It’s time for school! The Astro101 series will cover some of the most important questions in astronomy. In today’s lesson, we’ll have: What’s the difference between a “dwarf” and “giant” star? How are stars born? What happens when stars die? I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
Welcome back to class, everyone. I hope you had a good weekend. Unfortunately, the quiz results, were a little disappointing. And instead of putting the blame on myself for creating a poorly designed quiz or having too high of expectations, I'm going to blame you. Now we have no choice but to move forward.
And today, we're gonna talk about stars again. Why? Well, like I said said last last time, stars are kind of a big deal in astronomy. For hundreds of years, they were the only deal in astronomy, and so they deserve a lot of attention because, you know, there's been a lot of work about stars, and we like to think that we understand a thing or two about stars. Now when it comes to stars, there are a lot of stars in the universe, and they're very distinct and interesting.
Be I mean, look at the night sky. The night sky is either star or the blackness of the ultimate void. So you can see why stars get so much attention. They they're very eye catching. If you want a rough number for the number of stars in the universe, you can take the typical number of stars in a galaxy, which is a few hundred billion, and multiply by the number of galaxies in the observable universe, which is a few hundred billion up to 2,000,000,000,000.
Yeah. We're still working on that number. And when you multiply those two numbers together, you get, well, a lot. It's a large number that involves a lot of zeros, 24 of them to be precise. Look.
That's about the best guess estimate we're gonna ever gonna get for the number of stars because it's hard to count each individual star, so we just have to accept it for what it is. There are just a lot. Like, isn't that good enough there? Just how many stars out there? There are a lot of stars.
However, of those trillions upon trillions upon trillions upon trillions upon trillions of stars, you can only see about 6,000 of them with the naked eye. And that's only if you travel to both hemispheres of the Earth and you you find a nice clear dark patch every single time you go. If you have really good vision and get really dark skies, really great conditions, any particular sky you will see about 3,000 stars. So if you combine the two hemispheres together, you're gonna get around 6,000. You know, it might be a few hundred more or less depending on your vision and the conditions.
The farthest star but to give you a sense of just, like, how many stars there are actually in the universe because the stars we see on the night sky with the naked eye without the aid of a telescope, like, that's that's not gonna be a fair representation of all the stars. Those are gonna be the closest ones, the brightest ones, the biggest ones. To give you a sense of how many stars are actually out there, the farthest star you can see with the naked eye, which is I mean, this is a hard thing to quantify because everyone's vision is different. Every scene condition is different. So there are a few different candidates and maybe even a few dozen different candidates of what is the most distant star that we can see with the naked eye, but a very good candidate.
And and, of course, I'm gonna count, like, real stars, not like a supernova or something, something that cheats and gets super bright all of a sudden. I'm I'm talking about normal main sequence hydrogen burning stars. Actually, scratch that, because the star I'm about to talk about is not a normal main sequence hydrogen burning star, but it is still a star in a phase of its life cycle. It's not a temporary flare. A good candidate that a lot of people put forward is v seven six two Cassiopeia.
It's just a random star in the constellation Cassiopeia. It's about it's pretty far away. It's 16,000 light years away. And considering that the Milky Way galaxy is a hundred thousand light years across, getting, you know, getting to see something that's 16% of the distance across the entire galaxy with the naked eye, that's pretty cool. It appears to us as a super faint star, but really it is over a hundred thousand times brighter than the sun, which is pretty cool.
But in that volume so the most distant star you can see with the naked eye is 16,000 light years away. In that volume, you can see 6,000 other stars in that same volume. But if you take, like, in the volume I mean, if you take the distance to v seven six two Cassiopeia and take that as the radius of a sphere, like, there's a bubble with a radius 16,000 light years or a diameter 32,000 light years across. So in that bubble, we can see 6,000 stars with the naked eye. But in that bubble, there are really around 18,000,000,000 stars.
