Starts With A Bang #133 - The Milky Way's center

Here in our Universe, there are all sorts of places where stars are actively forming right now. Our galaxy is going through a relatively quiet phase at present as far as star-formation is concerned, but that rate was much higher in the past: perhaps 30 (or more) times higher than what it is today. If we want to understand what our galaxy was like earlier on, we have two ways to go about it: look farther away, which has the disadvantage that we can't really observe those places in such great detail, or look to the galactic center, which has the highest star-formation rate of anyplace in the Milky Way.The central molecular zone, in particular, is where the greatest amount of activity is: just two degrees across and half a degree high, centered on our supermassive black hole some 26,000 light-years away. This region was obscure to humanity for nearly all of cosmic history: blocked by all of the intervening neutral matter, and by cosmic dust in particular, that optical light simply can't penetrate. But in the 20th and 21st centuries, with the advent of multiwavelength astronomy, the secrets of the galactic center are finally being revealed as never before.I'm so pleased to get to speak with Prof. Cara Battersby of the University of Connecticut for this wonderful episode of the Starts With A Bang podcast. From space telescopes to technosignatures and a whole lot more that you might not have considered, the central molecular zone, or CMZ for short, is one of the most scientifically interesting regions in all of known space. Come find out what makes it so fascinating on this all-new episode!This multiwavelength view of the Milky Way's galactic center showcases the central molecular zone: the most active star-forming region in the entire galaxy. In purple, hot, massive stars illuminate clouds of neutral hydrogen, with cold, dusty molecular gas at just 20-30 K illuminated in orange. The cyan and colored points arise from stars and from complex molecules, such as polycyclic aromatic hydrocarbons. This whole region spans only around 2 degrees across. (Credit: NRAO/AUI/NSF; Adam Ginsburg and John Bally (Univ of Colorado - Boulder), Farhad Yusef-Zadeh (Northwestern), Bolocam Galactic Plane Survey team; GLIMPSE II team)

Whenever you look at the Universe in a more powerful way than you ever have before, either in a new set of wavelengths, at higher sensitivity, on wider-field scales, or with novel capabilities, you open yourself up to a remarkable possibility. In addition to the "obvious" science gains that you'll achieve from finding more examples and revealing greater details about the types of objects and events you already know about, you also give the Universe an opportunity to surprise you: by finding things, thanks to the new power of your novel observatory, that you never expected would be out there.This isn't necessarily restricted to flagship missions that "brute force" their way past the limits of our prior frontiers, but can sometimes, for certain specific use cases, be accomplished by much lower-cost missions that specialize in one particular set of applications: what I call "finesse missions" rather than flagship missions. In the far-ultraviolet part of the spectrum, there's very little that we can do for astronomy from down here on Earth; we have to go to space to access that portion of the spectrum. But other than NASA's Hubble Space Telescope, far-ultraviolet telescopes have been few and far between. Thanks to Principal Investigator of the ASPERA mission, Dr. Carlos Vargas of the University of Arizona, that's all about to change.In this fascinating conversation, we talk about the science that's awaiting us in the far-ultraviolet, including in one of the most poorly studied regions of our own galaxy: the circumgalactic medium. It's an important place, as it's quite likely where the majority of the baryons, or the components of normal matter in the Universe, are actually located. Have a listen to Carlos and I talking about the far-ultraviolet, the ASPERA mission, and more on this edition of the Starts With A Bang podcast!(This image shows a mock-up of the ASPERA mission, slated to launch in just a few months, in space, where it can probe the far-ultraviolet emissions in the outer halo of our own galaxy and a whole lot more. Credit: NASA/ASPERA/C. Vargas/University of Arizona)

