## Cleaned Transcript In 3, 2, 2… 1. Welcome to the Starcast for the week of **October 19th, 2025**. I'm your host, **Jay Shaffer**. And with me is my co-host, **Mike Lewinski**. Good morning, Mike, how you doing? I'm doing great, how are you, Jay? Great, I'm back in **New Mexico**, and while I was in **Florida**, I did get to watch the launch of the **KF-03** mission on a **Falcon 9** rocket, and when it finally launched last Monday night. Very cool. And I also got to see the **Starship** staging over the ocean there in Florida that evening. It was kind of an unexpected surprise. I wasn't actually watching for it, and kind of cut just to the tail end of it, and it was pretty impressive. And so what's impressive in space weather tonight over the next couple days there, Mike? Well, Jay, there is a **hole in the sun's atmosphere** that is streaming **solar wind toward Earth**. And we are expecting a potential **G1 class geomagnetic storm** when it arrives on **October 21st**. There was actually a **KP index hit 6** yesterday. And so there was a short duration geomagnetic storm from **CMEs** that had left the sun last week. And there's a great photo on the front page of **SpaceWeather.com** of a **comet** that is visible amid the **aurora display**, so I recommend our listeners head over there and check out the October 19th edition of SpaceWeather.com. As far as our **NOAA space weather forecast** for the next 48 hours here in mid-latitudes, we have about a **30% chance of active conditions**, **15% chance of minor geomagnetic storm**, and only a **5% chance of severe**. But if you're up at higher latitudes, that chance of a severe storm bumps up to **50%** for the next 48 hours, so I think, really, through the coming week here, or the beginning of the week, we have a chance of seeing something. So, with that, what's up in the night sky this week, Jay? Yeah, I'll definitely have to aim my camera north this week, with the possibility of **Aurora** as well. Well, the **Orionid meteor shower** should rain down in its greatest number of meteors for this year on the night of **October 20th** and through the 21st, so Monday night into Tuesday morning. So, keep an eye out toward **Orion** early in the morning as we pass through the remnants of the tail of **Halley's Comet**. It should be great viewing since it will be a **new moon** on Tuesday morning as well. And in addition, if we're expecting a geomagnetic storm, that should be just, it should be a really nice morning for viewing the night sky. And for those of you who are counting, that new moon will occur at exactly **8:26 Eastern Daylight Time**. And we're glad to report that the comet, **C/2025 A6 Le Momo** or **Lemon**, is continuing to brighten. I was able to image it last night with my telescope. And I think I was able to see it, like, as a faint smudge with my naked eye in our exceptionally dark skies here. And hopefully, over the next week, it'll be visible to everyone. So, let's take a look at some space news, Mike. What's happening? Well, Jay, in local space news, we had a **star party** here in **Crestone** last night, and we did point our telescopes and binoculars over at that **Comet Lemon**, and enjoyed viewing. We had very clear night skies last night, so it was a fun social event. In other news, astronomers have captured an unprecedented view of a spectacular celestial feeding frenzy far from the bustling core of a galaxy. This is an event known as a **tidal disruption event**, or **TDE**. And it occurs when a star strays too close to a **black hole** and is shredded by immense gravitational forces in a brilliant flash of light. TDEs are most commonly witnessed near **supermassive black holes** that anchor the galactic nuclei. But this newly observed event, **AT2024TVD**, took place about **2,600 light years** away from its galaxy's center. It involved a black hole with a mass of up to **10 million suns** that devoured a star in an **off-nuclear TDE**, an event scientists have only dimly hinted at. What makes this discovery truly unique is that it is the first such event to be clearly captured with a bright **radio wave signature**, providing a clear, fast-evolving signal, unlike anything seen previously. The exceptional nature of this has led researchers, including a team led by **Atai Zvar Adi** of the **University of California, Berkeley**, to dub AT 2024 TBD, quote, the **first Radio Bright, bona fide, off-nuclear TDE** and one of, quote, **fastest evolution observed to date**. The black hole's unusual location has prompted speculation that it may have been violently ejected from the galactic nucleus. Scientists theorize the black hole could have been gravitationally kicked out after a chaotic encounter with other larger black holes. While its history is tumultuous, this rogue black hole has certainly made a dramatic entrance into astronomical observation with its stunning distant, stellar meal. ----- And speaking of distant stellar events, our topic today is going to continue the discussion of **cosmology** leading up to an episode in coming weeks on the **crisis in cosmology** known as the **Hubble tension**. So, last week, we talked about the **cosmic microwave background radiation**, and how that has given us evidence for the expansion rate of the universe and possible eventual fate. Now, when we talk about knowledge and scientific consensus, in particular, there is a standard called a **knowledge-based consensus**. Where scientists are looking for three things. They want **consilience of evidence**, **social calibration**, and **social diversity**. And consilience of evidence means that we have more than one line of evidence that leads to a particular conclusion. A classic example is given for **climate change**, where we have evidence from meteorology, geology and geochemistry, atmospheric physicists, and so forth, that all point toward the conclusion that the atmosphere is heating. Here with regard to cosmology and the expansion of the universe, consilience of evidence is