Starts With A Bang podcast

You might not think about it very often, but when it comes to the question of "how old is a star that we're observing," there are some very simple approximations that we make: measure its mass, radius, temperature, and luminosity (and maybe metallicity, too, for an extra layer of accuracy), and we'll tell you the age of this star, including how far along it is and how long we have to go until it meets its demise.

This also operates under a simple but not-always-accurate assumption: that all stars of a given mass and composition have the same age-radius and radius-temperature-luminosity relationships. That simply isn't true! Stars vary, both over time as they evolve and also from star-to-star dependent on their rotation and magnetism. It's a funny situation, because just a few years ago, people had declared stellar evolution as a basically "solved" field, and now it turns out that we might have to rethink how we've been thinking about the most common classes of stars of all!

To help us explore this topic, I'm so pleased to welcome Dr. Lyra Cao (pronounced "Tsao" and not "Cow" in case you were interested) to the program, where she helps walk us through what we're only now learning about stars: particularly young stars, low-mass stars, and rapidly rotating stars. If you know nothing about stellar evolution, this will be a treat for you, as you won't have to un-learn a massive amount of information to make sense of the Universe!

Out there in the Universe, there's a whole lot more than simply what we find in our own Solar System. Here at home, the largest, most massive object is the Sun: a bright, hot, luminous star, while the second most massive object is Jupiter: a mere gas giant planet, exhibiting a small amount of self-compression due to the force of gravity.

But elsewhere in the Milky Way and beyond, numerous classes of objects exist in that murky "in-between" space. There are stars less luminous and lower in mass: the K-type stars as well as the most numerous star of all: the red dwarf. At even lower masses, there are brown dwarf stars, possessing various temperatures ranging from a little over ~1000 K all the way down to just ~250 K at the ultra-cool end.

These "in-between" objects, not massive enough to be a star but too massive to be a planet, have their own atmospheres, weather, and a variety of other properties. The thing that limits our knowledge of them, at present, is merely our own instruments. That's why, on this edition of the Starts With A Bang podcast, I'm so pleased to welcome Dr. Brittany Miles, an expert on ultra-cool brown dwarfs and a specialist in instrumentation technology. If you were ever curious about these "in between" objects, you won't want to miss this journey to the frontiers of modern astronomical science!

When we look at our nearby Universe, it's easy to recognize our own galaxy and the other large, massive ones that are nearby: Andromeda, the major galaxies in nearby groups like Bode's Galaxy, the group of galaxies in Leo, and the huge galaxies at the cores of the Virgo and Coma Clusters, among others. But these are not most of the galaxies in the Universe at all; the overwhelming majority of galaxies are small, low-mass dwarf galaxies, and if we want to understand how we formed and where we came from, it's these objects that we need to be studying more intensely.

So what is it that we already know about them? What has recent research revealed about these tiny galaxies in the nearby Universe, both inside and beyond our Local Group, and what else can we look forward to learning in the relatively near future? Join me for a fascinating discussion with Prof. Mia de los Reyes of Amherst College, as we dive into the science of the tiniest galaxies of all, and what they can teach us about our cosmic history as a whole!

We all knew, if Einstein's General Theory of Relativity were in fact the correct theory of gravity, that it would only be a matter of time before we detected one of its unmistakable predictions: that all throughout spacetime, a symphony (or cacophony) of gravitational waves would be rippling, creating a cosmic "hum" as all of the moving, accelerating masses generated gravitational waves. The intricate monitoring of the Universe's greatest natural clocks, millisecond pulsars, would be one potential way to reveal this cosmic gravitational wave background.

But not many expected that here in 2023, we'd be announcing the first robust evidence for it already, and that future studies will reveal precisely what generates it and where it comes from. Yet here we are, with pulsar timing taking center stage as the second unique method to directly detect gravitational waves in our Universe!
For this edition of the Starts With A Bang podcast, I'm so pleased to welcome Dr. Thankful Cromartie to the show, where she guides us through the gravitational wave background, the science of pulsar timing arrays, and the underlying astrophysics of the objects that we monitor with them: millisecond pulsars. It's a fascinating story and one that's more accessible than ever with this latest podcast, and I hope you learn as much as I did listening to it!

Sometimes, it's hard to believe we've come as far as we have, scientifically, in such a short period of time. We only began accumulating the first very strong evidence for supermassive black holes during the 1990s, and yet here we are, less than 30 years later, studying them, their effects, and their environments all across the Universe: from the present day to less than 1 billion years after the Big Bang.

We now believe that nearly every galaxy out there in the Universe not only produces black holes from the corpses of the most massive stars within them, but also supermassive ones that resides at the centers of these cosmic objects. Every once in a while, these supermassive black holes accrete matter and devour some of it, becoming active in a spectacular display. Just as we're learning all about how the Universe grows up in terms of stars, atoms, and gas, we're starting to learn how these supermassive black holes evolve and grow up, too.

Here to guide us through the latest and greatest scientific discoveries, I'm so pleased to welcome Dr. Allison Kirkpatrick onto our show. Allison is a professor at the University of Kansas and specializes in supermassive black holes, from X-ray to radio observations and well beyond. Join us on this exciting journey to the heart of one of our greatest cosmic mysteries, and see what it's like at the frontiers of science here on Starts With A Bang!

We have a pretty good idea of both what's in our Universe and how it grew up. But it's only because we have several different, completely independent lines of evidence that point to the same consensus picture that we actually believe that our Universe is 13.8 billion years old and composed of a mix of normal matter and radiation, but is dominated by dark matter and dark energy on the largest of cosmic scales.

In particular, we form large, cosmically bound structures on the scales of galaxies and galaxy clusters, but on larger scales, dark energy and the expanding Universe dominate, working to drive everything apart. The story of how we've come to know this information about the Universe and how we're using both old and new techniques to push the our understanding further is the subject of this edition of our podcast. It features PhD candidate Karolina Garcia, who's kind enough to walk us through a variety of types of research that all serve the same end: to reveal the story of the Universe and how it grew up to be the way it is today. Take a listen; you won't regret it!

Back in the 1990s, observations of type Ia supernovae were the key data set that led astronomers to conclude that the Universe's expansion was accelerating, and some new form of energy, now known as dark energy, was permeating the Universe. Over the past ~25 years, those observations have gotten so good that we now have a tension within the expanding Universe, as different methods of measuring the expansion rate yield two different sets of mutually incompatible results.

What's remarkable is that this result is robust even though we're still somewhat uncertain as to exactly how these type Ia supernovae occur. The original scenario, put forth by Chandrasekhar nearly a century ago, still has its adherents, but the evidence appears very strong that approaching and reaching a "mass limit" beyond which atoms are unstable can only explain a small fraction of white dwarf behavior. Instead, a new paradigm dominated by merging white dwarfs may explain nearly all type Ia supernova explosions!

On this episode of the Starts With A Bang podcast, we talk to UC Berkeley astronomer Dr. Ken Shen, a theorist whose expertise lies in type Ia supernovae, and learn how just the last 20 or so years have led to a revolution in how we conceive of these "standard candles" in the Universe, and just what observations might soon lead us to know, for certain, how these cosmic events are truly triggered!

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!

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!

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!

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!