What are blue straggler stars? How were they first discovered? What do they tell us about star clusters and the evolution of stars? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPTION (AUTO-GENERATED)

I'm warning you now that this is going to be one of those episodes that features two things. One a classification system that I made up on the spot as I drafted my notes on the subject and has absolutely no bearing on any professional work in astronomy at all. And two, a very long lead up to answering the actual question posed in the title of this episode. What are blue straggler stars? Because if I just give you the answer of, uh, while blue straggler stars are main sequence stars that are bluer and brighter than the main sequence turn off point for a stellar cluster that isn't very satisfying on its own. And while that answer is pretty short and to the point, we need to unpack it a little. And so, in this case, in this episode, the journey itself becomes the destination, and we should all just enjoy the ride. Disclaimer is over. Before I get to the question that drives this episode, I need to start with another simpler one. How do you tell how old a star is if I just gave you a random star, this guy a beetle juice Arye gel you know, a S.

You know, that's a great star name, by the way, A different story. Uh, you just point to it and say, How old is that star? How do you know? How do you be even begin to answer that kind of question? It's a difficult question, because at least at first we have no reference points. We have no way of measuring a cellar lifetime. We know for certain that stars live longer than human beings, and we know for certain that stars live longer than the entire recorded history of humanity. Because if stellar lies were shorter than that, we would see stars dying all over the place. Yes, I know we see stars dying in supernova, but those are very rare events compared to the multitude of stars that we can see. If stars had a lifetime of 100 years or 1000 years or even 10,000 years, we would be seeing stars die left and right. But beyond that, all we know is we can just use that to place a lower limit on stellar lifetimes. Yeah, stars probably live longer than 10,000 years or so, probably longer than 100,000 years.

But beyond that, we have no reference point. Contrast that to I don't know, dung beetles. We can watch dung beetles be born, live and die before our very eyes. We can measure that. No one really wonders how long dung beetles live, because it's a very easy and obvious measurement to make. Not so with stars, because stars have such enormous lifetimes greater than the entirety of the history of human written records. How do we develop the reference points needed to determine a star's age? Imagine you're an alien visiting the Earth for the first time and you meet one human being and the human being says, uh, hello, Alien Guess how old I am. I don't know why that's the intro, but, you know, let's go with it. And aliens, I don't know. You're obviously longer than a second or a minute or 10 minutes. Otherwise you would drop dead in the middle of this conversation so I can put a lower limit on it.

But I don't know, I have no reference points. An alien visiting the Earth would have a hard time guessing people's ages, but you probably have a pretty accurate gauge of someone's age. I bet you can pin it down to the decade or the half decade. Oh, yeah, yeah, that's a teenager. That's a 20 something. That's a 70 something, and and probably you're more right than you are wrong. You can do this because you have a lot of reference points. You have your own age and you have your own self in the mirror. You have your family, you have your friends. You maybe have kids or grandkids, and so you can start to get a judge. Once you have enough samples and you see enough people living and you have enough reference points enough benchmarks, you can guess people's ages. Kids have a lot harder time with this that my kids think I'm 60. They also think college students are somewhere in their forties. But as kids get older as you get older, you get a better handle on this. You get more reference points. You get better at guessing new people's ages.

You meet a random person on the street. You have a pretty decent chance of guessing their age to within 5 to 10 years. If we want to estimate the ages of stars. We need reference points. We need hooks. We need big giant signs that say I'm 500 million years old or we need to catch an over the hill four billionth birthday party in action. We need reference points. Even the age of our own sun was hard to pin down for a very long time. For for a long time we just assumed it was infinity or the age of the universe. The sun was simply a part of the cosmos the same way that rocks are a part of the cosmos, and stars and the air are simply a part of the universe. And so, for however long the universe has existed deep into the the dreamless past, that's how old the sun is. And it's meaningless to talk about the Age of the Sun just the way it's meaningless to talk about the age of the universe. It's long as we started to develop more scientific approaches to this in the late 18 hundreds, we started to develop some measurements and some estimates based on our knowledge of physics.

