The Star That Was Born Twice: Orion’s Nebula Mystery

0:00 Anna: Welcome back. And if you're new to our podcast, welcome aboard. In the middle of the Orion Nebula, there is a puzzle that has bothered astronomers since the first Hubble pictures came back in the 90s. The baby planetary systems there are being boiled away so fast they should be long gone. And yet there they are today. Avery: A team from Leiden and Chile thinks it knows why the star doing the boiling may have been born twice. Also coming up, a death star with two rings of planetary wreckage.

0:30 The magnetic field of a solar flare measured directly, and a surprising idea about giant impacts. Anna: I'm Anna. Avery: And I'm Avery. This is astronomy AstroDailyPod Series 5, episode 215 for Tuesday 27th October 2026. Let's get into it. Anna: Let's start with one of the most photographed objects in the sky. The orion Nebula, Messier 42, about 1350 light years away. The glowing patch in Orion's sword. At its heart is a tight knot of young stars called the Trapezium. And the brightest of them is a massive star called Theta1 Orionis C. It's roughly 33 solar masses and it floods the whole nebula with ultraviolet light.

1:17 Avery: And that's where the puzzle starts. In 1994, Hubble images of the nebula showed dozens of little dark tadpole shapes. They turned out to be protoplanetary discs, the dusty discs around young stars where planets form, being blasted by that ultraviolet light. Astronomers called them problets. Anna: The problem is the arithmetic. The radiation strips material off those discs very quickly. Some are losing something like a millionth of a solar mass every year. The stars in the cluster are about a million years old.

1:50 Run those numbers and the discs should have been gone long ago, unless they started out about as heavy as their own stars, which doesn't fit anything we know about discs. It's called the proplid lifetime problem, and it has been open for about 30 years. Avery: And there's a second puzzle that looks just like it. The nebula is surrounded by a huge shell of gas, inflated by the winds of the massive stars. Measurements published in 2019 and 2020 clock that shell expanding at about 13 kilometres per second.

2:25 Run that backwards and the shell is only about 300,000 years old. So the discs look young, the shell looks young, and, um, the stars look old. Anna: Two clocks disagreeing with a third. And when two things disagree with the same reference in the same direction, it's worth asking whether one cause sits behind both. Avery: That's exactly what Micah Wilhelm and Simon Portugi Swart at Leyden Observatory with Alessandro Trani at the University of Concepcion in Chile have proposed. Their paper is impress at astronomy and astrophysics, and Leiden announced it on 23 October, she said.

3:03 The preprint first appeared in September, so this is published this month rather than brand new this week. Anna: Their suggestion is that Theta1 Orionis C1, the big star, wasn't always one star. It was two stars in a close pair that merged into one. And here's the key physics a uh star's power output rises much faster than its mass. So 1 star of 33 solar masses shines considerably brighter than 2 stars adding up to 33. Avery: So before the merger, the nebula had a dimmer central engine. The discs could survive for most of the cluster's life.

3:40 Then a few hundred thousand years ago, the two stars collided. The radiation and the stellar wind jumped and and the discs started boiling off. The shell got a fresh shove at the same moment. Extrapolate today's fast rates backwards and you'd get the wrong age for both of them. Anna: And there's a nice detail in why the two stars merge. Theta 1 Orionis C already has a companion today, a star of about 12 solar masses on an 11 year orbit. The team suggests that companion once orbited an inner pair and its gravity slowly pumped up the inner pair's orbit, making it more and more elongated until the two inner stars touched.

4:21 It's a three body effect with a long the von Zepol Litov Cosi mechanism. And it's the same physics that can push comets and planets onto extreme orbits. Avery: They ran Monte Carlo simulations of nearly 2,000 possible starting triples. Outside the very tightest configurations, roughly half of them merged within a million years. So the starting arrangement isn't exotic. Most massive O type stars are in multiple systems anyway. Anna: And the merger story explains two other oddities about this star.

4:52 It has a magnetic field tilted relative to its rotation and a hint of extra nitrogen in its light. Both are things you'd expect if two stars had been stirred together, mixing core material into the outer layers. Avery: Now this is where we slow down, because the authors are refreshingly upfront about what doesn't quite work yet. First, the size of the jump. To explain the shell's speed, the star's wind power needs to have risen by a factor of about 100. A simple merger of two stars gives you at most about 30.

5:27 Anna: They propose the gap is closed by cool gas evaporating off those propolids and mixing into the hot bubble, which changes how efficiently the wind pushes. But that effect has to be strong enough, and they spell out the minimum strength needed. It hasn't been demonstrated. Avery: Second timing in their simulations, when these triples merge, they often merge too quickly within hundreds or thousands of years of forming. Not hundreds of thousands. The team suggests ways to delay it Stronger tides while the stars were still young and puffy.