Yes. Billion with a b, that is a real classic Sagan billion. In the same volume, the most I love just repeating this because it's so mind blowing. V seven six two Cassiope is 16,000 light years away. If you take a bubble, 32,000 light years across, there are about 6,000 stars that we can see with the naked eye, but there are actually 18,000,000,000 other stars that we can't see with the naked eye.
And as soon as you pop on a telescope or binoculars, you start to see all those in between stars. The reason that you can't see stars with the naked eye is that they have to be really, really ridiculously bright to be visible with the naked eye. And the vast majority of all stars are not ridiculously bright. They are small and dim and red. Remember the the red dwarf, also known as the m type stars, m type main sequence stars in that famous classification system of o b f o, I forget.
And then it ends with m, the smallest and reddest ones. 76% of all the stars in Milky Way galaxy are these small dim red dwarfs. And then there's, like, you know, 10 ish percent are mid range yellow stars like the sun, but you don't get to see the small red stars. You don't get to see the sun like stars. You only ever see the giant stars.
Like, for example, UV Scutty is one of the largest stars that we ever know. It's about 2,000 times wider than the sun and, like, a million times brighter. Like, that's the kind of star you need in order to be visible to the naked eye. That's what we're talking about. That's the real stuff.
The vast majority of stars in our galaxy and, therefore, our universe are simply invisible to the naked eye. Let that sink in. When you look in a particular direction in the night sky, even if it looks like the pure blackness of the infinite void, there are really stars and galaxies and nebulae all along that night line of sight that you simply can't see, inaccessible with the naked eye. Now these stars, like we discovered last time, these stars live and they die. They form.
They fuse hydrogen. They stop fusing hydrogen. They die. In the formation of stars is something that we kind of sort of basically understand, but then there are a lot of details that people still argue over. So I will give you the the broad brush basics of what we understand star formation to look like.
Stars form from the collapse of nebula. You have a cloud of gas and dust that collapses and makes a star. At least that's how it's done today in as far back as we can tell. The first stars appeared about thirteen billion years ago in an event that we call the cosmic dawn, the appearance of the first stars that formed from the first pockets of gas. Now there are potential ways for those first stars to have formed through different mechanisms that are not possible today, but I'm not getting it into that because I wanna talk about the stars of today and how they come on the scene.
There is a when we look at the history of stars and the generations of stars in the universe, you know, we've talked about these type o and type f classification schemes. There's another label that gets attached to stars that refers to what generation of stars they are in the universe. There's called population one, two, and three. Why they are labeled this way? Do not ask me.
Population one stars are like the present generations of stars. Like, our sun is a population one star. The previous generation of stars, stars that mostly lived billions of years ago but a few stragglers are still hanging on today are called population two stars. And then the very first generation of stars to ever appear, the ones that launched the cosmic dawn thirteen billion years ago are called the population three. Why this is backwards from what makes sense?
Do not ask me. Again, I'm just here to share it with you, not to justify its existence as a classification scheme. But that this population one, two, three just refers to the generation of stars. Population three stars, the first stars to appear on the scene, we do think don't exist anymore. We think they're very massive.
They live their lives very quickly, and then they died, and then they left behind. There are most population two stars are also dead, but there's still a bunch hanging around. And then the modern generation, the cool hip gen three or sorry, gen one. See, I always mix these up. Gen one is what's hanging around today, these young millennial stars.
Like I said, stars form from the collapse of nebulae, and I like to think of stars. This is just the own personal Paul Sutter, like, imagination, how I like to think of stars as the bright, intense, and temporary larval stage of a nebula. I'm about to describe how stars form from nebula, and then it turns out when stars die, they become nebula again. And nebula can hang out for a really long time just being clouds of gas and dust, and then stars are like this interesting temporary phase in the ultimate life cycle of a nebula, where you'll have a nebula, a cloud of gas and dust, portions of it will collapse, form stars, they live their life as stars, they fuse heavier elements, then they die and spew their guts, and they enrich the nebula again with some heavier elements, and then the cycle continues. And and so I like to think of the nebula as the real thing in the universe, and then stars are this this temporary thing, but, oh, they also happen to be hot and bright and easy to look at and have captured astronomers' attention for centuries.
