What powers Cepheid variable stars? What about Mira variables and pulsating stars? And are there variable stars that don’t actually vary at all? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPTION (AUTO GENERATED)

The Ghoul. The Demon Star. The Gorgon. The Head of the Ogre. The Mausoleum.

Today, we call it Algol. But through the centuries, this star has had many names. Over three thousand years ago, the ancient Egyptians called it Wedayat, or the raging one. According to one legend, if the star became too angry, it would destroy the earth. And to ancient Egyptians whose civilization depended on the orderly and regular flooding of the Nile, a chaotic agent who could unexpectedly doom the entire world was kind of a big deal.

Algol is a variable star. It's the second brightest star in the constellation Perseus. But roughly every three days, it dims considerably for a period of ten hours before returning back to normal brightness. Now, we don't know for absolute certain if the ancient Egyptians noticed this variability because they often didn't talk directly and candidly about astronomical phenomena. Remember, for these people and many other cultures, celestial objects were manifestations of divine beings and processes and other god like activities.

And it's kind of a religious no no to speak, well, directly and candidly about godlike activities, so instead, they would often use poetic illusions. Like, instead of just describing a solar eclipse, they would say that the sun god went into the underworld to battle a serpent, and we have to figure out by reading between the lines that they were just talking about a solar eclipse. So they never came out and said, hey. This star called the raging one, which, someday in the future you may or may not call Algol, regularly dims every three days. Instead, we get evidence that they notice the variability of algal in tables of lucky and unlucky days.

When algal happened to be bright at dawn, it was a good day. And when it was dim at dawn, it was a bad day. The Egyptians weren't the only ancient people to notice variable stars. Check out this intriguing story passed down through generations of oral tradition within several communities of Aboriginal Australians, and I'm taking this from a translation made by Duane Hamaker. A man named Nyiruna is a skilled hunter and vain womanizer who lives in the sky.

He comprises the stars of Orion in the same orientation as his Greek counterpart, meaning that he is upside down as seen from Australia. He pursues the Eugeoia sisters of the Pleiades across the sky each night in an attempt to make them his wives, a pursuit indicated by the relative diurnal motion of the two star groups. Nyruna is prevented from reaching the Eugeo Aurelia sisters by Kambuguda, their eldest sister who is represented by the Hyades star cluster. Kambuguda is protective of her younger sisters and is contemptuous of Nayruna. She stands before Nyaruna mocking and taunting him while blocking him from reaching the sisters.

Nyaruna is filled with lust and is angry he is being prevented from reaching the sisters. The club in his right hand, the star Beetlejuice, fills with fire magic ready to throw at Kumbuguda. She defensively lifts her left foot out the star Aldebaran, which also fills with fire magic. She kicks dust into Nyiruna's face, humiliating him. This causes the fire magic of Nyiruna's hand to dissipate.

Kamboguda then places a row of dingo pups in front of Nyiruna to shield her and her sisters from his unwanted advances. Before she leaves, in her last act of defiance, Kambuguda contributes to Patreon. That's patreon.com/pmsudda where you and all Aboriginal peoples going back generations can contribute to keep this show going, and I truly do appreciate it. That may or may not be an accurate part of the translation. I'll have to I'll have to triple check that after I record this episode.

Here we have vivid descriptions of two very specific stars occasionally filling with fire magic, which strikes me and anyone else who has heard these stories as more than a mere coincidence. Why these two stars, Beetlejuice and Aldebaran, specifically? Why not any others? And why do they seem to get brighter in the stories before returning to a normal state? Now we know that both of these stars vary in brightness, and these Aboriginal stories might be the oldest surviving accounts of our witnessing of that variability.

Now as usual, it took a while for the Europeans to catch up with everyone else astronomically. I'm only partially kidding here. The Europeans have always been skilled astronomers because in ancient times, every civilization needed skilled astronomers. But different peoples have different priorities, perceptions, and mythologies of the night sky, and this is before the standardization of science, and so people are always going to see what they want to see in the heavens. But what got the Europeans stuck behind everyone else in terms of noticing variability and paying attention to it was that for ages, Europeans were stuck with Aristotle, who said that the stars were perfect and unchanging and eternal, which honestly is not that bad for a first guess.

