The Stellar Compendium

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So today we will be going through all the various star types, from the tiniest category of brown dwarf stars up to monstrously huge Wolf-Rayet stars, and from the normal main sequence stars to some strictly hypothetical ones composed of Dark Matter or types of particles we’re not even sure exist. We will also be trying to simplify a lot of the concepts in astronomy that tend to get used a lot but generally not well explained. Often the biggest problem people have mastering astronomy and astrophysics is the terminology itself, and a lot of that comes from terms that have antique origins or had to do with how we discovered them, not so much how they function. We will try to clear that away. So loosely we will discuss the following topics in the following order. 1. The Harvard Classification System 2. The Yerkes Classification System & Lifetime 3. Population, Metallicity, and Formation 4. Abnormal Stars 5. Dead Stars 6. Hypothetical Stars         Okay, there’s a decent chance you have never heard of the Harvard Classification System but you have heard it used all the time. You’ve heard our own sun referred to as a G-type star or a G2 yellow star, that’s the Harvard Classification system. Some quick history is needed here, we have obviously known about the stars a long time but we only knew them by how bright they were and what color they were. We didn’t know how far away they were for instance, and we didn’t learn that as soon as we invented telescopes. We can’t even see the nearest star for instance, Proxima Centauri, with the naked eye, but we can see V762 Cas, which is over 16,000 light years away, 4000 times further off than Proxima. We can only see a few thousands stars with the naked eye, maybe double that if you’ve got great vision and great observing conditions, but there are a few billion stars closer to us than that most distant visible star, V762 Cas. So visibility doesn’t particularly indicate proximity.     Once we had telescopes though we could obviously see a lot more stars and we could also zoom in on one, by itself, and set up some instrument to measure the components of that light. How much red or yellow or infrared light was coming off it for instance. We could also measure its total brightness, its total luminosity, across all wavelengths even those invisible to our eyes by use of a device called a bolometer. This gives us the term bolometric luminosity, and it differs from normal luminosity in that the former is all the light given off by a star across every wavelength.     By and large stars give off light in a spectrum of wavelengths based off their surface temperature, but it generally is not a smooth curve because of various substances floating around on top of them absorbing some of that light and re-emitting it at a different wavelength. That results in some very jagged spectra, especially for hydrogen, the main component of stars, which sucks up light and re-emits it at specific frequencies. 4 of which are in the visible range of light and these were discovered Johann Balmer in 1885. So suddenly they could look at a star’s spectra and tell how much hydrogen it had. You can do the same for other elements and for frequencies outside the visible band, and also what they absorb, so you will often hear about absorption and emission lines, but for our purposes today we don’t care about that. That’s just how we identify the stars and how the system originated. The original system assigned each star type a letter based on how much hydrogen could be seen in their spectra, with A having the most, B the second most, and so on, for 22 in total, A to V. This original system, sometimes called the Draper Catalogue, just measured hydrogen, and was put together by Edward Pickering and his assistant Williamina Fleming, best known for her discovery of the Horsehead Nebula in 1888. Draper was an astronomer who did a lot of the groundwork for photography in astronomy and after he died his widow agreed to fund Pickering’s work, though Williamina Fleming did most of the actual classification. She got her career in astronomy started as Pickering’s house maid at Harvard and was the first member of what became known as the Harvard Computers, 80 or so women employed to do calculations back when computer was a job title not a device. Another one of them, Annie Cannon, formalized the classification system later on based on temperature, but the original version Pickering and Fleming made was a bit more cumbersome. Unfortunately she used it as the base so we got stuck with a bunch of letters that basically mean nothing anymore and is hard to memorize. From hottest to coldest they are O, B, A, F, G, K, and M, and if you are familiar with those and ever wondered why those letters were picked in that order, now you know. The original system, the one used in the Draper Catalog, focused on how much hydrogen they could detect in the absorption lines and it stuck around when Cannon made the modern system, the Harvard Classifications. These