Our knowledge of physics at the time was limited to thermodynamics and combustion. We thought maybe the sun was burning things in its core to generate its heat. But that led to a lifetime of around 100,000 years, which didn't really seem to mesh well with our knowledge of evolution, geology, all the other fields of science. But nowadays we have a pretty accurate handle on the edge of the sun. It's around 4.6 billion years old, and we get this through a few different ways. One is called and I love this word. Are you ready for this one? Hold hold on to something nuclear. Cosmo Chronology Nuclear Cosmo Chronology where you use essentially radioactive dating but for the age of the sun and the age of the solar system. If you assume that the sun is pretty much the same age as the as the rest of the solar system, we all condensed out of the same ball of goo. Then if you can find the oldest thing in the solar system, then that gives you a decent handle on the age of the sun, and we can find the ages of things in random objects in the solar system by looking at radioactive elements.

For example, iron 60 iron 60 is this isotope of iron that's very radioactive, and it's only produced in supernova explosions. And so if a supernova explosion which we do believe happened near our solar system and triggered the formation of our solar system, we got soaked with iron 60. But then, over time, iron 60 decays into nickel 60. And so, if you dig up some meteorite to go to a random bit of space junk and you find some iron 60 some nickel 60 the more nickel 60 there is compared to the iron 60. The older the solar system is, and then you can pin down a number. That's one way to get an age. Another way to get an age is science is, is looking at, say, evolutionary history and genetic codes and backtracking to to our last universal common ancestor and how long that takes in genetic drift. And there's this amazing science we can actually develop a clock of when Life first appeared on the Earth, just based on evolution.

That gives you a lower limit to the age of the sun because obviously the sun had to be around back then you can use geology. The physics of the Earth, and you can use our knowledge of the physics of the Interior of the sun. We know about nuclear reactions. Now, Now we know about, uh, neutrino production. We can measure the rate at which the sun is burning hydrogen in its core and we can backtrack to see how long it's been burning hydrogen in its core. And we can. We have all these tools available in the solar system to estimate the age of the son. But even with the age of the sun nailed down, we can't generalize that easily to other stars because the other stars, by and large do not look like the sun. There are small stars. There are big stars. There are yellow stars. There are red stars. There are blue stars. There are small red stars and big red stars. There are big blue stars and really big blue stars. There's all sorts of different stars with different temperatures, different brightness, different colors, different masses and presumably they all didn't appear At the same time.

It's a solid assumption that are all the stars in the galaxy didn't form at once, and so we can't do the tricks of nuclear Cosmo chronology in other stars because we can't access the radioactive elements, we just have a whole bunch of stars. So even having one reference point. If I tell you the age of one human being on the planet, you're gonna have a hard time generalizing that and using that as a tool to guess the age of other human beings on the planet. It's a mess, but thankfully, nature gave us a clue that allows us to unlock the ages of stars. And that clue works because physics is always physics throughout the universe, which is my favorite thing about physics. Stars need to fuse hydrogen to survive. That's how they get their power. They shine through nuclear fusion in their cores, but nuclear fusion has a tendency to blow things up. It releases an enormous amount of energy. Stars don't enjoy being blown up, and stars are obviously not blowing up right now. They also don't enjoy collapsing down into black holes through the massive weight of their their own gravity.

They're in balance, the balance between their own energy release from nuclear fusion and the pressure of gravity that wants to collapse stars in physics, we call this hydrostatic equilibrium. But you and me, we can just call it being chill. Starss are chill. Yes, they are giant factories of nuclear reactions in their cores that want to blow them up. And, yes, they are incredibly massive, with a lot of gravity that wants to compress and crush them. But they don't do either of those because they're chill. They're in hydrostatic equilibrium. If the nuclear reactions go a little bit out of control and release a little bit too much energy, then the the star swells and expands. But that relieves the pressure in the core, and it lowers the rate of nuclear reactions back down to what it was. And on the other side. If if, for some reason the star squeezes in too much and starts to collapse, well, that increases the pressure in the core, which increases the rate of nuclear fusion, which increases the amount of energy released, which which puffs out the star a little bit.

