Born Thick, Settled Thin

0:00 Anna: Welcome back, everyone. Lovely to have you. Join us for today's episode. 12 billion years ago, the Milky Way was not the thin, elegant spiral we pictured. It was puffed up, turbulent and restless. Avery: And now, thanks to some extraordinarily precise stellar ages, astronomers can tell us when it calmed down and how fast. Anna: Welcome to Astronomy AstroDailyPod. I'm Anna. Avery: And I'm avery. It's Wednesday 21st October, 2026, and this is episode 210 of series five today, Anna: how our galaxy's disc settled in two very different stages.

0:39 Then the gas around the black hole at the centre of the Milky Way turns out to be rich in metals. Avery: We'll meet two of the largest simulated universes ever built, find shadows shaping the chemistry of a stellar nursery in Orion. And it's Orionid Night, so we'll tell you when to look. Anna: Let's get into it. Avery: Okay, let's get stuck into our first story, then. Anna: Let's start with a picture of our own galaxy. If you could fly out of the Milky Way and look at it edge on, most of its stars would sit in a flat disc.

1:12 But it's really two discs, one inside the other. Avery: A fin disc and a thick disc. Anna: Exactly. The thin disc holds the younger stars, including the sun, packed close to the galactic plane. The thick disc is older, puffier, and its stars wander much further above and below the plane. Jerry Gilmore and Neil Reed identified it from star counts back in 1983. And ever since, the question has been, how did the galaxy end up with two? Avery: And there are two main stories, aren't, uh, there. Anna: There are.

1:47 Story one. Stars are always born in a thin, orderly disc. And over billions of years, they get shaken up. Encounters with giant gas clouds, spiral arms, the central bar. Each one kicks a star a little further from the plane. In that picture, the thick disc is just the old, well shaken part of Avery: the thin disc, like a stack of paper that gets messier the longer it sits on a busy desk. Anna: Nice. Story two is called the upside down model. Here, the disc is born thick because the gas that forms the first stars is wildly turbulent.

2:26 As that gas calms and settles towards the plane, each new generation of stars forms in a thinner layer. So the galaxy builds itself from the top down, thick first, then later. Avery: And, um, simulations can produce both. Anna: That's been the frustrating part. Both scenarios work in computer models. To tell them apart. In the real Milky Way, you need to know how the thickness of the disc changed with time. And that means knowing the ages of a lot of stars very precisely, including the oldest Ones, which is famously hard.

2:59 Avery: So how did this team do it? Anna: The study comes from David Mirabal, a, uh, pre doctoral researcher at the Instituto de Astrophysica de Canarias and and the University of La Laguna with Carme Gillart and colleagues in Spain, Italy and Canada. It was published online on 16 October in the Astrophysical journal Letters, with the Institute's release on Monday. Avery: And the data? Anna: Gaia, the European Space Agency's star mapping mission, which measured the positions, motions and brightness of more than a billion stars.

3:32 But Gaia doesn't hand you ages directly. That's where a project called Chronogal comes in. Instead of dating stars one at a time, it fits the whole colour magnitude diagram of a, uh, complete sample of stars in the solar neighbourhood, essentially reading the population's history from how its stars are spread across that diagram. Avery: Give me an everyday version of that. Anna: Think of a national census. You can't interview everyone about their birthday, but if you know how many people of each age a population should have depends on, and how births and deaths shape the numbers, the overall age pyramid tells you a great deal about the country's history.

4:10 Chronogal does something similar with stars, using models of how stars of different ages and compositions should be spread across the diagram and fitting the real distribution Gaia measured. Avery: And, um, how precise is that? Anna: Better than 5% for the oldest stars. That's an uncertainty of around half a billion years, but which at an age of 11 or 12 billion years is remarkable. The team grouped stars into what they call quasi monoage populationsstars of nearly the same age and chemistry, and then measured how far each group spreads above and below the plane.

4:49 Astronomers call that the scale height. Avery: So which story won? Anna: The data show a, uh, clear. The younger the stars, the closer they hug the plane. But the rate isn't steady. There's a turning point about 10.8 billion years ago. Avery: Before and after. Anna: Before it, the disc flattened very fast. In just 2 billion years, the scale height dropped from more than 4,100 light years to about 1,600. After the turning point, the thinning became very gradual, down to around 400 light years for the youngest stars today.