going to include **cosmic microwave background radiation** and **Cepheid variables**, which is our topic today. I will also mention that in the knowledge base consensus, we're looking for something called **social calibration**, which is to say that scientists agree upon the standard of evidence required for consensus. And **social diversity**, which is that it's not just all this one group of scientists from this one country and this one economic class who agree on a particular detail, or a particular theory, but that we have scientists from different economic, cultural backgrounds, diversity of age and ability, or with respect to a different ability, disability. And our most important person when we talk about **Cepheid variables** and the importance to cosmology is an astronomer named **Henrietta Swan Leavitt**. Born in **July of 1868**. Her big discovery came about in **1908**, and was actually published in **1912**. And Henrietta Leavitt was one of the so-called **human calculators at Harvard**. She was assigned a task that was probably deemed to be very menial and not a promising line of research, as women tended to not be very well respected at the time in this field. And so, it was kind of her triumph to have produced some of the very earliest evidence that eventually led to our understanding of the **Big Bang**. In fact, she would have been nominated for a **Nobel Prize**, but for the fact that by the time they realized they needed to nominate her, she had already died, and the Nobel is not awarded posthumously. But **Edwin Hubble** certainly believed and said so, that he believed that she deserved a Nobel Prize for her work, and I think there could be really no doubt about that. So let's jump into what the **Cepheid variables** are and why they're important. A Cepheid variable is a star that is approximately tens to hundreds of times the size of our sun. They're **giant, pulsating stars**. So their pulsation is a change in not just brightness or luminosity, but also in their physical size and their temperature. So they're, like, physically pulsing bigger, smaller, bigger, smaller, hotter, cooler, hotter, cooler. And Henrietta Leavitt figured this out. She was assigned a task of cataloging thousands of **variable stars in the Magellanic Clouds**. And she eventually noticed that the true luminosity of the Cepheids was predictable based on its **period of pulsation**. And from this, we eventually came up with a means to measure the distance using the Cepheid variables. So previous to Henrietta Leavitt's discovery, our only way to measure the distance outside of our solar system was using **parallax**. And we could only do this with stars that were relatively close. And so here is a case where once Henrietta Leavitt made this discovery of the relationship between period and luminosity, that we were able to use parallax distances to closer Cepheids to **calibrate** these things. So we can estimate a distance and parallax, if we remember back from our episode on measuring cosmic distances, is to take measures of positions of stars as we orbit the Sun at different times of the year, and from that, we can use math to calculate how far it is based on the apparent position shift over the course of those measurements. So, we use a couple of closer Cepheids, including **RS Puppis**. And **Polaris**, our Polestar, is one of these Cepheid variables, and it's one that is close enough to us that we are able to make a parallax measurement to get its distance, and from that, we can calibrate brightness. And from that, then we can start to use these additional more distant Cepheid variables outside of the Milky Way to estimate larger and larger cosmic distances. And we talk about the **standard candle** in our episode on cosmic distances, and this was the first standard candle, was the Cepheid variable relationship. I want to also talk just a little bit about what makes a Cepheid variable pulse. And don't let me forget to also wind this up with how do we know that these aren't **exoplanet transits**, to touch on another episode that we've had this year. So, when we are measuring the radiative opacity of a star, there is something called the **Kappa mechanism**. Using the Greek letter $\kappa$ to denote that particular opacity at any given depth of a stellar atmosphere. So the $\kappa$ opacity mechanism helps us to judge how much light and energy is escaping from that star. And in a normal star that is not a variable, there is an increase in compression that... and I'm basically summarizing the **Wikipedia** article for the Kappa mechanism here, because I'm not an expert in this. But there is an increase in compression of the atmosphere that causes an increase in temperature and density of the star, which produces a decrease in opacity, and you get this equilibrium condition where temperature and pressure are maintained in a balance. However, there are cases, such as Cepheid variables, where the opacity of the star actually **increases with temperature**. And so now the atmosphere becomes unstable. If a layer of the stellar atmosphere moves inward, it becomes denser and more opaque, causing heat flow to be checked. In return, this heat increase causes a buildup of pressure that pushes the layer back out again. And we get this cyclical process of the star repeatedly pushing out, and then collapsing back in on itself, not completely collapsing. And with the Cepheid variable, I think this is a super kind of fascinating mechanism here. It's believed that there is **doubly ionized helium**, which is driving this process. So this forms at high temperatures. And is doubly ionized helium is more opaque than singly ionized helium. So the outer layer of the star cycles between compression and expansion. Compression heats the helium until it becomes doubly ionized, so that energy is basically stripping off an electron, right? And now, because its opacity is increased when doubly ionized, the helium absorbs heat and expands, then cools. And becomes picks up its lost