So it's always in balance. They're chill. Once stars start using hydrogen at the beginning of their lives, they can be chill for a very long time when they are chilling. When they're in this state of balance of using hydrogen and their stable. We call them main sequence stars. So if you ever hear an astronomer at a bar or a party start talking to you about the main sequence, what they mean is a star that is chill and is burning hydrogen in its core to sustain itself. And it's maintaining a steady rate of this. The main sequence exists in a very abstract sense. It you you can't point to anything in the universe and say, Oh, yeah, yeah, yeah, this is the main sequence. What do we mean? What is the sequence and why is it mean if you make a plot of stars and you plot their temperature versus their brightness, you might imagine stars would be all over the place. And like you, you look at the variety of stars out there and there's all sorts of temperatures.

There's all sorts of brightness is out there, and you might imagine it's It's just all scattered crazy. Any combination of temperature and brightness is possible. You might think that, and you'd be wrong. And for a long time we thought that and for a long time we were wrong. It turns out that when stars are chill, when they are fusing hydrogen, they obey a very specific relationship between temperature and brightness. This appears when you, when you plot the temperature and brightness for a whole bunch of stars, that these dots that you put on your plot don't scatter around all over the place. They form a band, a diagonal band, in this plot of temperature versus brightness, and that band is what we call the main sequence. This is where stars that are chill that are burning hydrogen to survive, no matter its mass and no matter its age, as long as it's burning hydrogen, its temperature and its brightness will live in this little region.

It will have a specific ratio of temperature to brightness. It will be on the main sequence when stars are born. They first appear on the main sequence. That's when they show up. That's when they start to obey this relationship. That's when they start to be chill, and where a star appears on. The main sequence depends on its mass when it's first born, so a small star when it's born will have a low temperature and a low brightness. It will still obey a very specific relationship between its temperature and brightness. And so it will be on one end of the main sequence, this diagonal band that appears in plots of temperature versus brightness For a bunch of stars, you can do this at home with a telescope. You can measure the brightness and the temperature of stars in the sky, and you will rediscover the main sequence. A medium star like our sun when it's born, will be brighter and hotter. But it, too, will have the same relationship, the same ratio between temperature and brightness.

And so it, too, will appear on the main sequence, a different part of the main sequence. A little up the diagonal, a little bit, but still there, and then the very most massive stars. They will look blue and they will be insanely bright. They will still be on the main sequence. They will still be chill, just chill in their own way as stars evolve with time. This is where we start to get the clue to determining a star's age and how we can use the main sequence relationship. To tell a star's age is that stars evolve as they live as they burn hydrogen that hydrogen fuses into helium, and the helium just sits there in the core. It just hangs out that helium gets in the way of fusion. It blocks it. It the hydrogen has a harder time reaching each other to do that magical fusion dance. The helium gets in the way. There's ash, there's contamination. And so as stars age, they have to work harder to stay chill.

They have to fuse at a more intense rate as they age. And so as stars get older, they tend to increase in temperature and in brightness because the fusion reactions are going a little bit faster than they were in order to maintain that chillness because if they didn't do anything about it, then the fusion reactions would shut off and they would just collapse. So they have to increase their fusion rate to keep going. Our own sun is a little bit brighter and hotter than it was millions and billions of years ago, but they maintain this relationship. They maintain this ratio between temperature and brightness, so stars will appear when they are first born somewhere on the main sequence and that place is determined by their mass, and then as they age, they creep up in temperature and brightness. In astronomy, we say they move along the main sequence and again, that's in a very abstract sense on this plot of temperature and brightness. If you were to be able to watch a star over millions or billions of years and you check in with the star is like visiting your doctor, you check in, get your vitals with your temperature and your brightness, OK, million years later, it's time for my next appointment.

Temperature and brightness. Every time you do that, you put a little dot on this graph. You will see the dots moving up along this diagonal path that we call the main sequence moving towards hotter and brighter. So all stars do this? No. Regardless of their mass, their mass determines their starting point on the main sequence, and then as they age, they creep up along the main sequence. Eventually, though, a star runs out of hydrogen. In astronomy, we say it leaves the main sequence, but between you and me, we can say that it stops being chill. How long it takes for a star to eventually leave. The main sequence again depends on its mass low mass stars just sip at their hydrogen. They can hang out for trillions of years. In fact, we haven't seen a single red dwarf star leave the main sequence yet because the universe isn't old enough. Stars like her son will last 9 to 10 billion years before eventually leaving the main sequence and stop being chill. The most massive stars can do it in in a few million years. They race along the main sequence, and then they pop off of it.