And while Aristotle was a genius, he was not the most, how shall we say, experimentally or observationally motivated when it came to reaching conclusions. So sometimes, he was blisteringly wrong about everyday normal processes in nature. Honestly, it sounds like a typical theorist, often wrong, never in doubt. But at least in this case, the stars are always unchanging is a good enough statement that unless you're really paying attention and keeping very close records, you can easily fool yourself for a good millennium or two that that's the way things are. Oh, what about the comments?

We're not gonna talk about the comments. Then we get to the 15 hundreds in the eve of the scientific revolution, and what's precipitating that revolution is detailed record keeping of the heavens and a lot of talking and sharing across the European continent about what people are finding. And, yes, I really, really need to get around to doing a series on the scientific revolution, especially the Galileo affair. So please keep sending me questions about that to prod me along. I swear I'll get to it one of these years.

One of the things that people found was that stars are changing all the time and that Aristotle was dead wrong. We'll skip the religious, political, and cultural significance of those discoveries for today and head straight to the why. Why are all these stars changing? Astronomers, of course, have their own overly complicated categorization scheme for all the varieties of variable stars. I mean, seriously, I'm not even kidding around here.

This is this is here's an abbreviated abbreviated list of the kinds of variable stars. Are you ready? Please get your pencil and paper out. Here we go. Mirror variables, semi regular variables, slow irregular variables, long secondary period variables, beta Cepheid variables, RR Lyrae variables, Cepheid variables, Cepheid like variables, RV tauri variables, blue large amplitude pulsators, Herbig AE BE stars, Wolf Ray variables, the list goes on.

It's a lot. So as usual, when I can tell that the astronomy nerds are starting to get a little off the rails with categorization schemes, I develop my own. Remember, as is usual with Ask a Spaceman brand categories, this is not official, does not represent the consensus of the scientific community, should not be expressed publicly at astronomy cocktail parties, and if ever asked about it, I will deny any knowledge of or association with such ridiculous concepts. Disclaimers made. Here are my two, that's right, Just two categories of variable stars.

Boring and not boring. Just for completeness sake, there's also technically a third category, which is things so not boring that they actually blow up, which is the supernovae. But to me, that's not technically a variable star. That's a star blowing up, so not a subject of today's episode. Okay.

Okay. Good. So what's in the boring category? To me, these are the stars that aren't actually varying. They just look like they're varying, and astronomers or millennia ago couldn't tell the difference, so they ended up getting lumped into the variable star category even though they shouldn't, by all rights, belong.

Take our good friend, Algol, the demon star, the raging one. That's actually kinda mellow, not ragey at all, just misunderstood. The thing about Algol is that it isn't a single star. It's a triple system that from our distance, which is nearly a 100 light years, appears as a single dot on our sky. But these stars have an unlucky or lucky, depending on your point of view, alignment.

The plane of their orbit is along their line of sight, which means sometimes one star passes in front of the other, and this happens every three days. The third star in the system is is a small star that we don't really care about. What we really care about are the two bright stars in the center of the system. And when they're side by side, we get the full blast light of both of them. But then when one crosses in front of the other, yes, we get the light of the star in front, but we don't get the light of the star behind it.

It gets blocked like a little stellar eclipse. And so from our perspective, because we can't separate these two stars, we only see them as a single dot of light because they're so far away. It appears as this dot of light is getting dimmer. And then when the eclipse is over, we get the full blast brightness of both stars, and then Algol, that little dot on the sky, looks like its normal brightness again. Yes.

There is some interaction between these two stars. One of the stars is grossly distorted by the gravity of its companion, and so there is some transfer of mass between the two of them, which is pretty cool. But the the demonosity, yes, that's now a word, of Algol is purely because of our line of sight, which is neat, but kind of boring. Another example of neat, but boring variable stars are ones that are just kind of lumpy, or have more star spots on one side, or are rotating so rapidly that they are plumper around the middle. All of this causes variations not through any exciting physical process, but just because at different times, we're looking at different parts of the star.

And that's pretty much all we need to say about the boring category of variable stars. Yes. There are many subcategories, but like I said, this is an unofficial scheme, so we don't need to worry about it. And that means that the only thing left are the exciting variable stars, the not boring variable stars. These are the stars that vary because they're doing something other than just existing and having a weird rotation or orbital alignment.

These are the stars where some physical process is causing them to vary. Boy oh boy does the universe get up to some really funky stuff. But before we go on, we need to take a brief commercial break. Keep in mind, we've moved quite a bit a ways from our good friend, Aristotle. All stars vary to some degree.

