Awesome Astronomy - Extra: The Hubble Tuning Fork & Citizen Science

Introduction to galaxy classification

0:00It's the 365 Days of Astronomy podcast, coming in 3, 2, 1.

0:30Hello, and welcome to this Awesome Astronomy podcast extra about the recent paper on Hubble Tuning Fork and Galaxy Classification. This was a story that intrigued us at Awesome, because it's a fantastic example of citizen science and a major change in astronomy in an area that has been accepted science for almost a century. Joining me to discuss this is Karen Masters, Associate Professor of Physics and Astronomy at Haverford College, with a partial affiliation to the Institute of Cosmology and Gravitation at the University of Portsmouth.

1:00Professor Karen Masters, welcome to Awesome Astronomy. This is about, I think I can honestly say this was quite a surprise announcement. I didn't see this one coming. So this of course is about the tuning fork, I suppose, is the kind of familiar anchor that I think everyone will know about. It's Hubble's tuning fork, and that actually we should probably bin it, I suppose. What happened? Well, yeah, I don't know that I'd say bin it, but just be aware that how we put galaxies on it has changed over time. This is actually something we'd been sort of, we'd been sort

1:33of, I'm not going to say fitting on it, but we'd known about this for a little while with the Galaxy Z classifications. And we sort of saw it a few years ago, and we were sort of like, huh, that's strange. And then that's a really interesting place to be in research, but it's also a place where you want to do a lot of checking, right? Because I sort of knew, you know, it's a relatively big deal. We're sort of disagreeing with something that's been, it's in every textbook. And so, you know, you want to make sure you get it right. And then, so we did a lot of extra work, there are a number of other papers on similar topics. And then finally, we decided we're confident to sort of publish this.

2:06The bottom line is that when Hubble published his tuning fork, he said that spirals get placed on this sequence on the basis of two main properties, the size of their central bulge, which is a central bright bit in the galaxy, and how tightly wound the spiral arms are. And Hubble said that correlated very well, so that galaxies with big bulges had tightly wound arms, and those with small bulges had loose arms. And if you dig through the literature, you find all kinds of people classifying on the basis of the arms, because they're really obvious visually, and putting things on that sequence. What we found was there's no correlation

2:39in a big sample. And more than that, I think for astrophysicists, more than that, people are putting galaxies on the sequence on the basis of their bulge size. So they're putting them, it's all scrambled relative to how Hubble did it, you know, almost 100 years ago now. Yes. Yeah. It's, I say, this is, I mean, something when I studied astronomy, this was part of the degree, was to classify pictures of galaxies and actually try and put them in that framework. And when I think back to how I did it, I think, yeah, actually, when I was reading the paper, I was

3:09thinking, yeah, how did I do it? When I was looking at all these pictures, what was I actually doing, trying to fit it? And I think I was actually fitting it into the model rather than looking at what was in front of me. And I suppose that's what this is saying, isn't it? That there's actually, this correlation isn't quite right. Yeah. And in Galaxies, that was quite a deliberate choice. And we asked people to help us classify galaxies that we weren't saying, please put them on this classification sequence. We are asking descriptive questions. So if you look at the questions, it's like, how big is the bulge? How tightly wound are the arms? All of them are sort of separate. And that's why we're

3:43able to sort of find this or make this measurement. And then, you know, then we have to do a lot of checks and make sure that we're able to get good visual estimations of these features in the Galaxies. And we're now very confident of that. And so that's why we published the result. How long did this take before you were, you said you sort of sat on it for a while. I mean, how long was a while? I mean, I think the first draft of the paper was in 2014. Wow. I mean, that's not just the sort of waiting to be sure that it's right. That's also things

4:16like, you know, careers and jobs and other responsibilities. And research can take a long time because of all of that stuff. It really, it really needed, there was a PhD student at Nottingham called Ross Hart. He graduated last year. And his thesis was really looking a lot at the spiral arm measurements in Galaxy Zoo. And it really needed all of that enormous amount of work that he did for us to sort of be like, oh no, actually, we've got a really good measurement of how tightly round the arms are from Galaxy Zoo. So we can go with this.