electrons and becomes singly ionized again, and due to its transparency, when singly ionized, cools until it collapses. And the Cepheid variables become dimmest during the part of the cycle when the helium is doubly ionized. So we observed this relationship that is the brightness, the period, the temperature of the Cepheid variables. And using this, we're able to have our standard candle, our first known standard candle, for estimating distances beyond the closer stars to us. ----- So, we are, and I mentioned that this is not to be confused with the dimming that we see when there is an **exoplanet that is transiting**. And it really does have to do with the particular distribution of curve when we look at a Cepheid variable. It's kind of described as a **shark fin**, where it gradually gets dimmer and dimmer, and then we see this very sudden brightness peak relative to the period that it takes to get dimmer. And so, the shark fin has this kind of steep leading edge and then a long trail. And with an exoplanet, we're not going to see this same mechanism. It may be periodic, but you would expect, if this is just a planet that is large enough and relative to the star able to dim it, as the transit proceeds, it's going to be pretty much the same on one side as the other, and you're not going to see this shark fin plot as you plot the brightness. So, huge shout out to **Henrietta Leavitt**. Unfortunately, she died at a relatively young age, and did not live to collect the Nobel that she was rightly owed for this insight that led us, ultimately, to methods that **Edwin Hubble** picked up on her work. And was able to develop the theory of the **Big Bang** and cosmic expansion. The universe itself that we can observe is about **46 billion light years across**. Now, we believe, given that we can't observe beyond that **event horizon**, that the actual size is more like **92 billion light years**. But even taking that smaller figure of 46 billion light years and what we know about the age of the universe at 13.8 billion years, you can see right away that there has to be some sort of **expansion** happening. Otherwise, we would expect a universe that's about 13.8 billion light years in size, right? So, having these standard candles as mechanisms to measure is super important. And our **consilience of evidence** that we are looking for but missing here is that given by the **cosmic microwave background radiation** that we discussed last week and the **Cepheid variables**. When we initially started working with these figures, the margin of errors were within each other's range, and so there seemed to be some consilience, but over time, we have gotten more precise measurements. And there's now a **divergence** that is no longer within the margin of error for the rate of expansion given by cosmic microwave background radiation and the rate of expansion given by the Cepheid variables that we're able to, and we're continually discovering more and continuing to, with the discovery of more Cepheid variables, able to further refine our estimate of the rates of cosmic expansion. So, that's part two in our **Crisis in Cosmology**. Jay: Okay. So I wanted to talk a little bit, just real quickly, on the light and the **inverse square law**. And so this is something that actually I've encountered in photography and videography, and at one point in my career, I was a lighting director for television. And so, the inverse square law. And **foot candles** were a measurement that we used for light, and doing lighting for photography. And so, I just wanted to kind of show you how you can tell distance by the **luminosity**. And so, if we had a foot candle of light, which was a measurement of light, which basically says a candle of light, one foot away from the source, has this brightness of one foot candle. And so the inverse square law says at **twice the distance**, **you will experience square of the light**, or the light will reduce in the square of the distance, and so if we went to two feet away from my foot candle light source, we would actually be reading it as a half candle of light, foot candle of light. And we increased that distance further, to four times, that would be a sixteenth, and so forth, so on and so forth. And so, by the brightness of a light, we can tell if we know what the intrinsic luminance of that particular light source is. And we move that light source further away from us with the inverse square law, that's how we can measure the distance to that light by the reduction in the amount of **photons** that are actually being received at that distance. Does that help explain that a little bit better, do you think, Mike? Yes, yeah, that's perfect, Jay. I really appreciate you talking about the inverse square law, because that's a really important part of understanding how Cepheid variables are used. We are able to estimate by their period what their **true luminance** is, and then comparing our true luminance with **perceived luminance**, we now have inverse square law to give us distance. So, that's a great example. Thank you. Yep, so that, so basically, if you boil this down to my terms, **Cepheid variables are a cosmic yardstick**. Is that would be accurate? Yes, exactly. Okay, so we want to thank all of our listeners for checking out the podcast, and be sure to comment, like and subscribe, and let us know what you'd like to hear more about. And you can also check out our individual websites at **WildernessVagabonds.com** for Mike and **Skylapser.com** for me. And if you'd like to help us out, you can buy us a coffee at **[buymeacoffee.com/Skylapser](https://www.google.com/search?q=https://buymeacoffee.com/Skylapser)**. And any funds that we receive will go toward cameras and astronomical instruments, so that we can kind of add to the podcast. So, the intro music is **Fanfare for Space by Kevin MacLeod** from the **YouTube Audio Library**. And from the **Deep Sage 9 Observatory**. This is **Jay Shaffer** and **Mike Lewinski**. Wishing you all clear skies.