Milky Way's origin story revealed by 250,000 stars

Host: Shamini Bundell

Welcome back to the Nature Podcast. This week, putting a date on our Galaxy’s earliest stars.

Host: Nick Petrić Howe

And the pipes that feed Old Faithful. I’m Nick Petrić Howe.

Host: Shamini Bundell

And I’m Shamini Bundell.

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Host: Shamini Bundell

If you’re lucky enough to be living somewhere not blighted by light pollution, on a clear night you can see a smudge of stars crossing the sky. This is the Milky Way – the Galaxy we call home. And as galaxies go, the Milky Way is actually pretty run-of-the-mill, but that’s exactly why it’s so exciting for astronomers. They want to use it to understand how galaxies form and evolve. And this week in Nature, a new paper tries to unravel the origin story of our Galaxy by using a new method that focuses on a unique phase in a star’s life. To find out more, reporter Lizzie Gibney phoned up one of the authors, Hans-Walter Rix from the Max-Planck Institute for Astronomy in Germany, and he told Lizzie a little about the Milky Way’s makeup.

Interviewee: Hans-Walter Rix

So, the Milky Way, it is a disk galaxy. Most of the stars are in a flat, disk-like configuration, going around the centre of the Galaxy like a giant traffic circle. And that’s also why we see the Milky Way as a narrow band across the sky, because if you envision you sit inside a disk-like configuration and look around you, when you look out of the disk you see less than if you look in the plane of the disk.

Interviewer: Lizzie Gibney

And you wanted to look at the chronology, the history, of the Milky Way, over its formation. How did you go about doing that?

Interviewee: Hans-Walter Rix

So, we are trying to reconstruct the story of our Milky Way by measuring the ages, the compositions of the stars and the orbits in which they are. So, an analogy of if you want to study the historical evolution of a metropolis, finding out who moved when to where, who lived where, when houses were built – that’s kind of the analogy.

Interviewer: Lizzie Gibney

And how did you go about then figuring out the age of all these stars?

Interviewee: Hans-Walter Rix

The age is the hardest thing to measure because most stars, at least on human timescales, just sit there and shine. So, the question is how can you get the age of a star and the answer is only indirectly. Stars have a finite lifetime because they start out with nuclear fusion of hydrogen to helium at their cores. And once all the hydrogen at their core is fused into helium, they have to find other ways to shine, and they actually burn fuel at their shells and they become larger. They become so-called sub-giants. And so, if we see how bright a star is right after it’s used up all of its hydrogen at the centre, then we know how massive it was and how long it must have taken it to have used up all the hydrogen. So, by just measuring how bright it is and what it’s made of, we can best infer its age, and this is what we’ve done.

Interviewer: Lizzie Gibney

So, you’re just looking at this subset of stars in their brief subgiant phase, as you can accurately get a birthdate for them. Where did you actually get the data from to figure out their brightness and their composition?

Interviewee: Hans-Walter Rix

There are two data sources that are very pertinent here. One is ESA’s Gaia mission. What this mission does is measure very precisely the distances to stars across the Galaxy, and what that means is, if you look up at the sky, if a star looks bright to you, it may be luminous, or it may just be nearby. And if you have the distance independently you actually know what its luminosity is. The other piece that we used is a beautiful project in China, the LAMOST sky survey that has taken detailed spectra of 7 million stars, and that tells us what the composition of the stars are. And we were able to put this together for the first time, and what it’s done for us is that in the past, for large samples of stars, the gold standard age precision was about 35%. Now we’ve pushed it down to 7% – a five-fold improvement.

Interviewer: Lizzie Gibney

And you’ve used the positions from Gaia and the compositions from LAMOST to improve the accuracy with which we know the age of the stars. And you’ve done that for a whopping 250,000 of them. So, now you have this information on their ages, compositions and their movements, when you put that all together, what did it tell you about the Milky Way’s history?

Interviewee: Hans-Walter Rix

It basically, at first, gives us a clear empirical look at what we see in the teenage years of the Milky Way – the first 2, 3, 4 billion years. And so, the many things one can learn from it, the things we focused on initially was we saw that the oldest part of the Milky Way’s disk actually started forming only 800 million years after the Big Bang. That’s quite a bit earlier than previously seen or perhaps even suspected. So, the Milky Way started making stars earlier than we knew before. The second part that was very interesting was that we learned that the gas from which the Milky Way formed stars over the first 4 billion years must have gotten stirred exceptionally well by some form of turbulence or other mixing mechanisms. So, young stars, massive stars, they die, blow up under their own fusion at the centre, and pollute the surrounding gas with their nuclear products et cetera. Now, if that pollution doesn’t get stirred throughout the rest of the gas, then in some of the subsequent stars, we should find high levels of pollution and in others less. What we saw for the first time is that really the level of pollution in the Milky Way goes up by a factor of 10 over the first 4 billion years. But at any given epoch or age of stars, they all have nearly the same level of pollution, which means the gas must have gotten stirred, and how that actually should happen is not yet clear.