Whereas the nebula, you know, we didn't really start paying attention to them until the eighteen hundreds. So they seem like second stringers when, really, I think, nebula are the main story. But here's the main story. You start with something like a giant molecular cloud, which is exactly what it sounds like. It's a big cloud of molecules.
For big ones, a big giant molecular cloud can pop out like a few dozen or a few hundred stars. There are smaller versions of these clouds known as bock globules, and, no, I didn't just make that up on the spot. That is a real thing. Bock globules are small clumps of gas and dust that can pop out a few stars. The key ingredient you need to start formation of stars is you need to be cold or at least colder than average.
Because if you have a nebula and you're all really hot, it's really, really, really hard to get your stuff together and condense down into a very, very small point because your gas is just too busy flying around all over the place. So the key feature of giant molecular clouds and what separates them from other kinds of nebula is that they are cold. They are able to release their heat and that this allows them to shrink down. In order to really get the star formation process going, you need to introduce some sort of turbulence, like a shockwave from a supernova or just some, like, brushing up against another cloud. You need something to shake it.
You need something to to give you the impetus, the the desire to to start making stars, to really start shrinking down. And pockets of gas will collapse, shrink down, dump their heat, shrink down more, and shrinking down a gas causes it to heat up. So this is a very detailed process where, like, a piece of gas shrinks down, heats up, is hot for a little bit, but then is able to release its heat, say, through radiation, then can squeeze down more, heats up again because that's what squeezing down does, then it releases its heat, squeezes down, squeezes down. In this process, leapfrogs back and forth over the course of millions of years until you actually start forming stars. The first beginning stage of a star is called a protostar.
This is when you have a very hot, very high density ball in the center of this nebula or piece of a nebula. It is not quite fusing hydrogen yet, and so hence it's all only called a protostar. These protostars are very violent and ugly places. They'll be surrounded by accretion discs by forming planets. Gas can be falling onto the star.
There's a lot of complex electric and magnetic fields because, hey, that is one of the universe's favorite things to do is to introduce complex electric and magnetic fields. These can launch jets of material out perpendicular to the disk of a creation. It's very, very cool looking. You know, like, when we talk about active galactic nuclei or quasars or magnetars, it's like that except a very low energy version of that. If the protostar is small, it gets called astronomers have a label for everything.
They call these t Tauri stars. Why? Because there was a con there was a star in the constellation Taurus where they first spotted a protostar like this, and so it just became the name for everything. And if they're larger, they get more interesting. They're called Herbig AEBE stars.
Why? Just reasons. Either way it's a protostar. Eventually the densities inside the protostar reach a critical stage where it can ignite nuclear fusion, and that is when it becomes a proper star, not a pro pro protostar, a real star, a real star that is fusing hydrogen in its core. It is now what's known as a mean sequence star.
Main sequence, if you remember our lecture from last time about the Hertzsprung Russell diagram, this connection between luminosity and brightness of a star, if you do obey that mean that the central connection between brightness and luminosity, that if your and also I'm sorry, surface temperature and luminosity. If you're obeying that relationship, you're on the main sequence. This means you are burning hydrogen. Hydrogen fusion is our answer to explain this big clump of stars on the Hertzsprung Russell diagram. A star will spend about 90% of its lifetime here just hanging out burning hydrogen.
And then it will die. Like I said, it will move off this main sequence, and we'll explore what happens in a little bit. But I do wanna say at the end of a star's life, it it returns to a nebula. Like, a star stops existing. It stops being a star, releases the vast majority of its material back into interstellar life.
This mixes with everything else in the galaxy, which then condense down to form a new generations of stars. So there there there's this circle of stellar life, constantly trading places between stars and nebula. Like I said, I like I like to think of nebula as the long term players. They were here before stars. There were nebula before there were stars.
And then someday the universe will stop making stars. We'll get to that in a different episode. In which case, the nebula will just still just hang out. So so nebula are the long term players here. Stars are just interesting temporary features, life stages of a nebula.
