radcliffe, where we bring all the theorists who think they have ideas about how this could have formed and try to figure that out. So you’ll have to ask us again.

But the ideas that people have proposed have to do mostly with collisions of something falling onto the disk of the galaxy from what’s called the halo of the Milky Way, so the stuff right around the Milky Way or the collision of a very small galaxy from outside, but we have some constraints on the motions of these things. That means that whatever fell in had to do it in a kind of very vertical way, not in a super crazy collision from the side way.

But there’s other theories that have to do with not understanding the gravitational potential the galactic disk and dark matter and explosions, and we don’t know.

IRA FLATOW: I love that we don’t know line. And I’m going to use this opportunity to invite our listeners to ask their what’s-going-on questions. Our number 844-724-8255. We’re going to take a break, but when we come back, we’re going to expand our conversation to everything about the galaxy because there’s so much stuff going– and literally stuff going on in there. 844-724-8255. You can also tweet us at scifri, S-C-I-F-R-I. Because you know one of my favorite subjects is dark energy and dark matter, and I’m going to get into what all that dark matter might be doing inside the wave. Is it doing the wave? You’ve brought the dad jokes out on me about the Radcliffe. Wave.

So our number, again, 844-724-8255. You can tweet us at scifri. We’ll be back with Alyssa Goodman and Catherine Zucker, and we’re also going to bring on a researcher from Goddard in Greenbelt. Stay with us. We’ll be right back after this break.

This is Science Friday. I’m Ira Flatow. This hour we’re talking about the galaxies, how they form and some crazy things going on in our own galaxy with Alyssa Goodman, professor of astronomy at Harvard University, Catherine Zucker, an astronomy PhD student also at Harvard. And we were talking about this 9,000-year-long wave, flight year long wave spanning our home galaxy. It’s like the surf’s up in our home galaxy. You’ve heard enough of these, right?

ALYSSA GOODMAN: No, it was kind of fun because it was in Hawaii the announcement. And so there were a lot of surfing jokes. That’s great.

CATHERINE ZUCKER: And the best thing about that surfing joke is we actually think that it’s possible that we’re surfing the wave. Our sun has potentially crossed this structure about 13 million years ago, and we have the potential to cross it again in the future. So people are actually, we think, surfing the wave.

IRA FLATOW: I’m just speechless about this. Let me see if I understand. So this structure has been here. It sits here in space, and as our galaxy rotates around the universe, it just comes across the wave. It surfs the wave.

ALYSSA GOODMAN: As our sun rotates around the Milky Way, it crosses the wave because they’re both in the Milky Way. So the wave is in the up feature in the arm of the Milky Way that we’re almost in. And because the orbits are not exactly concentric and not exactly the same shape, they cross through each other very gently and slowly on tens of million year timescales, which is very small for the age of galaxies.

IRA FLATOW: So how long will we be continuing to surf this wave?

ALYSSA GOODMAN: Probably for as long as it exists and as long as the sun exists. And so the wave is likely to exist far less long than the sun. So the sun has 4 billion years to go or something. But the star-forming regions that are in the wave are probably, at most, 100 million years old, just as gas, so they are much younger.

And so we don’t know what caused this, and so we don’t know how long it will last. But another student at Harvard, Joshua Spiegel, worked with Catherine and others to do very careful fits, mathematical fits of the shape of this. And it shows that it’s damped. So in other words, damping wave is indicative of it dying out at some level or something else forcing it. And so it may just die down and turn into something else in the future and not in the very distant future on the scale of the lifetime of the galaxy.

CATHERINE ZUCKER: But one thing that we do know, following up on Alyssa is that so this structure is forming new stars. And those new stars will be our travel companions around the galaxy as they rotate around the center of our galaxy, but they’re probably going to live longer than our sun.

IRA FLATOW: Wow. You sound very excited.


ALYSSA GOODMAN: We also stated that we had the opportunity. Catherine did a fantastic job of putting all of the data that we used online. So there are two other papers that she wrote, and all of the data that present these distances and all the data from that and the Nature Paper and all kinds of interactive figures and the software is all open source and available. And so this is a wonderful example of open science, in addition. So people who are listening, go out and play with the data themselves and tell us what caused the Radcliffe Wave.