Interviewer: Lizzie Gibney

And there was a last finding in your paper that I found fascinating, which is learning about a quite cataclysmic sounding event that happened quite early in the Galaxy’s history when there was a collision. What has your data told us about that?

Interviewee: Hans-Walter Rix

So, indeed, there are two ways that galaxies can acquire gas. One is gas gradually concentrates at the centre, it settles into a disk, and then from that disk stars form. The other way is by a satellite galaxy having formed stars on its own actually gets dragged in on its orbit and then, by the gravity of the main Milky Way, gets shredded. And so, there had been evidence that such a major satellite had come in about 11 billion years ago, and the reason we can tell it’s come in is because there are stars of that approximate age with very radial orbits. They go in and out, not round and round, and so we could also determine their ages precisely. And if you look at the stars that were in the main body of the Milky Way at exactly that epoch, it was a tremendous boost in the rate at which new stars form, and the conjecture is that infalling satellite really brought the gas that was in the Milky Way out of equilibrium and tremendously increased the probability that the gas turned to stars. That had been speculated before, but now we can see it quite clearly.

Interviewer: Lizzie Gibney

And how do you go about figuring out that this kind of event must have happened from the data that you have?

Interviewee: Hans-Walter Rix

I think there are two ways of approaching that. One is to just argue qualitatively. If two events have the same age, they must have happened at the same time and probably were linked – one causing the other. But the more rigorous approach is, given that all these evolutionary timescales are billions of years, computer simulations of how galaxy formation works have now reached a level that they can ran many hypothetical Milky Ways forward in time over billions of years. And so, in the long run, the best way to interpret these results is to really go through all these simulations, ask which ones match kind-of the archaeological snapshot that we see and which ones don’t, and why and why not.

Host: Shamini Bundell

That was Hans-Walter Rix. Head over to the show notes for a link to his paper.

Host: Nick Petrić Howe

Coming up on the show, we’ll be hearing how scientists have mapped the waterworks beneath Yellowstone National Park. Right now, though, it’s the Research Highlights with Dan Fox.

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Dan Fox

Archaeologists have uncovered new evidence suggesting that a 2,500-year-old artificial lake on an Italian island was in fact one of the largest sacred pools in the ancient Mediterranean. The Phoenicians were master seafarers who ruled the Mediterranean between 1200 BC and 300 BC. The Phoenician island of Motya includes a large water basin that was long thought to be an artificial harbour. But recent excavations uncovered three temples, including one dedicated to Ba’al, the Phoenician god of tempests and fertility, next to the lake. Re-examining the area, researchers uncovered a pedestal in the pool that is a match for a statue thought to be of Ba’al found nearby in 1933. The team also found that many of the sacred structures adjacent to the basin are aligned with constellations. The authors suggest the Phoenicians might have tracked the movements of stars and other celestial objects on the pool’s surface. Dip into that research in Antiquity.

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Dan Fox

The Moon affects many things here on Earth, including, it seems, the altitude that some birds fly. The northern black swift nests mainly in Colorado in the US and winters in the Brazilian Amazon. Researchers interested in the bird’s migratory lifestyle captured seven swifts and equipped each one with a tracking device. The team found that the swifts spent more than 99% of their eight-month non-breeding season in flight. On nights before and after the new Moon, the birds’ average flight altitude was less than 1,000 metres. On moonlit nights, however, especially around the full Moon, the birds spent most of their time at altitudes of roughly 2,000 metres, occasionally ascending to more than 4,000 metres. And during a total lunar eclipse in January 2019, the birds dropped sharply when the Moon darkened, regaining altitude after the eclipse ended. The researchers think that as black swifts glide and wheel in pursuit of flying insects, moonlight might help to reveal their prey. Don’t wait until the next full Moon to seek out that research. Read it in full in Current Biology.