6:01 Rotation or a passing star nudging a stable triple into the unstable mode. They put the chance of that last one at, uh, roughly 10%. Plausible, not proven. Anna: Third, the best fitting version wants the two original stars to have been near twins in mass, and the authors estimate the odds of that at only 2 to 3%. And the nitrogen excess was measured from the combined light of C1 and its companion, so it still needs confirming in C1 alone. Avery: But here is what makes it good shrines it sticks its neck out.

6:34 There's an earlier claim from spectra that Theta 1 Orionis C1 has a small third companion, about one solar mass on a 61 day orbit. The authors say plainly that if that companion is confirmed, their model is effectively falsified. When they simulated that configuration, it merged in a median of about 135 years. So they argue it can't exist. An observation from the Gravity Instrument on the Very Large Telescope didn't confirm it, uh, but the resolution was borderline, which Anna: is a lovely clean test.

7:08 And because the cluster is so well studied, it may not take long. Avery: Co author Simon Porteghu Zwart says this would be the first time we've seen the consequences of a major stellar collision in our own cosmic neighbourhood, and that collisions may be more common than we thought. Similar triple mergers have been suggested for Eta Carinae and its great eruption in 1843, which is a southern sky object, and for the V838 monocerratus outburst. Anna: And there's a planetary angle. Most stars, very possibly including the sun, formed in crowded clusters near massive stars.

7:46 If a merger can switch on a much harsher radiation field partway through a cluster's life, then whether a young star keeps its planet forming disc depends not just on where it was born, but when. Avery: And one small housekeeping note the paper's acknowledgments. Thank Mordecai Mark Maclo for feedback. The same Maclo who co authored the Andromeda Superbubble paper we led with back in episode 199 Smallfield. Anna: And the best part, Orion is rising in the late evening right now for listeners everywhere.

8:19 So you can go and look at the scene of the crime tonight. We'll tell you exactly when in the skywatch. Avery: From the birth of stars to the death of one, White dwarfs are the cores left behind when sun like stars die. And a few percent of them show something remarkable wreckage from their old planetary systems. Asteroids and comets that strayed too close and were torn apart by tides into a disc. Anna: Usually we see that as a single ring of gas and the giveaway is in the spectrum. Gas orbiting in a ring makes an emission line with two peaks.

8:52 One from the side coming towards us and one from the side going away. Astronomers have found a couple of dozen of these over the past 20 years. Avery: Now a team led by Nicola Pietro Gentile Fusilio with Felipe Lagos Vilchas at the University of Warwick and Detlef Coaster at Kiel, has published a white dwarf that shows four peaks in its calcium lines instead of two. The paper came out in Nature Astronomy on 26 October and it's open access. Anna: The star is called WDJ 1328 -1450, a white dwarf of about 17,400 degrees and roughly half a solar mass.

9:30 And four peaks means two separate rings of gas, one nested inside the other seem fairly close to edge on the scale is wonderful. Avery: The inner ring sits about 100,000 to 240,000 kilometres from the star. The outer ring runs from roughly 380,000 kilometres out to more than a million. So the outer ring is about three times the distance from the Earth to the Moon. All of it squeezed around a star smaller than Earth. Anna: And the two rings aren't the same. The outer one is brighter in calcium, but the team finds magnesium is strongly depleted in it compared with the inner ring.

10:12 Different chemistry suggests different sources. Avery: That's the heart of their argument. The journal summarises it as two hidden planetary bodies shaping the two rings. One asteroid sized and one Pluto sized. Neither body is seen directly. They're inferred from the structure and the chemistry of the gas. Anna: Two honest notes. First, data age. The spectra come from the Very Large Telescope's X shooter instrument at Paranal in chile, taken in 2021 and 2023. And Gentile Fusillo first presented this object at a conference in 2022.

10:49 So the Star has been known as a puzzle for about four years. What's new is the published model. Second, the, uh, dust around the star is harder to pin down than the gas. And the authors note that several dust models fit equally well. Avery: There's a southern thread too. Besides Paranal, the team's brightness measurements draw on Skymapper, the survey telescope run by the Australian National University. At sighting Spring, the star sits at about minus 14 degrees declination, so it's reachable from Both hemispheres.

11:18 Though at around magnitude 17, it's a job for serious telescopes. Anna: And it fits a run of white dwarf stories from the second generation planet candidate we covered in episode 201 to the green white dwarf in 206. It's a reminder that our own solar system will one day be a white dwarf with a ring of whatever's left. Avery: Solar flares are magnetic events. Twisted magnetic fields above sunspots suddenly snap into a simpler shape and release energy. But measuring the magnetic field during a flare is genuinely hard.