also all correspond to colors, red is coldest and blue hottest. She was also kind enough to come up with a mnemonic for remembering those, “Oh be a Fine Girl, Kiss Me.” So now we had a system for temperature but still none for any other attribute, and an M-Type star, the coldest, can be either the smallest stars or the largest, for instance. It also is a bit vague even about temperature, so we follow each letter up with a number 0 to 9, 0 being hottest and 9 coldest. So the hottest star would be an O0 and the coldest an M9. Our sun is a G2, making it in the third hottest zone of the G-type stars, themselves the third coldest of the original categories. Again, these categories tell us nothing about how old a star is or how big it is or bright it is by itself, for that we needed something more. So in the 1943, at Yerkes Observatory in Wisconsin, Wilson Morgan, Philip Keenan, and Edith Kellman came up with an additional classification system to cover total brightness. This is sometimes known as the Yerkes Spectral Classification system, for the observatory, and sometimes the MK system for Morgan and Keenan, for the two men, and nowadays MKK to include Edith Kellman. Like the Harvard Scale this one has been added to over the years but originally they just had five, 1 through 5, done in Roman Numerals. And these just come after the Harvard scale of a letter, then a number, then a Roman Numeral. So our sun for instance is a G2-5, not a G2V, most stars talked about are V’s, or 5s, because most stars are 5s or Vs. This indicates a main-sequence star. 1, 2, 3, 4, and 5 are respectively 1, Supergiants, 2, Bright Giants, 3,  Normal Giants, 4, Sub Giants, and 5, Main Sequence. Again in Roman Numerals, and after the letter and number of the Harvard Scale. You might be wondering why there are no normal stars or dwarfs on that scale, and in fact we have added one to it in recent years, for sub-dwarfs, and another for white dwarfs, and broke up category one into various extremes of brightness, but again no dwarfs or normals. This is an example of terms changing as we learned more. Originally category 5 was dwarves, and that’s essentially a relic of when most stars we could see were unsurprisingly the brightest stars. Our sun gets classified as a Dwarf Star for this reason, even though it is bigger than more than 90% of other stars. And Main Sequence is a weird word too, you’ve probably heard it enough times that you don’t really think of it anymore but when we are talking about size and brightness, supergiant this and bright giant that, it does seem a bit bizarre to have the term ‘main sequence’, like you had a table for describing people’s height, giant and dwarf, but instead of just ‘normal’ or ‘average’ you have some weird term like ‘standard doorway clearance’ or ‘comfortable shelf access’. That’s basically what happened here. As we began measuring brightness of stars, their actual brightness, not how dim they are to us based on distance from us, we began plotting them. Back in 1910 Ejnar Hertzsprung and Henry Russel plotted a bunch of the stars by their Harvard spectral class, the O, B, A, F, G, K, and M thingy, and by their Luminosity, their brightness. Or basically by their temperature versus their brightness. This is unsurprisingly a logarithmic scale since the plot goes from things thousands of times dimmer than our sun to thousands of times brighter. You get some noticeable clumps this way and one is a big thick line we call the main sequence. Most stars were on that main sequence, and the name just stuck, basically as a way of saying that star was normal. As we learned more we added to it, but in the basic original system the biggest brightest stars were type 1, supergiants, and this could be a very massive hot star or a large, cooler, and lower mass red supergiant. A star’s total brightness is based on its temperature and diameter. Double the diameter, quadruple the brightness or luminosity. But for temperature it goes up even quicker, double temperature and you increase luminosity sixteen-fold. Some stars are very hot and tiny, like white dwarfs, other are huge and cool, like red giants. The Harvard scale tells us temperature, but Yerkes doesn’t tell us a specific dimension like age or width or brightness, it tell us which area of the HR diagram the star is in. It’s more like a map location, like continents or chains of islands. They’ve just got numbers and names, not just names like North America or Asia. Astronomers usually say the names and write the numbers. Now there are reasons why the stars are in these groups rather than randomly and evenly distributed, and it mostly relates to how old they are, in terms of how long they will live. Stars burn through their hydrogen at a rate based on their mass. Only the very smallest stars actually get to use all that hydrogen since they are fully convective, which basically just means stuff near the surface can float down to the core and stuff in the core where fusion is going on can drift up to the top. The smallest stars are fully convective, one