CATHERINE ZUCKER: And one of my favorite things about this structure is that it’s gigantic in 3D, but also if you look up in the sky in 2D, you’re probably looking at some piece of the Radcliffe Wave. It spans almost 150 degrees on the sky. And so famous regions like the Orion Nebula, that is part of the Radcliffe Wave. So when you see Orion, think of the Radcliffe Wave, please.

IRA FLATOW: Whoa! My favorite constellation. So at nighttime, we try to go out and see the Milky Way, on the plane of the Milky Way. Also, if I look at Orion, am I looking at the Radcliffe Wave, stars in it.

ALYSSA GOODMAN: The crazy thing is that when you look at the plane of the Milky Way on the sky, it looks very narrow, and this thing is so big in terms of how far out, it’s out of the planes. It’s 500 light years above and below the plane where it is, and because it’s so close to us, that means it’s a big angle away from that band that you see on the sky.

So there are some beautiful images. We have a website, tinyurl.com/radwave. So people can go there and look, and you can see Catherine made a lovely animation that shows you where on the sky this is, and because it’s so wavy out of the plane, Orion is quite far off that Milky Way that you see on the sky, and it’s the extreme of the wave at one end. And so it’s a big angle.

So actually, no, is the answer to your question. And if you look at the Milky Way, part of it crosses the Milky Way that you see on the sky, but a lot of it is out of the Milky Way. And it would have been impossible to identify the whole thing only looking in 2D, and it’s funny because it wasn’t until the reporter from The Guardian asked us what this looks like on the sky that we added that image to show that. Because we as astrophysicists think about it in 3D and think about what caused this and think about it the concept of galaxy, not about the sky. So we’re glad somebody has asked us that so we can tell you that, yes, it’s all over the place.

IRA FLATOW: To even increase our fun with galaxies we’re having this hour, I want to bring on another guest who studied some of the earliest galaxies that were formed just some 600 million years, 680 million years after the Big Bang. Sangeeta Malhotra is a researcher, an astrophysicist at Goddard Space Flight Center in Greenbelt. I think that’s the oldest NASA site. Welcome to Science Friday.

SANGEETA MALHOTRA: Thank you, Ira. It’s great to be here.

IRA FLATOW: Now, you were able to observe one of the oldest galaxies that formed right after the Big Bang. Tell us what did that galaxy look like. What features did it have in such a young galaxy?

SANGEETA MALHOTRA: It’s not just one galaxy, it’s three. So it’s a group of galaxies. And what’s amazing and unique about this group, that it’s a disruptive group of galaxies. What we see is actively disrupting the gas around those group of galaxies.

And these are the sort of signatures we were looking for because we were looking for this cosmic change called reionization, where most of the gas that’s in between galaxies gets disrupted by like particles coming from within the galaxies.

IRA FLATOW: Does it look like a galaxy or these groups of galaxies, do they look like something we would recognize in older galaxies like our own Milky Way?

SANGEETA MALHOTRA: Oh, they’re tiny, and they’re so far away they’re tiny. I wish I could tell you. There was a huge wave, no, but we see three small dots, and then we confirm those dots. And then because we see this special light called Lyman-alpha, which is a hydrogen light, and then we are able to see it from all of that, it’s like seeing specks and inferring everything.

We infer that the gas around these galaxies has been ionized, and this is the signature we’ve been looking for for a while.

IRA FLATOW: I want to bring all of my guests in here. Let me bring back Alyssa and Catherine. You all seem very, very excited about studying galaxies, and you’re astrophysicists. Sangeeta, do you share that excitement?


IRA FLATOW: Why? Why is it so exciting? I spoke to Vera Rubin many years ago when she was still here, and she talked about her excitement in studying galaxies. Do you share that kind of excitement, Sangeeta?

SANGEETA MALHOTRA: Oh, absolutely. Yes. I gave a talk at Vera Rubin’s Institute once about these very searches we are doing. And she was like, afterwards she taught me, Sangeeta, you’re going to be studying these galaxies all of your life.