In order for stars to form, you need the right ingredients to make it happen: gravity, mass, time, and of course the right type of matter in the form of baryons. Shortly after the Big Bang, the Universe had plenty of them, but they were all very simple: protons, deuterons, helium-3 and helium-4 nuclei, and a tiny bit of lithium-7. These nuclei, made out of protons and neutrons, were all that the Universe gave us to work with prior to the formation of stars. But, in clumps of material small and large, from individual star clusters to enormous galaxy cluster scales and everything in between, these baryons went to work and, across the Universe over the past 13.8 billion years, created sextillions of stars within the observable Universe.In a very real way, the story of how we ourselves came to be is the story of how baryons evolved, matured, and wound up deducing our own cosmic history. Yet so many mysteries about that process still remain, from the baryon life cycles within galaxies to identifying the right conditions to trigger star-formation in the vastly different environments where they formed throughout cosmic history. Yet, it's by examining so many different environments where stars do and don't form, in galaxies of all different masses and metallicities, and in the modern, nearby Universe as well as far back in ancient cosmic history, that we can begin to piece the full story together.Here to guide us through this topic, and starring in this episode of the Starts With A Bang podcast, I'm so pleased to welcome newly-minted Dr. Ava Polzin to the show. Ava got her Ph.D. from the University of Chicago and is about to begin a postdoc at the University of Toronto, and has a whirlwind tour of information in store for us all on this episode of the podcast. Plus, at the end, there are a few bonuses she was eager to highlight, and you can find the links to two fabulous papers she's written for the good of the world below. Make sure you tune in; this is one episode you won't want to miss!Astronomy as a Field: https://arxiv.org/abs/2312.04041Picture an Astronomer: https://arxiv.org/abs/2512.24465(This image shows a composite of Hubble and JWST data for gas-rich spiral galaxy NGC 7496, which highlights not only the starlight you're used to seeing in optical images, but heated dust that shines brilliantly in JWST imagery, showcasing the locations of near-future episodes of star-formation yet to come. Credit: NASA / ESA / CSA / Judy Schmidt)

One of the most foundational questions we know how to ask in astronomy is simply this: given a cloud of gas of a given mass, what types of stars will form? How many stars of a given mass will you wind up with, and what factors does that depend on? The answer to this question, if we can give an answer, is known as the "initial mass function," and is generally very difficult to measure except in the most nearby of places: within our own Milky Way.It's possible that every time we form stars, we have a different initial mass function to reckon with. It's possible that in a different environment, perhaps with less dust, fewer heavy elements, or earlier in the Universe (when the background temperature was hotter), things behaved very differently from how they do in the here-and-now. Yet because of the extreme difference in brightness between high-mass and low-mass stars, we can only measure both high and low mass stars together nearby. It's as though we're only measuring the tip of the cosmic iceberg as far as stars go, where we're compelled to use what we know to draw conclusions about the rest of the Universe.In a very exciting new development, University of Missouri professor Charles Steinhardt, along with his undergraduate students Carter Meyerhoff and Alexander Luening, just put out a paper (link here: arxiv.org/abs/2603.23594) that could wind up revolutionizing what we think about star-formation across the Universe. Astronomers have long considered a top-heavy mass function as a possibility, but early on, perhaps "bottom-light" is a better answer. Have a listen and a good think for yourself in this truly remarkable episode of the Starts With A Bang podcast!(This image shows the Eagle Nebula, Messier 16, in a three-color composite that closely approximates the colors a very sensitive human eye would be able to see. Although the gas and dust makes prominent features, those are transient; they will be blown away in only a few million years. Although the new stars inside have formed across all different masses, the majority of the new starlight is dominated by massive, bright, blue, short-lived stars. Credit: ESO.)

We often think about the Solar System as being our own cosmic backyard, and in many ways, it is: these are the closest objects to us in all the Universe, and our only opportunity to study lunar and planetary systems in situ. However, when it comes to the objects beyond Saturn, including the Uranian and Neptunian systems, as well as everything that lies in the Kuiper belt and beyond, the only probes we've ever sent their way are Voyager 2, which flew by Uranus and Neptune in the late 1980s, and New Horizons, which flew past Pluto in 2015.That means, unlike Jupiter and Saturn, we've never had a dedicated orbiter, lander, or atmospheric probe around the outermost planets or lunar systems even in our own backyard. Moreover, there are no such planned missions that are funded and slated to fly, which is really too bad, as there's so much to learn about these planets and worlds that are so well-represented in exoplanet analogues all across the galaxy and Universe. In particular, one moon stands out as the largest body with a solid surface: Triton, the 7th largest moon in the Solar System and which represents more than 98% of the mass of all the moons that orbit Neptune.Here to guide us through the far reaches of our Solar System, I'm so pleased to welcome PhD candidate Lana Tilke to the program. There's a whole lot of ground that we cover, and the conversation left me inspired with the questions that we're asking today, and brimming with hope that we take the steps we needed to answer them. If you'd like to know where we are and where we're headed next, you just might love this episode too!(This image shows a composite of Neptune's giant moon Triton, assembled from Voyager 2 imagery at the highest possible resolution. The dark streaks come from cryovolcanic geysers, also known as black smokers, from Triton's south polar region. Credit: NASA/JPL)