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Interviewer: Nick Petrić Howe

If you think of Yellowstone National Park, you’re probably conjuring up images of the Old Faithful geyser spewing hot water into the air, or bizarre volcanic features full of colourful mineral deposits. For geologists, it’s a bit of a wonderland.

Interviewee: Carol Finn

Yellowstone is a very special place. It has 10,000 famous thermal features, geysers, hot springs. And you might go to the park and say, ‘Where does this hot water come from? Where does it go?’ And that’s what we can see.

Interviewer: Nick Petrić Howe

That’s Carol Finn from the US Geological Survey and, this week in Nature, she has published the first map of Yellowstone’s plumbing. Yes, you heard me right – the naturally formed pipes that feed Yellowstone’s famous landmarks. I gave Carol a call to find out more, and started by asking her why she was interested in where the water in the park comes from.

Interviewee: Carol Finn

Wow, I mean, it’s a wow factor kind of project, in the sense that there’s been a lot of recent work on understanding the deeper magmatic system, and a lot of detailed studies at the surface for looking at geysers and their chemistry, but there’s a big gap between the very surface where we see the water coming out and the deeper parts of the system. The geochemistry told us what it would look like, but nobody’s ever seen the paths by which the fluids come.

Interviewer: Nick Petrić Howe

This is, as you sort of alluded to, quite an intensely studied site. Yellowstone is of a lot of interest for a lot of people. So, how come we didn’t know much about its plumbing before your paper?

Interviewee: Carol Finn

Exactly. I tried for many years to get this data, and part of it is because the data are complex and relatively expensive to acquire. It produces a lot of information so sort of cost per information is low, but the initial costs are high. There aren’t a lot of people that can do this kind of work because it takes a team to be able to carry out these kinds of surveys and then make the geological or groundwater interpretations that we did.

Interviewer: Nick Petrić Howe

That bring me onto the sort of nuts and bolts of it. How are you able to figure out something that’s happening well below the surface, the plumbing as it were?

Interviewee: Carol Finn

Well, we have these remote systems, right. It’s a spectacular image of the helicopter with the 25-metre-diameter roughly loop dangling down beneath it, and it flies about 50 metres off the ground. And the system works like the wireless chargers that we have for our cell phones now or like an induction stove, where current goes in the loop, it generates a magnetic field that in turn generates currents in the ground that generate another magnetic field that’s sent back at the sensor. And that allows us to make models, that we show in the paper, of electrical resistivity. And that’s interesting because water conducts electricity, dry volcano rocks don’t. There are clays that are important in this system that tell us about the plumbing that are also formed by hot water, and those also conduct electricity and are not very magnetic.

Interviewer: Nick Petrić Howe

So, you were able to take these measurements of electromagnetic data in the air, and that tells you about things well below the surface?

Interviewee: Carol Finn

That’s right. Depending on how much electricity can go through the rocks, we can see almost to 700 metres depth in the deepest parts, and maybe to 125 metres in the shallower parts.

Interviewer: Nick Petrić Howe

And what’s the sort of output of this? What do you see? Does it resolve like a map or does it just give you an indication of, ‘This kind of thing is here, this kind of thing is here.’

Interviewee: Carol Finn

So, it’s a map, and that’s what’s so powerful about it. In the paper, we show two ways of looking at the data. One is cross sections, so you can imagine if you had a wall that was 700 metres tall and standing back, you’re kind of looking at a cut in the ground. And we can see these layers of different resistivities, so the higher resistivities in the layers are groundwater and the lower resistivities are hot thermal fluids. So, we can see them all in the system. We can see vertical breaks in our data that indicate probably vertical conduits for water coming from depth. And so, we can look at it in those cross sections and then we also can make some map views.

Interviewer: Nick Petrić Howe

Obviously this is an audio medium, so I won’t ask you to tell us step by step what the map showed, but was there any particularly interesting features you can tell us about?

Interviewee: Carol Finn

Well, the first thing that happened was because people don’t do these surveys, we’re always wondering if they’re going to work. So, I mean, it’s a big risk, right? And when we saw the first images over Old Faithful, we were just so ecstatic because we could see these vertical striping in our models that indicate that there are faults there, and we can see that they’re bringing up hot water. People didn’t know that because the faults are all covered at the surface, so people thought water was mostly coming into, for example, Old Faithful from the side. So, we could see it coming up. But then we could see these different layers which, in our models, have different colours, and it took me a while to realise, ‘Oh my goodness, we’re seeing groundwater pathways at the shallow surface and these hot fluid pathways underneath.’ So, that means that we could see what the geochemistry was telling us, that there’s mixing of fluids.