They all get brighter and dimmer here and there. They all go through periods of more or less activity or more or less intensity, and they all change with time as they age. For example, our own Sun has variations in brightness that last about five minutes at a time. Every five minutes, our Sun gets a little bit brighter and then a little bit dimmer and then a little bit brighter and then a little bit dimmer. You don't notice this on a day to day basis or in five minute by five minute basis.

It's not something you catch by eye, but when we record the brightness of the sun with high precision. We see these variations of every five minutes. So sorry, Aristotle. There are exactly zero stars that remain perfect forever. But a star does have to cross a certain threshold to make that categorical jump to variable star.

And as you might imagine, making that jump requires quite a bit of energy. And where there's energy, there's not boringness. Take Betelgeuse, which the Aboriginals found occasionally flared up with fire magic and deemed those events important enough to record in their oral traditions. It's what we call a semi regular variable star, which as the name suggests means that it doesn't have a fixed period to its changes in brightness. And what's causing these changes in brightness is the sun's heartbeat, which pulses roughly every four hundred days.

And then there's a second heartbeat that pulses every five point six years. Think of the forces involved inside of a star that keep it alive. You have gravity pulling in. You have the weight of the star that is constantly pulling the star in. And if if it weren't for anything else, the gravity of the star would crunch it down all the way to a black hole.

But what's resisting that contraction, what's resisting that pulling is the giant nuclear bomb happening in the core of the star that is constantly exploding it outwards. And with nothing left to resist it, that explosion would just continue and the star would blow up. But these two forces are in balance, something we call hydrostatic equilibrium. This is what keeps stars alive for millions or billions or, in the case of small stars, trillions of years. This constant battle between gravity pulling in and explosive energies pushing it out.

But this equilibrium is not perfect. All the time all the time, Gravity is winning by just a little bit. Or the radiation, the explosive energies are winning by just a little bit. And then gravity grabs hold of it and pulls the star back down. And then it the fusion reactions heat up just a little bit and release a little bit more energy to push the star back out.

And then that wins for a little bit, but then and then everything cools off, and so gravity can pull things down. The and there's this constant seesaw back and forth. But it's not a perfect straight line. There are rhythms and variations to it, and it all depends on the star, its mass, how rapidly it's spinning, how old it is, how many heavier elements are mixed in. And for Betelgeuse, there are two fundamental rhythms.

One rhythm is roughly every four hundred days, and another rhythm is roughly every five point six years. Every time the star has one of these heartbeats, the surface is a little detached. The star has expanded because when the explosive energies win, the star expands. But then that makes the surface farther away from the nuclear core in the center which means it cools off and so it appears dimmer. And then when the star is in between heartbeats, the star is a little more contracted.

It's a little more compact, which means the surface is closer to the nuclear fires in the center, which means they heat up, which means they glow brightly. So there are these these semi regular variations, but then sometimes even these heartbeats turn into full on heart attacks. And the regular pulsating rhythms tip way too far in one direction or another. This is what happened in 2019 and 2020 where a pulse went way too big. It went so big that it threw off a massive amount of material that coalesced into dust which blocked the light from the star.

Betelgeuse was, like, a quarter as bright as it is, like, some huge fraction of of of dimming, and we called it the great dimming. The opposite can happen where it can flare up, and this may have been the the fire magic that the Aboriginals, recorded in their oral traditions because you can just have these these dramatic events happening. Or they may have been recording and noticing these four hundred day long cycles or the five point six year long cycles where they would notice that the star Betelgeuse was brighter than it was last year. And then the same thing happens to Aldebaran. So maybe they recorded this.

It's it's certainly plausible. Sometimes these heartbeats and these heart attacks that that really tip this balance one way or the other can be so dramatic that the outer layer of the star looks like it's disappearing altogether because it cools off so much that it shifts to emitting mostly infrared light instead of visible light. And these are called the Mira variables named after the star Mira, aka the miraculous one, because every eleven months, the star almost disappears completely from the naked eye. Sometimes the heartbeats change the fundamental chemistry happening within the stars layers, especially if the star has a lot of helium. Helium is weird.

I'm maybe I need to do a whole episode on helium. Someone asked about the the role of helium in the universe. So helium can have up to two electrons. And if it loses one of its electrons, it's called ionized. And if it loses both of its electrons, it's called doubly ionized.

It takes a lot of energy for the helium to lose both of its electrons to become doubly ionized. So if you pour a lot of energy into a big vat of helium, they will lose both of their electrons, and they'll be doubly ionized. Helium is almost always transparent. If you have a big old blob of helium, you're just gonna look right through it. It it's transparent to light.