And I was like, OK, no problem.

IRA FLATOW: Well, let me bring in one of my favorite topics, which is I want to talk about dark energy. I’m going to talk about dark matter because it was Vera Rubin, who got all that data that said there’s a lot of dark matter in all these galaxies. The stuff you are discovering, does it give us any more hint, even the wave, does that tell us anything more about dark matter?

ALYSSA GOODMAN: It could someday. So there’s yet another student at Harvard. His name is Gus Beane, who while he was an undergraduate, did some work simulating the collision of the Sagittarius dwarfs, a tiny little galaxy colliding with the Milky Way, and it makes these big waves.

The wavelength of what he published is a little bit too long, so it’s the wrong magnitude, but it’s kind of the right sort of phenomenon. And so I am not saying that it definitely is something having to do with dark matter. But if we have the amount of dark matter, either in the halo, or the disk of the galaxy wrong or the distribution of it, how clumpy it is, that could ultimately have something to do with the explanation.

So there are hints that this probably has something to do with a collision. There are not hints that we definitely need dark matter to make everything work out mathematically, but it’s not nuts to think that that might have something to do with it, but it’s a big might.

IRA FLATOW: All right. I’m going to get very geeky now, and go to the phones, because a lot of astrophysics geeks are on the phone. Let’s see if we can understand, if all my audience can understand what they’re asking. I’ll start with Ron. Hi. Welcome to Science– Rob in Nevada City. Let’s go to you. Yes.

AUDIENCE: Yeah. Hello. I have a question about the harmonic series that we experience in our everyday life, especially if we’re musicians, I guess. But does that apply to this wave, and also, do you have an idea of where the origin of the harmonic series might be?

IRA FLATOW: Good question.

ALYSSA GOODMAN: Actually, that’s a really good question. Gus, the student who I mentioned who did that simulation of the collision, he’s mentioned already that higher order harmonics– and for those of you who don’t know what that means, if you pluck a string, there’s one sort of major sine wave that’s given by that pluck, but then there’s a lot of smaller vibrations that are these so-called higher order harmonics that would also happen.

And so it’s possible that there’s a phenomenon that happens on many skills at once that has these harmonics and that we’re just seeing one that happens to manifest in the dust, in the gas. There could be something else going on on different timescales or in the stars, and we’re looking for all of that. So the next release from Gaia, Catherine maybe can talk a little bit more about that, will come later this year, and we’ll have even more information about the velocities of stars.

So this wave is seen in the gas and the dust, but we don’t know exactly, not for all of the stars anyway, what they’re doing. And so it’s a really good question. And yes, the reason we’re so interested in a collision is because that might be like plucking a string-like thing that was– and I don’t mean superstring. I don’t mean that kind of string, analogy string, something that sort of was this region of the galaxy and had an aura-like structure, and if you pluck it, it will wave.

IRA FLATOW: Let me ask you this question then from our caller. Our caller was talking about harmonics, which in music we know are different frequencies. If you have found this big wave, does that mean there are harmonics of it, other smaller waves or different frequency waves somewhere else, or maybe we haven’t found them yet?

ALYSSA GOODMAN: Very possibly. One of the other papers in the meeting this week was actually talking about a much larger scale wave in the stars from Gaia. And so what I’m saying is that this could be a harmonic of something even larger or smaller. In other words, there could be a whole set of waves at different frequencies, and this is just one of them, but we don’t know.

IRA FLATOW: And so we think this happened because of a collision. Is that what you’re saying?

ALYSSA GOODMAN: I think that’s the majority opinion. Catherine, what would you say?

CATHERINE ZUCKER: Yeah, I would say that’s my favorite scenario, but it’s not the only scenario. And so that’s why we need to bring all these theorists together from all over the world to figure out exactly how you form this structure, and then we can start to answer those questions.

IRA FLATOW: I have another question after that. I want to go to Justin in San Antonio. Hi, Justin.

AUDIENCE: Hey. How’s it going?

IRA FLATOW: Hey, there. Welcome.