Whenever a new star forms, several processes appear to be nearly universal. A cloud of cold molecular gas contracts, fragments, and rapidly collapses in certain places. The densest, coldest clumps of gas contract first, drawing in larger and larger amounts of matter onto them. A large, massive enough clump will heat up and have a random shape: collapsing along the shortest axis first, forming a protostar at the center surrounded by a disk of material. That's where the story of planet formation begins.Assuming the conditions in the disk are sufficient, clumps will begin to form, and over hundreds of thousands to millions of years, the first protoplanets and then full-fledged planets will arise: a relatively rapid cosmic process, that's usually all complete within a mere 10 million years: a blink of a cosmic eye in the history of our own 4.5 billion year old Solar System. However, by looking at the youngest stellar and planetary systems, we can uncover many details that are common to planetary systems in general, and in turn, we can learn how our own Solar System grew up.This fantastic episode of the Starts With A Bang podcast features observational astrochemist Dr. Charles Law, and takes us inside one of the most remarkable young stellar systems ever found: the edge-on system known as Gomez's Hamburger, complete with a first-of-its-kind exoplanet known as GoHam b. Come find out the incredible science behind planet formation, and meet our first-ever proto-protoplanet in the process!(This JWST NIRCam image shows many never-before-revealed details in the dusty disk of the edge-on protoplanetary system known as Gomez's Hamburger, with a massive, unique exoplanet within the disk known as GoHam b. Credit: NASA/ESA/CSA JWST; Francois Menard et al.)

When most of us were children, and we went to a rural area with clear skies overhead at night, we were all greeted by the same familiar sight: a dark night sky, glittering with many hundreds or even thousands of stars. Depending on how dark your sky was, you could spot up to 6000 stars at once, as well as deep-sky objects, the plane of the Milky Way, and only the rare, occasional satellite streak. As time went on, more and more satellites were launched, bringing us up to around 2000 active satellites as of 2019.And then we entered the era of satellite megaconstellations, beginning with the launch of the first Starlink satellites. Now, nearly 7 full years later, there are over 17,000 active and defunct satellite payloads in orbit, with approximately 100 times as many satellites proposed in the coming years. From satellite communications to direct-to-phone links to the proposition of AI data centers in space, the number of proposed use cases has exploded. However, as the environment around Earth becomes more crowded, the risks, the harms, and the potential for disaster all grow evermore severe, with woefully insufficient (or, sometimes, no) mitigation measures in place.Is this a cause for despair? Or could this be our finest hour in terms of combatting these new forms of pollution. I've brought expert Dr. Meredith Rawls onto the podcast this episode to discuss satellites and space pollution, and the conversation ranges from thoughtful to passionate to pessimistic to hopeful many times over. Have a listen; you don't want to be underinformed about this one!Helpful links:IAU's center for the protection of dark and quiet skies: https://cps.iau.org/NRAO/VLA's paper on radio telescope operations coordinating with satellite providers: https://arxiv.org/abs/2502.15068v1Vera C. Rubin's public alerts stream: https://rubinobservatory.org/news/first-alertsAn article on Rocket plumes: https://www.nature.com/articles/s43247-025-03154-8Meredith's Nature News and Views piece regarding streaks in space telescopes: https://www.nature.com/articles/d41586-025-03725-x The latest on the CRASH clock: https://outerspaceinstitute.ca/crashclock/Astronomers argue for astronomy on the ground and in space: https://spacenews.com/the-future-of-astronomy-is-both-on-earth-and-in-space/ and Yvette Cendes's previous appearance on the SWAB podcast: https://soundcloud.com/ethan-siegel-172073460/starts-with-a-bang-77-stellar-destruction and https://open.spotify.com/episode/4xnBB0Ma4SzHk8ulziOidk(The illustration shows all tracked objects in space as of 2025, as shown by the European Space Agency. The size of the objects, including intact satellites as well as space debris, is greatly exaggerated, but the number of objects shown is actually far less than the number of objects in space now in 2026, just one year later. Credit: European Space Agency)

