Starts With A Bang podcast Ethan Siegel

When stars are born, they can come with a wide variety of masses. But there are only a few ways that stars can die, and only a few types of remnants that can be left behind: white dwarfs, neutron stars, and black holes. Neutrons stars and black holes are most frequently created from core-collapse supernova events: the deaths of massive stars. Somewhere, even though we're not sure exactly where it is, there's a dividing line between "what makes a neutron star?" and "what makes a black hole?" Somewhere out there, there's a heaviest neutron star, and someplace else a lightest black hole.

But the dividing line might not be so clean, after all. It turns out that when neutron stars merge, they can form another neutron star, a black hole, or a third case: an in-between scenario. In this third case, you can temporarily form a hypermassive neutron star: a neutron star that's too massive to be stable, but that collapses in short order to a black hole, but only after persisting as a neutron star for a detectable amount of time.

To help guide us through the science of hypermassive neutron stars, I'm so pleased to welcome Dr. Cecilia Chirenti to the show, a joint scientist at NASA Goddard and the University of Maryland, College Park. There's a whole lot of cutting-edge science right at (and even over) the horizon of what we know today, and you won't want to miss this information-rich episode!

(This image shows the illustration of a massive neutron star, along with the distorted gravitational effects an observer might see if they had the capability of viewing this neutron star at such a close distance. Credit: Daniel Molybdenum/flickr and raphael.concorde/Wikimedia Commons)

One of the great advances of 20th and 21st century science has been, for the first time to show us two things: how the Universe began and what the Universe looks like today. The modern frontier is all about the in-between stages: how did the Universe grow up? How did it go from particles to atoms to the first stars and galaxies to the modern Milky Way, Local Group, and Universe-at-large? It's a question that, the more deeply we answer it, the greater the number of details that emerge, requiring us to make a special effort to pin each one down.

For this episode, I'm so pleased to welcome Dr. Ivanna Escala to the podcast: an expert in how stars and stellar properties within the Local Group can reveal not only its stellar history, but its history of galactic assembly. While the Milky Way has had a few major mergers, its most recent was a whopping ~10 billion years ago. Andromeda, our Local Group's other large galaxy, has a remarkably different story: with a major merger that occurred only 2-4 billion years ago!

Have a listen and enjoy, and thanks to Avenues Online for being our sponsor!

(This image, assembled from very long wavelengths of light of the neighboring Andromeda Galaxy, shows features within Andromeda's galactic disk as well as the gas clouds of neutral hydrogen found in Andromeda's galactic halo. By examining these features, as well as streams and stars in and around Andromeda, we can reconstruct precisely how this galaxy came to be the way it is today. Credit: NRAO/AUI/NSF, WSRT)

For life on Earth, there's no more important source of energy than the Sun; without it, it's doubtful that life would have arisen on Earth, and it certainly wouldn't have evolved to give rise to the wild diversity of biological organisms seen today. But the Sun is more than just a constant source of heat and light; it also emits particles, and there's a darker side to that activity: flares, coronal mass ejections, and the threats this space weather poses to living planets like our own.

It turns out that for technologically advanced civilizations like our own, the threats that arise from the Sun are far greater and more dangerous than at any time prior in Earth's history, and despite the knowledge we have of what the Sun can do to the Earth, we're woefully unprepared for the inevitable. Thankfully, there are not only people studying it, but many of them are also fighting and advocating for solutions and planetary protection, including Sierra Solter, a plasma physicist specializing in solar plasmas, who joins us on this edition of the Starts With A Bang podcast.

Welcome to a glorious 2023, and may we learn the needed lessons for what must be done before we're left with the sad alternative of simply picking up the pieces!