AUDIENCE: Yeah. I was actually just about to ask about that, whether that was a structure that maybe we ran into and then drug along with us with our gravity from the galaxy or if it was something maybe leftover no man’s land material that hasn’t formed into stars yet.

CATHERINE ZUCKER: Yes. So that’s a great question. And so what we do know about this structure is that it’s 3 million solar masses, 3 million times the mass of our sun and gas, and about 1% of that mass is in young stars, so tens of thousands of young stars across this structure. And so this material is a lot younger than the type of material that formed our sun, which is a lot older.

IRA FLATOW: I’m Ira Flatow. This is Science Friday from WNYC Studios. Sangeeta, when you study these very young stars, do you expect to find a wave going through them yet?

SANGEETA MALHOTRA: I wish we could see that wave, but what we see actually, is one small galaxy, and these galaxies are typically 100th the size of the Milky Way. They haven’t fully fledged yet. And the stars are extremely young. We’re looking at a few million years young stars. And a whole lot of star formation, star burst, even going on in this very small size.

IRA FLATOW: The stars, the universe started out with a lot of hydrogen. It’s still the most abundant element in the universe. Have you figured out in these young galaxies how that hydrogen turned into galaxies?

SANGEETA MALHOTRA: Well, that’s a very good question, and that’s why we’re observing them.

IRA FLATOW: Any guesses?

ALYSSA GOODMAN: It’s important though. It’s important to point out to your listeners that there are many, many generations of stars in the universe. And so the process of star formation is ongoing all the time. And so some stars do live for billions of years, but a lot of young stars like the ones you really like in Orion only live for millions of years. And so there’s been many generations of those. And so the early galaxies have maybe the first stars, but our galaxy has hundreds, thousands of generations of stars.

IRA FLATOW: Well, while we’re talking about Orion, I want you to weigh in on Betelgeuse. Where do you stand in it’s going to become a supernova or not?

ALYSSA GOODMAN: I’m passing that to Sangeeta.

IRA FLATOW: OK, Sangeeta, you’re up. You’ve got the spotlight on you. Who are you going to pass it to?

SANGEETA MALHOTRA: Oh, I wish it would explode. I’d love to see that in my lifetime. It would it be all there.

IRA FLATOW: What would it look like? I heard it would be as bright as the moon. Is that right? There would be something during the daylight time that would be bright in the sky or only in the evening.


IRA FLATOW: I want to see that too.

SANGEETA MALHOTRA: Yes, I want to see that.

ALYSSA GOODMAN: The problem with astronomy is that there’s these little uncertainties, but the little uncertainties, when you multiply a small percentage by a really big number, that turns out to be a large number of years that you’re not sure about something. So unless it’s some process that has to do with gravity at extremely high masses like it’s the black hole at the galactic center where the timescales involved are years, it’s really hard when the timescales could be years, could be tens of years, could be hundreds of years. We’re not really sure. So that’s why I’m not answering any questions.

IRA FLATOW: You’re such a party pooper.

SANGEETA MALHOTRA: No, but this is great. I mean, as Alyssa mentioned, this stellar birth and death and all of this happens in galaxies. And the dark matter that you mentioned maintains this sort of stable place where all of this stuff, the heavy elements that come out of stellar beds, they stick around because dark matter makes this stable place for galaxies and these gas to stick around. And then they get into the next generation of stars. So it’s a lot of recycling. It’s a lot of exciting stuff happening.

IRA FLATOW: Well, we have to put a little pause on our excitement cause we’ve run out of time, but we’re talking about one of my favorite subjects. And I want to thank you all for delighting us today. Sangeeta Malhotra is a research astrophysicist at Goddard Space Flight Center in Greenbelt. Alyssa Goodman, professor of astronomy at Harvard. Catherine Zucker, astronomy PhD student also at Harvard. Thank you all for talking about astrophysics. Love it.

CATHERINE ZUCKER: Thank you for having us.

IRA FLATOW: You’re welcome.

SANGEETA MALHOTRA: Thank you for having us.

IRA FLATOW: Have a good weekend.

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