But doubly ionized helium is opaque. If you get rid of the electrons and you heat up helium so much that it loses its electrons, it actually becomes rather good at blocking light. Blocks light much more effectively than regular non ionized helium. This leads to something called the Eddington pump or the kappa mechanism. Both these terms refer to the exact same process.

This was first proposed by sir Arthur Eddington, great astronomer in the early twentieth century. If a star happens to have a layer of helium in its body, you know, it doesn't have to be near the top or near the surface. It can be somewhere in the middle. But if there is an area, if there's a region of the star with with higher than average helium, it doesn't even have to be 100% helium. It can just be more than average helium.

There's a layer of helium, which tends to happen when stars get pretty old. They start to develop these layers of helium within them. At first, everything's normal. You've just got a lot of helium going on. But that helium, if it absorbs a lot of energy, which it tends to do, you know, being inside of a star and all that, if it absorbs a lot of energy, it becomes doubly ionized.

It loses its electrons. This makes the helium opaque, which means the light coming out of the core that it's working its way out of the core trying to punch through to the surface of the star runs into this opaque helium and physically hits it. The light just punches it. It creates a source of pressure from the light, from the radiation itself inside the star, which pushes the helium outwards which pushes the whole rest of the star along with it outwards. Then the helium reaches an an a greater distance from the core.

Now that it's far from the core, it can start to cool off because the helium gas has expanded and is far from the nuclear core. It cools off. The electrons come back to the helium. It becomes normal helium now. That makes it transparent, which means the light shines through, and we see a nice bright star.

Then once the helium cools off, it slinks back down, goes closer to the core where it heats up, loses its electrons, becomes opaque again. Now the light is being blocked and we see a dim star, but then the helium gets pushed out, cools off, recombines, gets its electrons back, becomes transparent, the light shines through, we see a bright star, and then the cycle starts again. This kind of cycling can last anywhere from a few days to a few weeks. It turns out that with smaller, dimmer stars, the cycles are pretty quick. And with larger, brighter stars, the cycles take a long time.

The classic example of this kind of mechanism, which appears in all sorts of different kinds of stars and all sorts of different variations leading to many of those subcategories. But the number one category, the first kind that we encountered is, are called Cepheid variables because we saw a star like this in the constellation Cepheus, hence the name Cepheid variables. In the early twentieth century, astronomers saw a lot of these Cepheid variables, and a wonderful astronomer by the name of Henrietta Swan Leavitt discovered this fundamental relationship between how overall bright a Cepheid variable is and how long it takes to cycle up and down in brightness. Now she didn't know what was causing this. That would come a couple decades later with Eddington's proposal of the the pump mechanism.

But to her, that didn't matter because what mattered was that she was the first to discover a standard candle in the universe because you can find any Cepheid variable in the universe you want. All you have to do is record how long it takes to vary in brightness, and then you can convert that into a true absolute brightness of the star. So now you know how bright the star should be, and you compare that to how bright the star looks. And it's going to be dimmer than it should be because it's really far away. And you can do some math and figure out that distance.

And it's by finding Cepheid variables in the Andromeda Nebula that Edwin Hubble was able to realize that the Andromeda Nebula is actually the Andromeda galaxy sitting millions of miles away from the Earth, which unlocked all of modern cosmology. So I would say that, variable stars are not demonic after all. Thank you to Mike d and Matthew s for the questions that led to today's episode. Keep those questions coming. That's askaspaceman.com for the website, or you can email me at askaspaceman@gmail.com.

You can send my those questions directly to me. Please keep the reviews coming on your favorite podcasting platform. That really helps to show visibility. And, of course, thank you to all all who contribute to Patreon. That's patreon.com/pmsutter.

And I'd like to thank my top Patreon contributors this month. They're Justin g, Chris l, Alberto m, Duncan m, Corey d, Michael b, Nyla, Sam r, John s, Joshua, Scott m, Rob h, Scott m, Louis m, John w, Alexis, Gilbert m, Rob w, Jessica m, Jim l, David s, Scott r, Heather, Mike s, Pete h, Steve s, Watwat Bird, Lisa r Koozie, Kevin b, Michael b, Eileen g, Don t, Steven w, Brian o, Deborah a, and Michael j. That's patreon.com/pmsutter. Thank you so much for listening, and I will see you next time for more complete knowledge of time and space.

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