11:55 And most of what we know is inferred indirectly. Anna: Here's why. The main tool is the Zeeman effect. A magnetic field splits a spectral line into components. In visible light, the split is tiny compared with the line's width. So instruments measure the polarisation of the light and then infer the field with models and assumptions. Avery: But the splitting grows faster with wavelength than the line's own width does. Go far enough into the infrared and the split becomes wide enough to see directly.

12:26 There's a magnesium line at 12.32 microns. That's one of the most magnetically sensitive lines known on the sun. Anna: China built a telescope specifically to use it. It's called ames, the Accurate Infrared Magnetic Field Measurements of the sun telescope at about 4,000 metres altitude at Langhu in Qinghai Province, a high dry site that keeps water vapour out of the way. It was declared operational about a year ago as the world's first mid infrared solar magnetic field telescope, with accuracy better than 10 gauss.

13:00 Avery: And now the first big result in nature astronomy. On 23 October, Yongyang Deng, Xianyang, Bai, Sholin Wei and colleagues at the National Astronomical Observatories of China report AMES observations through an X 1.5 flare. By measuring the Zeeman splitting directly, they saw the magnetic field weaken beneath the flare ribbons during the impulsive phase, the most violent part of the flare. Anna: That's exactly what you'd naively expect. A flare releases magnetic energy, so the field should drop somewhere.

13:34 Seeing it in a direct measurement is a genuine step, but notice the word Avery: in the title apparent. The authors offer two the field may really have been rearranged locally, or the flare's heating may have lifted the layer where this magnesium line forms to a greater height, where the field is naturally weaker. In that case, you're measuring a different altitude, not a weaker field. Anna: And that distinction matters because flare models depend on knowing which it is. Separating the two will take more flares and comparisons with other instruments looking at different heights at the same time.

14:11 Avery: This joins A solar thread we've followed for a while. From the superflare potential story in episode 192 to the radiation storm measured by weather balloons in 211, better magnetic measurements are the foundation for better space weather forecasting. And, um, that matters to everyone with a GPS receiver or a power grid. Anna: And one practical reminder, because we're talking about looking at the sun, never look at the sun directly or through binoculars or a telescope without proper protection.

14:42 Eclipse glasses must meet the ISO 123122 standard, and telescopes need certified solar filters over the front of the instrument. Avery: One of the clearest patterns in exoplanet science is a gap among small planets close to their stars. There are lots of rocky super Earths up to about one and a half times Earth size, and lots of puffier sub Neptunes around 2 to 3 times Earth size, but surprisingly few in between. It's called the Radius Valley, first clearly measured from kepler data in 2017. Anna: The usual explanation is that many of these planets started with thick hydrogen atmospheres and some lost them, stripped by their star's radiation or boiled off by the heat of their own interiors, leaving bare rock planets that kept their hydrogen stay puffy.

15:37 Avery: Hence two groups and giant impacts. Collisions between young planets are usually thought of as atmosphere strippers too. Smash two planets together and you'd expect to blast gas away. Anna: A new paper in nature astronomy on, um, 26 October argues it can go the other way. Darius Modir Rastagalian and Yoon Kornaga at Yale find that giant impacts can help rocky planets retain hydrogen rich atmospheres. Their argument is that the mass the planet gains in a collision increases how tightly its gravity holds the atmosphere by more than the collision's heat dries gas away.

16:15 Avery: In other words, it's a tug of war between a heavier planet and a hotter one. And they find heavier tends to win. And they suggest this could help explain why exoplanet sizes split into two groups in the first place. Anna: What's interesting is that the same pair argued in a preprint last year that very hot surfaces after an impact can drive off hydrogen remarkably fast. So this isn't a team that assumes impacts are gentle. They're weighing both sides of the ledger. Avery: The caveats? This is a theoretical result and we're reporting it at the level of the Journal's own summary.

16:52 And there are several competing explanations for the Radius Valley, including the idea that some sub neptunes are water worlds rather than rocky planets wrapped in hydrogen. This adds a mechanism to that debate rather than settling it the test will Anna: come from planets, ages, masses and atmospheres measured together, which JWST and the coming PLATO mission are well placed to provide. Avery: A few quick updates. First, NASA is changing how it picks its science missions under what it calls Competition Model 2.0 announced on the 21st.