All stars have a core of nuclear fusion because it's kind of a definition of a star. That is where the party happens. That's where the energy is released to power the whole thing. All stars have some atmosphere, some layers of hydrogen, helium. It's, like, usually three quarters hydrogen, one quarter helium, and a small percentage of other.
All stars have these layers enveloping the core. In small stars, these these layers are what we call fully convective, which means, material, this this plasma, this gas is constantly cycling like conveyor belts up and down going from the core, then out to the outer layers, then back to the core, then out to the outer layers. You're constantly getting these up and down motion. For larger stars, there'll be a mix of these convection zones where materials moving up and down are in and out, in and out, in and out, and then radiative zone where radiation is just blasting through. For our own sun, we have a core surrounded by a layer of radiative transport where heat and energy are being carried by radiation, and then a layer on top of that or surrounding that of convection where material is sloshing up and down.
For stars much bigger than the sun, these layers reverse for various complicated physical reasons. Now as to these stars, you remember those stellar classifications in last episode, like o type, m type, whatever type, and how there is this neat and tidy connection between color and brightness and age and all that. Do you remember in that episode how I gave a little caveat that that only applies to stars in the main sequence? It doesn't count that, this type relationship between letter of the alphabet that apparently is randomly chosen and color of the star, temperature of the star, luminosity of the star, age of the star, all that that that cool relationship that Annie Jump Cannon accidentally discovered, that only applies to stars on the main sequence. That only applies to stars that are burning hydrogen in their cores.
From now on, it's gonna get hairy. And to describe this hairiness, we need to introduce a new pair of jargon words, which don't really have any strict definitions and have a variety of meanings depending on who you ask. If you ask 10 astronomers what these jargon words mean, you're likely to get 11 answers. And the names are really, really lame. And and, like, we've explored a lot of very lame names here in the history of astronomy on Ask a Spaceman.
These are gonna be top contenders for lame names, but once again, I'm not in charge of naming things. If you want to change that, if you want me to be in charge of naming things, give Patreon a shot. That's patreon.com/pmsutter. It's how you can keep these shows going. I can't thank you enough for all your support.
I don't know if I can change anything in astronomy. But, hey, if you contribute to Patreon and you say, hey, Paul, can you rename o type stars as, like, Greg type stars? I'm perfectly willing to do that. At least file a petition or at least a social media post. But, anyway, these jargon words that I need to introduce to describe what happens when stars move off the main sequence, I need to introduce two words.
The two words are dwarf and giant. No. I have not started reading the Fellowship of the Ring to you. You are still listening to Ask a Space Man. This is still about stars and astronomy and physics, but we are talking about dwarfs, and we're talking about giants.
And in the hilariously nonsensical world of astronomy, dwarfs describe basically any star that is anywhere ranging in size from very small to pretty much normal in terms of size and brightness. Yeah. You can have a star that is one tenth the size of the sun, and it will be called a dwarf. The sun is technically a dwarf star. Stars that are, like, three times more massive than the sun are also called dwarf stars.
Suns that are stars that are 10 times more massive than the sun are also called dwarf stars. It I mean, come on, but sorry. Giant stars are stars that are somewhat vaguely larger and or brighter than the dwarf by some unspecified amount. That's it. Dwarf stars are just stars.
And then if you're larger than stars and still a star, you get called giant. And that's that's it. That's what we're dealing with here, folks. And, of course, astronomers being astronomers found stars eventually. Like like, they like, oh, wow.
We've got giant stars. Okay. And then eventually, they found stars even bigger than the giants, so they found supergiants. But then they found stars that were even bigger than the supergiants, so they called those hypergiants. Oh, oh, and then we found some stars that are bigger than the dwarfs but smaller than the giants, so those are called subgiants.
And I'm sure there's more, but I stopped paying attention because after that, I was too busy rolling my eyes into the back of my head. So we have dwarf stars. We have sub giant stars. We have giant stars. We have super giant stars, and we have hyper giant stars.