5:26 Avery: And why does that point to upside down? Anna: Because slow, steady heating can't do that. If stars were all born thin and gradually kicked upward, you'd expect the disc's thickness to change slowly and smoothly with age. A, uh, collapse that steep, that early in a young galaxy that was regularly colliding with smaller ones fits gas that started turbulent and then settled in the Author's words. An initial highly turbulent epoch gave birth to the thick disc and a calmer period of gradual gas settling built the thin disc.

6:03 Avery: Mirabal's pretty confident about it. Anna: He says the result resolves a classic problem in astrophysics and calls it the first clear observational evidence favouring one of the two models for the galaxy's early stages. Avery: This lines up with a few things we've covered. Anna: It does. Back in episode 199, we looked at Andromeda, where clustered supernovae turned out to be enough to keep a, ah, galaxy's gas turning. This is the other half of that story. What happens to a disc as that turbulence dies down?

6:33 And in episode 205 we covered the Sagittarius dwarf galaxy and its own disc history. The Milky Way's neighbours are being dated the same way. Avery: 10.8 billion years ago. Isn't M that around when the Milky Way swallowed a big neighbour? Anna: It's in the right neighbourhood. Gaia data showed years ago that the young Milky Way merged with a sizable dwarf galaxy, usually called Gaia Enceladus, or the Gaia sausage, somewhere around 10 billion years ago, or a bit earlier. It's tempting to say the merger ended the turbulent phase, but this paper doesn't make that claim.

7:10 And the dating of the merger has its own error bars. So we'll leave it as an open question and a good one for the next study to test. Avery: And, um, the sovereign angle. Anna: Honestly, there's no Australian team on this one. But two things are worth knowing. Gaia's data came home partly through the European Space Agency's 35 metre antenna at New NORSHA in Western Australia. And the same question is being attacked from the chemical side by gala, the survey run on the Anglo Australian Telescope at Siding Spring, which measures the makeup of hundreds of thousands of stars.

7:46 Ages and chemistry together are, um, how galactic archaeology gets done. Avery: What should we be careful about? Anna: Three things. First, this is the solar neighbourhood, a complete local sample, not the whole disc. Other parts of the galaxy may have settled differently. Second, the authors strongly support the upside down scenario. That's not the same as ruling heating out entirely. Later heating still happened. It just can't explain the steep early collapse. And third, everything rests on those ages.

8:17 They're the best available, but they come from stellar models. And the next test is whether independent age methods like astroseismology find the same turning point. Avery: So, born thick, settled, thin. Anna: Born thick, settled, thin. Our galaxy's calm, flat disc is the end of a much messier beginning. Avery: From the whole disc to Its very centre. Sagittarius A, the 4 million solar mass black hole about 26,000 light years away, is surrounded by streamers of hot gas called the telescope to work out what that gas is made of.

8:56 Anna: And it's rich in metals. Avery: Metals in the astronomer's sense, anything heavier than hydrogen and helium. Nicole Ford, a PhD student in Daryl Haggard's group, led the study in the Astronomical Journal. Webb's mid infrared spectrograph picked out the separate gas structures, the bar, the northern arm and several compact blobs within about a tenth of a light year of the black hole, some racing towards us at more than 600 kilometres per second. And the composition measured from neon, argon, sulphur, nickel and iron.

9:29 The gas comes out mildly super Solar, around 1.8 times the sun's metal content. The team says the enrichment probably comes from stars that exploded and seeded the region over time. And the gas looks a lot like the stars orbiting the black hole, so they may have formed together. They also found nickel and iron being freed from dust. And the compact blobs show signs of fast shocks. Anna: Why does metal content matter for a black hole? Avery: Because metals help gas cool. Cooler gas clumps and clumps can fall inward.

10:05 And Sagittarius A is a strangely light eater. Ford puts it neatly. The black hole is eating a lot less material than other supermassive black holes and more slowly. Knowing what's on the menu is part of working out why. Caveats two, A companion paper led by Mayura Balakrishanan found these infrared lines are shaped by local conditions, so the gas can't serve as a reliable record of the black hole's past outbursts. And um, the preprint first appeared in May, so this is the journal version arriving rather than a brand new result.