Out there in the Universe, we're most aware of what we see: of all the forms of light that arrive in our eyes, instruments, telescopes, and detectors. Much more difficult to see, as well as understand and make sense of, is the wide array of "stuff" that's present, but that isn't readily apparent to the apparatuses we normally use to reveal the Universe. From the dark bands of the Milky Way to the light-blocking materials in nebulae and clouds, all the way to lining the arms of spiral galaxies and the heavy, long-chained molecules found in protoplanetary disks, cosmic dust is perhaps our most enduring mystery.Sure, it gives absorption signatures that we can leverage, and at long enough infrared wavelengths, dust that gets heated has its own emission signatures, but we can generally only observe it in detail up close: within our own galaxy or in the nearest galaxies of all. That poses a huge challenge, because the origin of dust, including from a cosmic perspective, remains only very poorly understood. We may have identified many dust-producing sources in the Universe, and we may understand that the young Universe was a lot less dusty than our modern cosmos, but we still lack an understanding of how this has come to be the case. Thankfully, we have scientists on the case, like this month's guest: Dr. Elizabeth Tarantino of the Space Telescope Science Institute.In this fascinating interview, she takes us on a journey spanning gently dying stars, the formation of new stellar systems, the outskirts of our cosmic backyard, and to the farthest reaches of JWST as we try and piece this mysterious cosmic story together. Buckle up for an exciting and informative ride; you'll be glad you tuned in!(This image shows the Pillars of Creation within the Eagle Nebula, as assembled by two entirely different data sets. On the upper-right, a visible light view showcases how this dusty region obscures the stars behind it. On the lower-left, an infrared view showcases the stars, although reddened, that can be seen behind the dusty cloud. At still longer wavelengths, the dust would glow due to the heat inside of this region. Credit: NASA, ESA, CSA, STScI, J. DePasquale, A. Koekemoer, A. Pagan (STScI), ESA/Hubble and the Hubble Heritage Team)

One of the most exciting developments in modern astrophysics isn't merely our standard "concordance cosmology" model, but rather the cracks that seem to be emerging in it. Sure, we've said for some 25 years now that our Universe is 13.8 billion years old, is made of mostly dark energy with a substantial amount of dark matter, and only 5% of all the normal stuff combined: stars, planets, black holes, plasmas, photons, and neutrinos. But more recently, a couple of cosmic conundrums have emerged, leading us to question whether this model is the best picture of reality that we can come up with.We don't merely have the Hubble tension to reckon with, or the fact that different methods yield different values for the expansion rate of the Universe today, but a puzzle over whether dark energy is truly a constant in our Universe, as most physicists have assumed since its discovery back in 1998. While "early relic" methods using CMB or baryon acoustic oscillation data favor a lower value of around 67 km/s/Mpc, "distance ladder" methods instead prefer a higher, incompatible value of around 73 km/s/Mpc. Now, on top of that, new large-scale structure data seems to throw another wrench into the works: supporting a picture of evolving dark energy, and specifically one where it weakens over cosmic time.Here to guide us through this is Dr. Kate Storey-Fisher, a cosmologist whose expertise is exactly on this topic, and who herself has recently become a member of the very collaboration, DESI, that provides the strongest evidence to date for evolving dark energy. The story, however, is only just beginning, and with current and future observatories poised to collect superior data, we take a look ahead as to what's in store for the Universe, and for those of us who are working oh so hard to try and understand it.(This image shows a "slice" through 3D space of the galaxies mapped out by the DESI survey, and color-coded by their distance/redshift from us. Features such as "great walls" can be seen even by eye within the data. Only 600,000 galaxies, or about 0.1% of DESI's total data, is displayed in this figure. Credit: DESI Collaboration/NOIRLab/NSF/AURA/R. Proctor)

All across the Universe, stars are dying through a variety of means. They can directly collapse to a black hole, they can become core-collapse supernovae, they can be torn apart by tidal cataclysms, they can be subsumed by other, larger stars, or they can die gently, as our Sun will, by blowing off their outer layers in a planetary nebula while their cores contract down to form a degenerate white dwarf. All of the forms of stellar death help enrich the Universe, adding new atoms, isotopes, and even molecules to the interstellar medium: ingredients that will participate in subsequent generations of star-formation.For a long time, however, we'd made assumptions about where certain species of particles will and won't form, and what types of environments they could and couldn't exist in. Those assumptions were way ahead of where the observations were, however, and as our telescopic and technological capabilities catch up, sometimes what we find surprises us. Sometimes, we find elements in places that we didn't anticipate, leading us to question our theoretical models for how those elements can be made. Other times, we find molecules in environments that we think shouldn't be able to support them, causing us to go back to the drawing board to account for their existence.Where our expectations and observations don't match is one of the most exciting places of all, and that's where astrochemist and PhD candidate Kate Gold takes us on this exciting episode of the Starts With A Bang podcast! Have a listen, and I hope you enjoy it as much as I enjoyed having this one-of-a-kind conversation!(This image shows the fullerene molecules C60 and C70 as detected in the young planetary nebula M1-11. This 2013 discovery was the first such detection of this molecule in this class of environment. Credit: NAOJ)