(This illustration shows a massive space weather event, larger than a typical solar flare, known as a surface mass ejection. Although SMEs have the capacity to entirely destroy a planet, they're thankfully limited to occurring on red supergiants, a class of star that will never include our Sun or anything it will evolve into. Credit: NASA, ESA, Elizabeth Wheatley (STScI))

For a cosmologist like me, "cosmic dust" is a thing that's in the way, confounding our data about the pristine Universe, and it's a thing to be understood so that it can be properly subtracted out. But the old saying, that "one astronomer's noise is another astronomer's data," proves to be more true than ever with cosmic dust, as how it's produced, where it came from, and how it comes together to form planets, molecules, and eventually creatures like us, are some of the most essential elements necessary for us to exist within this Universe.

In visible light, cosmic dust is normally just a starlight blocker, but in other wavelengths of light, its composition, distribution, density, grain size, polarization, and many other kinetic and thermal features can be revealed. Here to guide us through the ins-and-outs of cosmic dust, with a special view towards millimeter, submillimeter, and radio wavelengths, I'm so pleased to welcome PhD candidate Carla Arce-Tord to the show. Enjoy this far-ranging tour of cosmic dust, and perhaps by the end you'll walk away inspired about all there is to know as well as the remarkable people making it happen!

(The image shows the magnetic field lines imprinted by the galaxy on the cosmic dust in the interstellar medium, as revealed by the Planck CMB experiment. These field lines are of microgauss strength and can be coherent over hundreds or even thousands of light-years. Credit: ESA/Planck Collaboration. Acknowledgement: M.-A. Miville-Deschênes)

The supermassive black holes at the centers of galaxies is a tremendously interesting area of research, advancing rapidly over the past few years. While most of these observations focus on either high-energy or radio emissions from them, there's a recent push to see what these objects are doing in other wavelengths of light, as well as how they vary in time.

Once, it was thought that supermassive black holes would become "activated" at a certain point in time, would remain on for hundreds of thousands or even millions of years, and would then turn-off. But our observations have shown us that there are remarkable variations in what types of light and energy these objects emit over time, and with new studies being conducted at the South Pole and other places studying the Universe in millimeter-wavelength light, we're about to get an unprecedented amount of high-quality data.

Here to guide us through what we've learned so far about these active galaxies and where this research might take us in the future is Dr. John Hood, a postdoctoral research associate at the University of Chicago. It's a wild ride here at the frontiers of science, and I hope you enjoy every minute of it!

(In this artistic rendering, a blazar is accelerating protons that produce pions, which produce neutrinos and gamma rays when they decay. Lower-energy photons are also produced, allowing blazars, a form of Active Galactic Nucleus (AGN) to be seen all across the electromagnetic spectrum. In recent years, we’ve advanced to the point where we’re detecting neutrinos from billions of light-years away, beginning with blazar TXS 0506+056. Credit: IceCube collaboration/NASA)

All throughout the Universe, we see stars and galaxies everywhere we look. But as we look to greater and greater distances, we're only seeing the light that's the easiest to see: the ones from the brightest, most visible objects. But the most numerous objects of all are exactly the opposite: less luminous, smaller, and lower in mass. How can we hope to find and catalogue them all if they're the hardest ones to find?

The answer lies in measuring the closest stars to us. If we can measure the stars that persist in our own backyard, cataloguing them and taking as complete a census as possible, we can then combine what else we know about stars and starlight and the environments in which new stars form to reconstruct precisely what we believe is out there: not just here-and-now, but elsewhere and all throughout cosmic time.

Here to bring us up to speed on how this attempt to catalogue and categorize the stars in the Universe, I'm so pleased to welcome PhD candidate at Georgia State University Eliot Vrijmoet to the show, who takes us on a fascinating journey to the edge of our knowledge, and from there we'll peer over the horizon to what just might come next. Enjoy the latest episode of the Starts With A Bang podcast!

Star density maps of the Gaia Catalogue of Nearby Stars. The Sun is located at the centre of both maps. The regions with higher density of stars are shown; these correspond with known star clusters (Hyades and Coma Berenices) and moving groups. Each dotted line represents a distance of 20 parsecs: about 65 light-years. (Credit: ESA/Gaia/DPAC - CC BY-SA 3.0 IGO)