17:27 Proposals for discovery, New Frontiers and the new Astroprobes class compete on the science first with detailed engineering and cost deferred. NASA says selection time drops from about 35 months to about 19 and a half. Selected concepts then go through implementation studies with Goddard or jpl. And NASA warns that breaching cost or schedule thresholds could mean termination. The community town hall was held On Monday afternoon, US time. Um. Anna: Second Swift NASA formally closed the mission on the 23rd.

18:01 The observatory reached about 275 kilometres on the 20th, fell into safe mode and the team at Penn State sent the command that ended operations. Reentry is expected within the next few weeks. Its orbit is inclined about 21 degrees, so it will come down somewhere in the tropics, a band that includes northern Australia. Most of it should burn up. Avery: Third ESA's HERA spacecraft. ESA says the second braking burn on the 22nd was a success, cutting its speed relative to the Didymos asteroid system by 70 metres per second.

18:34 Three smaller manoeuvres follow before arrival in November and the planetary science community is gathered in Spokane, Washington this week for the annual Division for Planetary Sciences meeting. We'll bring you the best of it. Anna: Let's go and find the Orion Nebula. And this is one of those lovely objects that genuinely belongs to everyone. Because it sits almost exactly on the celestial equator at about 5 degrees south, it's well placed from both hemispheres. Avery: Here are the times for Wednesday night, the 28th.

19:05 In Sydney the nebula rises at about a quarter to 10 in the evening, AEDT by 2 in the morning it's about 50 degrees up in the northeast and it reaches its highest a, uh, very comfortable 61 degrees just after 4am Anna: In Los Angeles it rises at about 10 past 10 and in New York at about 20 past 10. By 2am it's around 40 degrees up in the Southeast from LA and 36 from New York and London, where the clocks went back on Sunday, sees it rise at about half past nine, climbing to about 33 degrees at uh, best Avery: around 3am so the south gets the higher view this time about 60 degrees against 30 to 50 in the north.

19:52 But here's the fun part. From the north, Orion rises on its side with red Betelgeese leading from Sydney, blue white Rigel and The Sword lead the way and Orion appears upside down with the Sword above the belt from Sydney. The nebula is up nearly an hour before Betelgee's. Anna: Look below the three belt stars in the north or above them in the south for a short line of three fainter stars. That's the sword and the fuzzy middle one is the nebula. Binoculars show the glow. A small telescope shows four stars and a tiny trapezoid at its heart.

20:29 And the brightest is Theta 1 Orionis C. Today's lead, you'll be looking at the star that may have been born twice. Avery: One complication the Moon. Monday was the full hunter's moon and this week it's a bright waning gibbous rising in the evening, about 90% lit. On Wednesday it'll wash out the faint outer nebula, but the bright core shows through moonlight. Well, last quarter is on the 1st of November and the darkest skies come with the new moon on the 9th. Anna: Now the planets in the evening.

21:01 Saturn is the steady yellowish light high in the east to northeast after dusk at Ah, magnitude half. From Sydney it's about 31 degrees up at the end of Twilight. From LA, about 19 from New York 16 and from London 11 Southern listeners Avery: also get Mercury's Last Call from Sydney. It's about 9 degrees up in the west at the end of civil twilight around magnitude 1.7. Find a flat western horizon in the next few evenings. From the north it's already lost in Anna: the glare and the north gets the pre dawn show.

21:38 Mars and Jupiter are drawing together in the eastern morning sky at nautical dawn on Thursday. They're about 60 degrees up from Los Angeles and New York and about 50 from London, but only around 25 from Sydney. They're about 7 degrees apart now closing to under 6 by 1 November, 3 by the 8th and just over 1 degree around the 16th. Jupiter is the brilliant one at magnitude -1.9, with Mars much fainter and redder beside it. Avery: Venus passed between us and the sun last week and is still too close to it to see.

22:15 Look forward to emerge low in the east before dawn over the coming weeks. Anna: And one standing reminder Swift may come down anytime in the next few weeks and re entries are sometimes seen as fireballs. If you see something spectacular in the tropics, note the time and direction and report it to your national space or meteor network. Avery: That's the show today. A possible stellar collision behind Orion's age paradox, a white dwarf ringed by two bands of planetary wreckage, a flare's magnetic field read directly in the infrared and a case that giant impacts can help planets keep their air.

22:54 Anna: Full sources and links are in the show notes at, uh, astronomydaily IO, where you can also sign up for the newsletter and send us your questions through the contact form and keep them coming. Listener questions have shaped segments on this show before. Avery: Find us as astrodaily pod on social media and on, um, the bytes.com podcast network. And if you get a clear night, go and find the Orion Nebula. It's a remarkable thing to look at a star and wonder whether it used to be too. Anna: I'm Anna.

23:24 Clear skies, everyone.

23:40 Avery: Sam.