And guess what? Dwarves are basically main sequence stars. Basically. If you are living on the main sequence, you're burning hydrogen in your in your core. No matter how big you actually are, you are called a dwarf star.
Why? I gave up. Dwarf stars are basically the main sequence stars, and then giant stars are what happens to stars when they evolve past hydrogen fusion. When a star is done burning hydrogen in its core, it leaves the main sequence and it becomes a giant. And here's how you make a giant star a giant.
You start fusing hydrogen and or helium in a shell. So basically what happens is towards the end of a star's life it stops fusing elements in its core and starts fusing elements in a shell surrounding the core. This could be for a variety of reasons. It could be because it's built up too much of a heavier element in its core and so you can't it just can't fuse anything there. It could be because it is fusing heavier elements there, and then the lighter elements are sitting on top of it and also burning Massive dumpster fire on top of a dumpster fire inside the core of a star.
But the key point is that near the end of a star's life, It stops burning, fusing elements in its core, and instead fuses elements in a shell around its core. And this makes the hot zone the place that is actually powering the star. It's not deep down buried in the center. It's now around the center, and then what this does is it inflates the rest of the star. Right?
If you're an atmosphere of a star and you're minding your own business and you know it's hot on the inside and cold on the outside, but then all of a sudden the hot on the inside part keeps getting closer and closer and closer to you, you're gonna go, oh, oh, oh, oh, oh, oh, oh, oh, get away from me, and you start inflating away. So that it's this process towards the end of a star's life, any star's life, when this fusion starts happening in a shell, it makes the star swell. And I just made up that rhyme on the spot, and I'm very proud of it. If the star is massive enough that even though it inflates because with the natural thing that's gonna happen, that you want that the star wants to do is the the outermost layers are gonna swell a balloon, like, hundreds of times or thousands of times the radius of our own sun. Like, our own sun will swell to reach the orbit of the Earth.
Those layers are now so far away from the core that even though they're large, they're relatively cool. The surface temperatures will drop, And cooler temperatures on the surface mean red. And this is what happens when a star is big and the outer layers swell too much. They cool off because that fusion shell is, like, way down there. And I'm all hanging out here at Earth orbit, you become a red giant or a red supergiant or a red hypergiant depending on how big you are.
But if you're a very massive star, like more than 40 times the mass of the sun, then when you enter this stage, you're still burning fuse, hydrogen in a shell or helium in a shell. You're still doing that dance, but you are capable of pumping out so much energy that even though the surface is huge and distended and bloated, it is still super hot because you are just capable of generating that much energy. You will be both large and you will be blue. You will be a blue giant or a blue supergiant or a blue hypergiant. So the giant blue stars tend to be more massive than the giant red stars.
In the wonderful quirks of astronomy, the giant red stars actually turn out to be larger than the giant blue stars just because of the way physics works. Either case, they are insanely bright. They are insanely bright. Even the red giants, even though they're relatively cool on their surface I mean, they would still melt your face off, but relatively cool as stars go. They are so large.
They have so much surface area that they can't help but be bright because there's just so much stuff pumping out so much light that they appear incredibly, incredibly bright. And this is what breaks that main sequence relationship on the Hertzsprung Russell diagram. On the main sequence, there is a connection between surface temperature and brightness. If you're hotter, you are also brighter, and if you're cooler, you are also dimmer. That is why all of so many stars live on that main diagonal sequence.
But once you enter a giant stage, once you are near the end of your life, you can be hot and bright or you can be cool and bright. Either way, you're bright, but you can be red or you can be blue. And so you have entered this land of the giants on the HR diagram, the Hertzsprung Russell diagram. Usually, these stars are incredibly variable because it's a kind of an unstable situation where you have, like, layers of fusion and shells of fusion happening inside of you. You have these huge bloated distended atmospheres.
Things can go haywire in a moment. So as you might recollapse and then reinflate and then collapse again. You might dim for a while and then brighten, and a great example of this is Betelgeuse in the constellation Orion. Betelgeuse is a red supergiant. It is massive.