10:40 Next step, the team says, is matching the observations to long timescale simulations. Anna: And there is a southern angle you can see for yourself. Avery: There is. The galactic centre is a southern sky object. From Sydney it climbs to about 85 degrees nearly overhead. From Los Angeles it peaks, uh, at 27 degrees, New York 20 and London under 10. Tonight at the end of civil twilight, it's 52 degrees up from Sydney. You won't see the black hole of course, but you can point at it Anna: now to a universe that doesn't exist.

11:16 Or rather two of them. Researchers at the US Department of Energy's Argonne National Laboratory have completed two of the largest cosmological simulations ever run. Avery: How large? Anna: Each one follows 13.4 trillion dark matter particles across a region 28.6 billion light years wide. They ran on Aurora, one of the world's largest supercomputers. Each run used about 8,100 nodes and roughly 50,000 graphics processors. And the simulations, plus much of the analysis took about four days. Avery: Um, and the data?

11:52 Anna: The two runs wrote more than 85 petabytes, mostly as restart checkpoints in case something crashed. About eight petabytes were kept for science. Avery: What's it for? Anna: Cosmic inflation. The brief burst of extreme expansion in the first instant of the universe. The standard picture says inflation left behind density ripples with a simple Gaussian kind of randomness. If the real ripples depart from that even slightly, what's called primordial non gaussianity, it could point to extra fields or physics at work during inflation.

12:28 Avery: And the signal is tiny. Anna: Tiny and hidden on the largest scales, where cosmic variance, the fact that we only have one universe to samplemuddies everything. So you need enormous volume and enough resolution to see the dark matter halos that host galaxies. That's the hard bit. Avery: Here's the clever part, though. Anna: Two clever parts. The two runs are statistical mirror images. Same physics, but with their initial ripples flipped. Analyse them together and a lot of the random noise cancels out.

13:04 And the strength of the non Gaussian signal planted inside is blinded. Teams working on DESI, the Dark Energy Spectroscopic Instrument and NASA's SPHERE X mission, can build mock skies from it and test whether their pipelines recover the hidden answer before they go anywhere near the real data. Avery: Like marking an exam where you don't know the answer key exactly. Anna: A quick note on what this is and isn't. It's a computing milestone announced by Argonne on 19 October. Not a published scientific result.

13:37 The science comes when survey teams use it, but it fits a theme on this show. In episode 193, we covered the dark energy supernova compilation and how much of modern cosmology is about not fooling yourself. Line tests are how you do that. Avery: Since it's Orionid night, let's stay in Orion. About 1350 light years away, inside the Orion Nebula sits a ridge of gas called the Orion Bar. It's where ultraviolet light from young massive stars is eating into the molecular cloud. Anna: They were born from a, uh, photo dissociation region, right?

14:12 Avery: A zone where starlight breaks molecules apart. And we see the Orion Bar almost edge on, so its layers line up like a cross section. That makes it the textbook test case. The trouble is that Alma and Webb images show the layers don't behave Like a textbook. The front where hydrogen molecules are destroyed overlaps the front where ionised carbon recombines and the emission comes in arcs and filaments. Anna: And the models were one dimensional? Avery: Mostly, yes. Tidy layers in a row. M A team led by brank gaetjes@ uh, the University of Duisburg Essen with colleagues at Zhejiang Lab in China has now built what they describe as the first fully three dimensional high resolution model of an Orion bar like region.

14:59 It's in Astronomy and Astrophysics, published online on 16 October. Anna: What does 3D change? Avery: Shadows. Real clouds are clumpy. Dense knots block the incoming radiation and leave shielded pockets behind them where molecules survive and the chemistry runs differently in the Model A uh, box about a tenth of a light year across with cells around 160 times the Earth's sun distance. Those shadows naturally produce the overlapping fronts and the arc like hydrogen emission Webb Cs it's an analogue, not a reconstruction.

15:35 The team uses statistically realistic fractal clouds rather than the actual Orion bar's shape. So it shows the mechanism works, not that every filament is explained. And Orion is one of the few deep sky targets both hemispheres share up after midnight from everywhere this month. Upside down from Sydney Quick hits. Anna: ESA's Hira spacecraft fires its second braking burn, BRM M2 on Thursday. It's a shorter follow up to last week's 93 minute BRM1. Fine tuning Hira's approach to Didymos and Dimorphos.