Crowdsourcing data with Galaxy Zoo

4:45And like a lot of papers, they say you mentioned Galaxy Zoo, there's a lot of people that contribute to this paper. But in some ways, there is a lot of people that contribute to this paper. Because of course, it's Galaxy Zoo. For those who don't know, what is Galaxy Zoo? Oh, right. So Galaxy Zoo is a crowdsourcing project. It's a website. It's www.galaxyzoo.org. And you can go and look at images of galaxies and answer a series of simple questions about them. And what we do behind the scenes is we collect those data, those answers from sort

5:17of 40-ish people per galaxy. That number changes a bit. And then we make a consensus answer. And we've shown it's been running for more than 10 years now. We've shown that those classifications we get from that consensus method are, you know, as good as any other way of getting morphology better in some cases, I think. So yeah, so there's a whole lot of people contribute to this paper through the classifications. One fun thing, actually, was we had a little side project where we invited people who classify to read some astronomy papers and help us find

5:48some examples of how people have applied the Hubble classification. So three of the Galaxy Zoo volunteers are also listed as co-authors because they helped us write the paper and we acknowledge everyone. Yeah, there's a separate link to all the people who did classification. When you go on the paper, there's like another paper, all these people that classify it. It's fantastic. The story goes at the very first Galaxy Zoo paper. I wasn't actually part of this at that moment, but they tried to submit it with everyone as a co-author. So it was like, you know, 20 pages

6:19of co-authors. But the journal said that was not okay. So we just had to go with acknowledgement. So this lack of correlation, so basically, what are you now saying that is being suggested?

Reevaluating spiral arm models

6:32Well, so what's interesting is that the fact of a correlation, I guess I'm doing air quotes, the fact of a correlation was used to justify one particular model for how spiral arms form. And so this was a longstanding problem. So the spiral arms in all these galaxies everywhere and galaxies rotate. And so actually, if the spiral arms were a physical thing, if it was like a string, they'd wind up over time. Yeah. And they'd kind of disappear. So it's called the winding problem. And in the 60s, some people came up with an idea that, you know, they're not physical, they're a density wave. So that's a bit like a traffic jam. Cars move in and out of the traffic jam,

7:06but you've got that sort of over density that you can see. And there's a very particular version of that called the static density wave model that predicts the pitch angle, how wound up the arms are depends on how big the central mass is. So, you know, the fact, again, air quotes that bulge size and arm winding correlate was used to say, okay, most galaxies must be static density waves. Now we see that that's not true. Then we think, you know, it's most galaxies probably not static density waves. Turns out the modelers have kind of moved away from that anyway. It's a bit odd to

7:37think that it would be static. There's sort of the traffic jam moves slowly around the galaxy. And that's okay. You can do models of that where bulge size and arm winding don't correlate. So it's still quite likely that a lot of these are density waves in some form. But we've now sort of lost one of these constraints on the model that was needed before. So the discussion in the paper says that potentially the arms do wind up. Yeah. And the sort of the pattern breaks down after a while and they get tighter and tighter, then it destroys and then probably restarts.

8:08Yeah. What sort of timeframe are we looking at for that sort of event? Yeah, they sort of form on hundreds of millions of years timescales. So it's a long timescale for humans. Galaxy timescales are billions of years. So, you know, the universe is about 15 billion years old. Galaxies have been around for many billions of years. So potentially the sort of spiral shape of, say, the Milky Way has formed, collapsed and really formed within its lifetime then? Yeah, I mean, I wouldn't really think about it completely going away.

8:41It's more that, you know, a particular arm will wind up and another one is forming kind of at the same time. Right. Okay. If you look, if you look out, spirals most have two arms, but you do find them with like three or four and you can see sort of bigger arms or older arms and minor arms. There's actually a lot of debate about the Milky Way, whether it has two arms or four. Yes. Because if you look at different ages of stars, you get different answers. And that, you know, again, could be telling you that the arms themselves actually have different ages, right?

9:12Yes. Yeah. Yeah. Yeah. That would make sense. In terms of like the bar, it was always that the bar seemed to suggest sort of looser arms and wider. Is that still the case? Is there a correlation there or is this now changed as well? No, we did see, we did see that. The bars, the galaxies that have bars did have leoser arms and much stronger two arm patterns as well. So bars, I think, do change how the arms are formed. So it seems much more, you know, much closer to the density wave model when you have

9:44a strong bar because it's sort of stirring the pot almost and driving these arms. So that's interesting. Yeah, it is very. And what initially started this search? When you say sort of a while ago, you sort of noticed, what was the trigger for this project? Oh, I don't know. I was probably just curious one day. Occasionally go in and make a few plots of different morphology things against other things. A lot of the work I've done with Galaxy Zoo has been about the bars, actually. And I think maybe at some point I realised spiral arms were interesting too. And it would be interesting to have a look and see what

10:16we could quantify with the spiral arm classifications. We really feel quite strongly about making use of the classifications, you know, doing science with them. So we're sort of always I'm an observational astrophysicist. That's what I would call myself. I'm always looking for ways to test models. So if I read a paper that says a prediction, I'm really like, oh, how could I test that? Sometimes you can think of ways to test it in Galaxy Zoo. So that's fun. If you wanted to get involved in Galaxy Zoo, how would you do that? Well, so we are collecting classifications right now, galaxyzoo.org. There's a couple of