They stop being chill when stars stop being chill. They go on to fuse other heavier elements, like helium or carbon and oxygen. And it starts getting really funky, like becoming a red giant. When stars become things like red giants, it doesn't have the right chill relationship between temperature and brightness. That main sequence stars do, and so they're not chill anymore. They're funky. They've left the main sequence, so we know how stars evolve with time, how their temperatures and their brightness is evolve with time. We call this relationship the main sequence, and all stars obey it because physics is physics all across the universe, they all. They're all using hydrogen in the exact same way, and that governs how they're gonna evolve. So you might be tempted to think that we're done, that the main sequence can give us the age of a star. We can put together a star's mass, its temperature and brightness and figure out where it is on the main sequence. And once you know where it is on the main sequence, the star's Mass tells you where it started on the main sequence, and then you can compare that to where it you finally observe it on the main sequence, and you subtract those in a way and you can figure out the star's age.

But it's not perfect. In fact, it's far from perfect, especially for individual stars. This show is sponsored by better help. One of the most awesome things about physics is that it's like a user manual for the universe. You can literally use it to predict the future Now. Now humans are a little bit more complicated. Believe it or not, people are more complicated than quantum physics. I am not joking, and and life and dealing with people does not come with a user manual, and the next best thing is therapy. I have been using therapy for years. It's such a powerful tool for me to to answer life's questions when those questions don't come in the form of of of physics problems. And I think you will benefit a lot from it, too. And that's why I'm proud to have better help as a sponsor. Better help is the world's largest therapy service. Better Help has matched 3 million people with professionally licensed and vetted therapists available 100% online.

Plus it's affordable. Just fill out a brief questionnaire to match with the therapist. If things aren't clicking, you can easily switch to a new therapist any time. It couldn't be simpler. No waiting rooms, no traffic, no endless searching for the right therapist to learn more and save 10% off your first month at better help dot com slash spaceman. That's better. Help HE LP dot com slash spaceman. The problem with individual stars is is that if I hand you a star or point to a random star and you measure its mass and its brightness and its temperature, you're not exactly sure if it's a chill star that's hanging out on the main sequence or a funky star that's begun to move off of it without other clues. You don't know exactly where in its life cycle a star is. You have to compare what you measure to a theoretical main sequence track for that particular star.

And once you start getting into the details of Star, it's more than just mass and brightness and temperature. Things like the amount of metals in a star, the amount of heavy elements in a star. It's environment, whether it's part of a binary pair. All these other factors influence a star's life, and so you can only develop a rough estimate, which is good, but not good enough. You don't know if you're looking at a chill star or a funky star, because those details matter at the level of precision that we want. If we want to call it within, you know, plus or minus a billion years, yeah, we can do it. But if you wanna get more fine grains than that, you need more information. You need a bunch of stars, like if I told you everything you needed to know about human aging patterns. And I said, OK, it's basically summarized if you want to figure out how old a random guy is of your baldness is a good indicator.

The more bald a guy is, generally the older he is, and then you can collect data points from all over the world. You can develop theoretical models, and you can have a pretty decent relationship. But if I give you a random person a random guy, maybe you can get it right. Maybe you'll be really wrong because you don't know if this is a normal person that's aging along the, uh, main sequence baldness track. Or if something funky happened to him like premature balding. I lost my hair when before I was 20. And so if you developed this fancy model the main baldness sequence and you met me when I was 25 or now you would look at me according to my model and and according to your model and you say, Uh, you're approximately 75 years old and you'd be way off because there's something different about me that that doesn't fit the trend. And there are plenty of old dudes out there with massive manes of hair and you would look at them and say, according to my model, you're approximately 16 years old and you'd be way off.

You'd be right a lot. But you'd also be wrong, and you'd be wrong enough that it wouldn't be good enough for you. But if I gave you a group of people, a group of guys and I told you that they were all born in the same year, then you could use your knowledge of human aging and baldness to make a very reasonable gasses to their age. Because all the funky oddities like premature, balding or having a full head of hair when you're 95 they average out. So the power here of using that this technique of the main sequence and the evolution of stars along the main sequence doesn't come with looking at individual stars. But when you look at a group of stars and when, especially when you know that that group is all roughly the same age, then you can average out all the weirdness you can average out all the, uh, the metal content and and binary interactions and all the other stuff. If I gave you a group of guys. Some of them might be completely bald like me.