16:08 The asteroid's dart hit in 2022. Close in characterization from about 25 to 30 kilometres follows in November and December with tracking through ESA's new Norsha Station in Western Australia. Avery: Uzbekistan has signed the Artemis Accords, the 77th nation to do so at a ceremony in Samarkand on Monday. Anna: And NASA will give an update on Artemis on Thursday at 2pm Eastern. That's 5am Friday in Sydney. Including Artemis 3, which NASA now plans as a 2027 crewed test in Earth orbit of rendezvous with its commercial lunar landers ahead of a landing attempt targeted for 2028.

16:47 We'll bring you what said Skywatch. Avery: And tonight is the peak of the Orionid meteor shower. Dust from Hallie's Comet. Under dark skies it typically gives up to about 20 metres an hour. The catch this year is the moon. It's waxing gibbous, around 3/4 lit, so you'll want to wait until it sets. Anna: Remind us why Hallie's dust makes good Avery: meteors speed Orionids hit the atmosphere at around 66 kilometres per second, among the fastest of any annual shower. So the brighter ones often leave glowing trains that linger for a second or two.

17:23 And Earth crosses Hallie's debris twice a year. In May we get the Eta Aquarids, which favour the southern hemisphere. And in October the Orionids, which this year favour the north. Anna: And when do they set? Avery: On Thursday morning, 3:38am in Sydney, 3:22 in Los Angeles, 3:12 in New York and 2:49 in London. After that you've got dark sky until dawn. Anna: And this one favours the north? Avery: It does. And it's worth being straight about it. From Los Angeles you get more than two hours of dark sky after moonset with the radiant near orion's raised club, 59 to 70 degrees up.

18:03 New York is similar. London gets nearly three hours with the radiant 43 to 54 degrees up from Sydney. There's only about an hour between moonset and astronomical dawn at 4:37 and the radiant sits 35 to 40 degrees up in the northeast. Still worthwhile, just shorter. Lie back, look about 45 degrees away from the radiant and give your eyes 20 minutes. Anna: What does the south get? Avery: The evening Mercury is about 16 and a half degrees up at the end of civil twilight from Sydney around 7:39 and sets just after 9.

18:41 From Los Angeles it's 4 degrees up, New York too. And from London it's already gone. And as we said earlier, the centre of The Galaxy is 52 degrees up from Sydney at uh, dusk. Sagittarius and Scorpius high in the west against 21 from Los Angeles and 16 from New York. Anna: And Venus. Avery: Venus reaches inferior conjunction on Saturday the 24th, passing between us and the sun only about 6.5 degrees from the Sun's direction. So not one to hunt for. And never um, look anywhere near the sun with your eyes or any optics unless you're using a certified solar filter that meets the ISO 123122 standard.

19:22 Anna: Anything for the weekend? Avery: On Saturday evening the nearly full moon sits close to Saturn, about 6 degrees apart from Sydney and London. Eight from New York, nearly 10 from Los Angeles. Full moon follows on Monday the 26th at 4:11am, UM Universal Time. That's 3:11 in the afternoon in Sydney. So it'll look full on both Sunday and Monday nights. And in the pre dawn east, Mars and Jupiter keep closing in 10 degrees apart now about 7 by the 28th and 4 by the 5th of November. Anna: And a uh, practical note for our Avery: northern listeners clocks, the UK moves back to Greenwich Mean Time early on Sunday the 25th, and the United States follows on Sunday the 1st of November.

20:08 So for a week, the gap between London and New York shrinks by an hour. If you're planning a pre dawn session next week, check which time your app is showing. Here in Australia, we're already on daylight time, so nothing changes. Plenty to see, which is always a good thing. Anna: That's Astronomy daily for Wednesday, 21 October. Today, the Milky Way's disc was born thick and settled thin. The gas around our central black hole is rich in metals. Two mirror image universes are ready to test the next generation of surveys and shadows are shaping the chemistry of the Orion Bar.

20:46 Avery: Find show notes, sources and the Fullback Catalogue at astronomydaily IO and you can reach us through the contact form there. Your questions really do shape what we cover. We're astrodaily pod on social media. Anna: Tomorrow we'll check in on Hera's second breaking burn and anything else the universe throws at us. Avery: I'm Avery. Anna: And I'm Anna. Uh, thanks for listening. And if you're up for the Orionids, clear skies.