10:48little sort of side projects that we're working on that are close to my heart at the moment. We've got one going where you're actually tracing over spiral arms for a set of galaxies where that will be useful because we can then use that to get spectra, get more information about the arms. That's called Galaxy Zoo 3D. All of these are part of the Zooniverse, which is a platform for citizen science projects. So if everyone just goes to zooniverse.org and look for the space projects, they should find these. They can find them there. Brilliant. Looking back at the sort of the Hubble tuning fork. So

11:20it's not dead completely. This is, you're saying it's... Yeah, I don't think it'll die. I just think we need to describe it differently and say that in the modern usage, the spiral sequence is to do with bulge size. That's how people are using it. However, in the sort of traditional sequence, arm winding was folded in. And so people need to just be careful about not comparing apples and oranges, right? Yeah. Pairing different types of galaxies to each other. So should... Do you think this will eventually lead to a different terminology? So this sort

11:52of, you know, thinking of the SBCs and SA's and things like that. Is this going to lead to a redesign of that model in terms of how we talk about what a galaxy is and what type?

12:04Maybe it'll be part of that. I think we're moving... I mean, what we're trying to get at with morphology is the way the stars are moving in the galaxy. So sort of like the motion and, you know, spiral galaxies are disks that are rotating and elliptical galaxies are blobs. And I imagine that we're more likely to move towards a more like physical classification. So it's not just looking at the image, but information. You know, if we could measure information about how the stars are moving, that's how we want to classify a galaxy. We can't do that for as many galaxies as we like at the moment. And so we're kind of using morphology as a way

12:38of getting at that. But if we're going to completely make a new classification, we should do it on a new type of, you know, a new way of classifying. Yeah. Someone said to me the other day, talking about this, that it feels like it's the change that happened in biology when they went from visually just looking at creatures to then actually realising that there was DNA that related different creatures in different ways that weren't obvious from their sort of visual characteristics. Is that a good analogy that

13:09was that we were trying to work out if that was a good analogy or not? I have to be a bit careful though, because in the animal world, there is an answer. There is a type that an animal is. Galaxies, I personally think galaxies are much more of a continuum than a set of boxes. So I like to think of morphology as a continuum. There's another reason I really like the Galaxy Zoo way of doing it, because we don't put them in boxes, we put them on a continuum of properties. Yeah. So that's why I'm not so keen on the biology analogy. But going back to it, I mean, it is about getting into like

13:40the actual physics or the actual sort of the mechanism that causes the differences you see. So I guess in that sense, yeah, if you can get more directly at the mechanism that makes those differences, that would be a better way of classifying. If you had to, I say guess, but how are spiral arms formed? Huh. I think that every model that has correct physics happens somewhere in the universe. Wow. So there's a galaxy somewhere that forms with spiral arms that forms using every method you can think of that actually works.

14:11There are enough of them, I suppose. Yeah, yeah. So I think probably most of them are this sort of swing amplified, so the model where they kind of, they pop up as small densities, they get amplified as density waves, they're slowly moving density waves that slowly wind up, and then that just keeps continuously happening. But then you've got, you know, tidal triggering drives these really big density waves, like in the Whirlpool Galaxy. Bars drive big density waves. And then there's clearly ones where it's all flocculent, it's a lovely word, flocculent, stochastic star formation, where it's all just sort of bubbling

14:45up and winding around. That's my phrase of the day, I think.

14:49It's a big universe, so there's probably, there's one of every type. Where does this leave the sort of relationship between spirals and ellipticals and irregulars? Because I think slowly over time, the tuning forks, people have mistaken it as an evolutionary diagram. So curiously, to make an elliptical, we actually think you need many, many mergers. If you just sort of collide two spirals, you're not really going to lose enough angular momentum, so they're sort of orbiting each other. And so you can end up actually with an elliptical

15:20that looks quite like a disk galaxy, just a bigger one, eventually, being like, you know, hundreds of millions of years. But if, you know, if this happens repeatedly, you do, everything will get scrambled and you end up with a giant elliptical. We still think that's happening, absolutely. It's not the case that every single spiral galaxy is going to end up as an elliptical, though, because that's going to depend on where they are in the universe and, you know, the other things that are happening to them. And then irregulars are always such a weird mixed bag, right? Really low mass galaxies can be irregular just because they haven't, they don't have enough gravity to kind of make them smooth and nice

15:53patterns and set up spiral arms and stuff. But we also call mergers while they're going on irregulars, so, or can do, so.

Credits and closing notes

16:01To finish, what's your favourite galaxy? Oh, I'm really a big fan of the Whirlpool Galaxy. That's my favourite. It is beautiful, isn't it? The first one where the spiral structure was recorded. That's the camera masters. Thank you very much. It's been really interesting. The pleasure. Awesome Astronomy is produced at Orbital Sound Limited by Ralph, Paul, Jenny, John, and Damien, and is free to use and distribute with attribution. We promote general science, astronomy, space

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