Some of them might have full heads of hair. But if I told you that they were all the same age, then you could average all that out. And you can get a pretty good estimate of the group's age. And so when nature gives us a star cluster, we can finally get to work. Star clusters are exactly what their name suggests. They're groups of stars. They're open star clusters, which are groups of newborn stars that have not drifted away from each other. They're globular clusters, which are chunks leftover chunks of either preformed Galaxies or torn apart Galaxies that were consumed by the Milky Way. In either case, clusters generally have stars of the same age because clusters tend to form at the same time. If you take a cluster, there will be all sorts of different stars in it. There'll be small red ones, medium white ones, big blue ones. You know that these different colors and brightness or luminosity are due to the star's mass and their ages.

And if you didn't, if if all the stars had different ages, then you would be lost. But if you know that the stars have roughly the same age, then now you have a clue. And when it comes to clusters, stars generally have the same age. Since all the stars were born within roughly the same amount of time for our purposes, you know anything plus or minus a a million years is good enough because we like to keep things loose and fun in astronomy. Then you know that all these stars, regardless of their mass, started being chill by entering the main sequence at the same time, they were all born roughly at the same time, they all ignited nuclear fusion in the cores. At the same time, they all entered the main sequence at the same time. They all started being chill at the same time. But the more massive stars, because they're massive, will run out of hydrogen first, and they will start to get funky. They will leave the main sequence and this can help us reconstruct the age of an entire cluster. Imagine taking a snapshot right When the cluster forms, all the stars will be chill.

They're all burning hydrogen. They're all on the main sequence. And then you take another snaps snapshot a little while later. Imagine you can follow this cluster instead of following one star for billions of years. Follow an entire cluster for billions of years. At first they're all chill. Then a little while later, the most massive ones will have run out of hydrogen and they're gonna be funky. They have left the main sequence. They're not on this nice, neat, orderly diagonal band. The relationship between temperature and brightness. They have something weird. They have something funky. The most massive ones have left, but most stars are still on the main sequence. And then you take a snapshot a billion years later. Now, even more stars have left the main sequence. Yeah, you're chipping away. Uh, first, the most massive star that one stops being chill starts getting funky, and then the second most massive one, then the next most massive one. Then the next most massive one and so on and so on. And so, you know, if all the stars started on the main sequence at the same time, then you can compare how many stars have left in the main sequence versus how many stars are still on you now have the platform you need and you now have the reference point.

You have all the hooks you need because now you can combine that with your knowledge of physics and you're averaging out all the weirdness because you're looking at a group of stars instead of just an individual star. And you have the extra added knowledge that all these stars were born at the same time. You don't know when yet, but you know that they were born at the same time. Boom. You're done Where the turning point happens from chill to funkiness from on the main sequence to off the main sequence tells you the age of the cluster. You know that all the stars of the same age you know how individual stars evolve. And you can be sure that because you're looking at a group, you can average out any weirdness. You can point to the place where the stars switch from chill to funky and calculate an age, and it works. Astronomers use this method all the time to estimate stellar ages and all the stars in a cluster obey this relationship of being either chill or funky except the blue stragglers. The blue stragglers aren't chill. They aren't funky. They're weird.

They look like massive stars. They are big, they are blue, they are bright. They're big enough that they should have left the main sequence a long time ago. They should be funky. They're so massive that they they should be dead. They should not be burning hydrogen anymore, but there they are, hanging out on the main sequence. They're still chilling, even though they're too old for it. It's like grandparents dressing like teenage social media influencers. Imagine going to your grandparents house, you know, drapes, doilies, little plate of cookies, music playing in the background. Everything's quiet, very well organized, very tidy. And then they start showing off their neck tattoos and using all the slang that kids are using These days. I'm not even going to give examples of slang that the kids are using these days, because I know I'll get it wrong. And there is a limit to how much I'm willing to embarrass myself for your entertainment and education. These blue straggler stars are big and blue and bright, and they are on the main sequence, even though they shouldn't be they're not funky.