Interviewer: Nick Petrić Howe

And I guess one thing is now that you’ve got this map, what could it be used for?

Interviewee: Carol Finn

Well, for example, one of my microbiology colleagues, he said, wow, we’ve known about the geochemistry of Yellowstone for a long time, but this is the first time I’ve ever been able to see the pathways for the fluids that come to the surface and result in the great biodiversity that he studies. So, we’re going to try to link the pathways that I see with microbiology. Another thing is to be able to produce groundwater models, qualitative groundwater models, using the layers from our data to understand much more about the flow of the fluids. We have a static picture. I can only guess at sort of where the fluids are flowing away from, for example, the geyser basins. Because the water grows in the spaces between lava flows, you actually are getting a signal of the thickness of some of the lava flows, so geologists can use those to map volumes of volcanics. I would like to work myself on more details of the system in Norris, Old Faithful and Yellowstone Lake.

Interviewer: Nick Petrić Howe

That was Carol Finn from the US Geological Survey. To find out more about this research and to peruse the map yourself, I’ll link to the paper in the show notes.

Host: Shamini Bundell

Finally on the show, it’s time for the Briefing chat, where we discuss a couple of stories that have come up in the Nature Briefing. So, Nick, what have you got for us this time?

Interviewer: Nick Petrić Howe

So, I was reading an article in Nature about a funder that’s barred a university from its grant programme.

Host: Shamini Bundell

So, a grant funder has barred an entire university from getting any of its grants?

Interviewer: Nick Petrić Howe

That is indeed correct. So, this funder is called Snow Medical, and it’s decided to suspend the University of Melbourne from its million-dollar fellowships. So, this was in response to a photograph which showed six people receiving an honorary degree, which is quite a high award, and they were all white men.

Host: Shamini Bundell

So, the university has made a decision on its honorary degrees this year, to award them entirely to white men.

Interviewer: Nick Petrić Howe

Well, not quite, at least according to the deputy vice-chancellor of the university. According to him, he said there were actually four other people who were invited for these honorary degrees – three women and one Indigenous man. So, according to them, there was greater diversity in the people there, but it was just that the six people who were able to come were all white men, then this photo was taken and, well, as the deputy vice-chancellor put it, ‘It backfired rather badly.’

Host: Shamini Bundell

Wow, this is a pretty bold move to sort of bar the entire university because of this photo. What was the reasoning given?

Interviewer: Nick Petrić Howe

So, the chair of Snow Medical, Tom Snow, said that he saw the photo and realised that everyone was white and male, and his heart sank. So, they approached the University of Melbourne to explain what had happened here, and they had a conversation – we don’t know what happened during that conversation – but basically Snow Medical said that their reasoning for this to have happened was unsatisfactory and it would not be considering fellowship applications from the university until it had demonstrated better outcomes. And the university itself has responded to that and the deputy vice-chancellor said that this idea that they’re not committed to addressing gender and diversity inequality is unfair, and 35% of honorary doctorates at the university have been awarded to women since 2017.

Host: Shamini Bundell

And what kind of context does this come in, in terms of the sort of current gender and racial and other signifiers of diversity at this university and in Australia in general?

Interviewer: Nick Petrić Howe

So, it comes amidst a backdrop of broader reckoning for women’s rights in Australia. So, in the past year, there have been thousands of people protesting against the government for their response to a sexual assault that took place inside the parliament building in Canberra. And in academia, there was a breakdown of government spending last year, and it revealed that, even though men and women were applying in the same numbers, men were still getting the majority of the medical research funding in Australia. And so, it’s against this sort of backdrop that then this picture was sent out, and researchers have largely applauded Snow Medical for taking this quite stiff response to it.

Host: Shamini Bundell

And do people think that this kind of action is the kind of thing that is going to bring about change?

Interviewer: Nick Petrić Howe

Yeah, so there’s a couple of people who have been interviewed for the article, and they think that this is going to make universities do a stop and think about what they’re doing and whether enacting policies quick enough. As you said, this is quite a bold step, so it may promote other universities to take more aggressive stands against gender and racial inequality. The question will be whether this translates into broader action that goes on into the future or whether this will just be a flash in the pan for this particular thing. There also could be some unintended effects. The people who get these fellowships aren’t the people who are making the decisions about who gets honorary degrees. They’re not very senior people. These fellowships from Snow Medical are for mid-career researchers, so this may end up disadvantaging the people who the fellowship is actually meant to help. Where we’re at now is the University of Melbourne is going to review its policies, and we’re basically waiting to see what will happen and whether this will translate into effective action.