It is bloated. It is about ready to die. Shortly before recording this episode, like in the past few months, there were a lot of stories about Betelgeuse, like, unexpectedly dimming, and astronomers were perplexed. And is Betelgeuse about to go supernova, etcetera, etcetera, etcetera? No.
This is just what giant stars do. They don't care. They wanna be dimmer for a little bit, they'll be dimmer for a little bit, and you can't tell them otherwise. They wanna be brighter for a few weeks, they'll be brighter for a few weeks, and so we're gonna see stuff like that. There's another star in the constellation Orion Orion, Rigel, which is a blue supergiant.
So right there, you've got two supergiants. In fact, blue and red supergiants are incredibly rare in real life. These are stars that are near the end of their lives. These are stars that are massive. So that's just gonna combine to make them very, very rare, but they dominate the night sky.
Almost every star you see on the night sky is either a blue or red, giant or supergiant. Why? Because they're bright. They can be thousands of light years away or even just hundreds of light years away and be so bright that you can see it with the naked eye. You know, of those 6,000 stars in the night sky visible to the human eye, the vast majority are blue and red supergiants because those are the brightest ones.
You don't get to see all the 18,000,000,000 normal stars minding their own business. No. You got all these 6,000 hogging all the attention. And when it turns out these are actually an incredibly rare form of star, but if you were to just trust the naked eye, you would think these are the only star there is. Both blue and red giants are huge.
We are talking hundreds to thousands of times wider than the sun. They are incredibly bright. They are thousands to millions of times brighter than the sun, and they are near the end of their life, and they are ready to die. And so we see that the Hertzsprung Russell diagram is like a fortune teller for stars. When they're born, when they emerge from that protostar stage, they land somewhere on the main sequence, and where they land depends on their initial mass.
You know, if they're very, very small, they'll land on that dim and red side. If they're medium, they'll land on that whitish, yellowish, brightish side. And if they're massive, they'll land on that bluish, brightish side. The as they live, as they burn fusion, turning hydrogen into helium, they steadily grow brighter and hotter as they age. So they steadily creep up the main sequence.
Red dwarf stars, the small stars will spend trillions of years on the main sequence. Stars like our sun will spend billions of years on the main sequence, and then the giant stars will only spend millions of years on the main sequence, which is like nothing at all. Eventually, they will stop burning hydrogen in their cores. They will evolve into another stage, their last life life cycle stage. They will evolve into one of the several categories or branches of giants, you know, red giant, blue supergiant, maybe a hypergiant here, they can do so several times because they might enter a giant phase and then collapse down.
And then a new round of fusion kick starts, and they enter another giant phase, and they might do this again. Our own sun will become a subgiant at first, then collapse down, and then be kinda normal for a million few million years, and then it will enter a true true red giant phase. Eventually, if its star is massive enough, it will go out with a supernova bang. If it's not, it will go out in a gross looking planetary nebula. Either way, they will scatter most of the material out into interstellar space where it will enrich and become a part of another nebula and it will leave behind something.
These somethings will be the remnants. These will be the white dwarfs. These will be the neutron stars. These will be the black holes. These are the stellar leftovers.
As for those dead, dying, and decaying remnants, well, that's for another class. Class dismissed. Thank you so much for listening. And I would really, really like to thank my top Patreon contributors this month. That's patreon.com/pmsutter.
We got Matthew k, Justin z, Justin g, Kevin o, Duncan m, Corey d, Barbara k, Neuter Dude, Robert m, Nate h, Andrew f, Chris l, Cameron l, Nalia, Aaron s, Tom b, Scott m, and Rob h for all your contributions. I really do appreciate it. Go check out my book, How to Die in Space. It's available at bookstores nationwide, worldwide too, or on my website, autograph copies, PMCenter.com/book. Go ahead and leave an iTunes review if you can.
I really do appreciate it. It's it's good to hear from you. I really appreciate hearing from you. And, hit me up with some questions. Hashtag ask a space man, ask a space man at g mail dot com.
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