They're trying to be chill and then just end up just being weird. You know, like patreon patreon dot com slash PM Sutter if you It's like the worst one I've ever done. If you're not weird and not chill and not funky, you're a patron at patreon dot com slash PM So it is how you keep supporting the show. I do appreciate it. What are blue straggler stars? Blue straggler stars are weird grandparents. They're stars that look old but are acting young or look young but are acting old Whatever way, it it just doesn't fit. All the other stars of the same mass have left the main sequence and are now over there being funky. But these things are still hanging on and still burning hydrogen. How do we get stars that are bright and blue and massive but still on the main sequence? Uh, one explanation is that it's an just an accident of perspective that these are stars that happen to lie within the line of sight or are near the cluster, but not really a member of it.

They didn't really they weren't born at the same time. They're just different stars. Another explanation is that they were captured. Uh, the cluster formed first and then captured these stars, and so they're not born at the same time. Uh, these explanations are hard to reconcile with. The fact that blue stragglers are tend to be found in the center of clusters, not in the outskirts. In the centers are is not where captured objects go. Captured objects stay in the outskirts. And maybe it's a coincidence of observational alignment. Uh, but then, like every cluster we look at has blue stragglers near their center, and that's that seems like an odd a lot of odd coincidences. Maybe it has something to do with the cluster itself and the dances that stars can go through. Clusters are incredibly dense environments far more dense than, say, the solar neighborhood, you know, 1000 times 100,000 times a million times more dense than the density of stars around us. And that leads to a lot of extra interactions that you don't typically get in stellar neighbor or in neighborhoods like the ones around the sun.

There are two models for how we generate blue stragglers stars. I will say that we don't really know for sure what causes blue stragglers, these weird grandparents. So take both of these with a grain of salt, one is called, uh, the collision model is pretty much exactly what it describes. You. Take two low mass stars. They get captured in each other's orbit. They rotate around each other. They merge. This leaves behind a massive, rapidly rotating star. All that angular momentum got sucked up is now spinning the star because of that incredible amount of spin, that incredible amount of angular momentum it swells when stars swell. They tend to turn red because the power source in the center tends to stay the same because it's just hydrogen fusion in the center at the same rate. But then there's something else in this case angular momentum that's flinging the outer surface away from the core, and so that gives it a lower temperature. So it'll it will look red, but these stars tend to have because of the collision in extremely strong magnetic fields, one of my favorite things in the universe.

In this case, the magnetic fields act like giant sails that slow down the rotation rate of the star. Eventually, the star shrinks back down with a higher temperature now and becomes a bright blue star. Another model. It's called the slow coalescence model, which is the same thing as above, but it just happens more slowly and instead of a violent merger. Instead, one star just cannibalizes the other. In either case, the star gets a second shot on life. Blue straggler stars first enter the main sequence as something else, and then something happens to them. A merger, an interaction, a cannibalization And it gets more mass not because it was born with that mass, but because it stole it. And so it restarts its journey on the main sequence in a different point. But it really it's the same age as everybody else in the cluster is just acting too young for its true age. It's a weird grandparent, thank you to Flat Biscuit on the website for the question that led to today's episode and thank you to all my patreon contributors, All of you.

I really do appreciate it. It is your contributions that keep this show going. If you want to support the show, if you ask, like what can I do best to support the show. Patreon dot com slash PM Sutter is what you do. I'd like to thank my top Patreon contributors. Justin G, Chris L Barbeque Duncan M Cordy, Justin Z, Andrew E, NAIA Scott M, Rob H, Justin Lewis, M John W, Alexis Gilbert, M Joshua, John S, Thomas D, Simon G and Aaron J. Those are the top patreon contributors this month, and I really do appreciate it. If you want other ways to support the show. If you don't have money, don't have money to spare. It's fine. It's cool. I understand. Instead, leave a review on iTunes or Spotify, or however you are consuming this podcast. Tell friends and family about it, shout outs on social media, always appreciate it and keep asking me questions, because that's how the show keeps going. And you can send those questions to ask us spaceman at gmail dot com or the website. Ask us spaceman dot com, and I will see you next time for more complete knowledge of time and space

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