Host: Shamini Bundell

Well, I can imagine that this might make other universities and institutions consider their honorary degree lists a bit more carefully in the future. Thanks, Nick. That’s fascinating. I’ve also got a Nature article that I’ve been reading today that I found in the Nature Briefing, which has the excellent phrase ‘COVID toes’ in the title. Have you heard of COVID toes?

Interviewer: Nick Petrić Howe

I’ve heard this doing the rounds, but I’ve not really looked into it, so I don’t really have much of a concept of what it is. Something to do with toes?

Host: Shamini Bundell

This has been a whole thing where there was, particularly in that sort of like early pandemic in 2020, a sudden sort of rise in the occurrence of chilblains, which always reminds me of Enid Blyton novels. But it’s this sort of condition you can get on your feet, particularly in cold weather, probably down to poor circulation – it’s not very well understood – which gives you sort of itchy, red and purple patches, for example, on your toes. And suddenly a load of people were turning up to doctors with this condition. So, one of the dermatologists in the article said that usually she’d see one or two cases each winter, but suddenly she was seeing 15 or 20 patients a day. So, quite a notable effect.

Interviewer: Nick Petrić Howe

Right, and obviously in early 2020 there was something else going around, so the association, I’m guessing, has been made between this weird toe thing and COVID.

Host: Shamini Bundell

Exactly, and several sort of dermatologists who were the ones that all these sort of patients were coming to were completely puzzled by this, and people have really thought there’s got to be some link, but we don’t know what it is. So, at the time, people were tested for having COVID when they came in with this condition, but most of them tested negative. And subsequently, they did antibody testing on people who had these COVID toes and found no SARS-CoV-2 antibodies that you would usually expect to be there if they’d previously had COVID, at least in most of the people, apart from a couple who did have COVID. And now, a new study has gone even further and has looked at T cells, whether the body has produced T cells against SARS-CoV-2 and again, no sign, sort of potentially adding to the idea that maybe these chilblains had nothing to do with COVID after all.

Interviewer: Nick Petrić Howe

Oh, so if they weren’t anything to do with COVID, then what might be causing them?

Host: Shamini Bundell

Well, one possibility is that they’re somehow a result of a change in behaviour. So, of the things that were happening in spring of 2020, one of them is that we were all suddenly staying home. So, one idea is that people were not wearing socks and shoes while in the house and maybe getting cold feet that might cause chilblains. They’re not really sure. If I sound hesitant it’s because this is just an idea and it’s really not clear what’s caused this.

Interviewer: Nick Petrić Howe

Okay, but we’re now sort of a couple of years into this. Do we have a better idea now of what’s causing it?

Host: Shamini Bundell

You’d have thought, wouldn’t you, but this paper is just one more bit of a clue, and it’s absolutely not convinced everyone, and one of the researchers quoted in the article said, ‘This is a very small study and maybe not generalisable.’ Another piece of research found the people who had developed these chilblains during the pandemic showed evidence of a strong innate immune response, and potentially someone with a strong innate immune response could have fought off the virus without leading to the rise in antibodies and T cells that would be picked up, so that’s another sort of plausible explanation, that maybe it was COVID, but what we’re doing right now can’t pick up those signs.

Interviewer: Nick Petrić Howe

I see, so what do we need to do to sort of resolve this question, I guess?

Host: Shamini Bundell

Well, I mean, as you can see, it’s all sort of very unresolved in many ways and people aren’t sure. But the chilblains thing, not so much of a problem anymore. It seems to be back to the sort of background rate of chilblains toe issues right now, and the article ends with a quote from dermatologist Patrick McCleskey who says ‘Honestly, I think maybe we can chill out about chilblains.’

Interviewer: Nick Petrić Howe

Thanks, Shamini. And listeners, if you want more stories like these, make sure you check out the links in the show notes, where there will be a link to sign up to the Nature Briefing and all the articles we’ve discussed.

Host: Shamini Bundell

Well, that’s all for this week. Don’t forget, you can follow us on Twitter. We’re @NaturePodcast. Or you can send us an email – podcast@nature.com. I’m Shamini Bundell.

Interviewer: Nick Petrić Howe

And I’m Nick Petrić Howe. Thanks for listening.