just a bit bigger actually have a big drop off in this, and it rises again when you get to stars in our sun’s mass range, but is still pretty low. But the bigger stars don’t live nearly as long, some live only a few million years, while the smallest stars can live trillions of years. You probably already know that and you might even be familiar with a rule of thumb that let’s you determine a star’s lifetime based off its mass. Most folks think a main sequence star when it runs out fuel becomes a red giant but there’s more to it than that. For instance our sun gets brighter every year, about 1% brighter every hundred million years. I’ve never heard anyone phrase it this way before and it’s probably from my own big focus on megastructures and Dyson Spheres but the sun’s brightness increases every five years by about what is needed to illuminate the planet Earth. So if you are building a giant dyson swarm around your sun you can add about a planet’s worth of new living area every five years, or a space the size of the US or Europe every month or so. Eventually it will run out of hydrogen in its core, just the core though, if our sun used all its hydrogen it would live closer to a hundred billion years. Many stars, as they begin getting to the end of the main sequence, growing brighter, have a sub-giant phase, class IV on the Yerkes system. This is generally a relatively short phase though its exact characteristics depend on the star’s mass, but basically the star just gets decently brighter, noticeably more than it was already doing just from aging. All stars have a main sequence phase, their main phase of their life essentially, and will pass through a subgiant phase as well before becoming a giant, though the lightest stars never become giants. At least we think so, the fact of the matter is that virtually no stars less massive than our own sun have ever left the main sequence. The Universe isn’t old enough for them to have even if they were among the first to form 13 billion years ago. No M or K class main sequence star, red or orange dwarf stars, has ever died of old age, and they are the supermajority of stars. The only stars to leave the main sequence yet are ones more massive than our own sun or those just a little bit less massive which also formed way back near the beginning of time. Most stars are red or orange dwarfs and partially because they have longevity, big stars don’t live long. Anyway we always have to be a bit speculative when talking about phases for things less massive than our own sun since even though that is most stars the universe just isn’t old enough for us to have actually witnessed what happens to them when they leave the main sequence. As I said though, current thinking is the very lightest of stars don’t even have a giant phase. Leaving the main sequence, and going through type 4, subgiant, they arrive at normal red giant phase, type 3. Now here we have another example of bad terminology. A red giant is always an old star, one that left the main sequence and expanded and cooled. It might have been a G or F or A type star beforehand but it is an M-type now, and it has expanded so much it can have enveloped its inner planets and even a close binary neighbor. But we often use the term giant to refer to very large main sequence stars too, like a blue giant, and this can get kinda of confusing. Also, amusingly, while we think of things getting swallowed and vaporized when this happens those planets or stars would just keep orbiting inside it, albeit that orbit would decay and those planets would be roasted, but slowly roasted. This is because while the red giant is quite hot, it’s thinner than air. So properly insulated you probably could dive a spaceship into one for a while. Type II are the bright giants, and these can come in any color, any letter on the Harvard Class, O through M, blue through red. A Blue Bright giant would be a main sequence star while a red bright giant would be a dying star that had been fairly large to begin with. When stars leave the main sequence they don’t just expand overnight or contract afterwards, with the exception of the really big ones that outright explode, which is not an instant process either but the timelines are a lot shorter. The next category, type 1, are supergiants and again these can run the whole spectra, but they are inevitably a very massive star on the main sequence or a pretty massive star near the end of its life. We also further subdivide this category by how bright they are and you will sometimes see a type 0 called a hypergiant. It’s not that there are so many of these stars out there that we actually needed a new class, it’s just that we can see them so far away that they are pretty easy to catalog and they are often yellow, because stars that massive have stability issues that tend to make them physically larger and cooler than we’d expect. So it isn’t that color, or temperature, of a star doesn’t tell you a lot about that star, it's just that unless you are an astronomer you are almost always better off looking at stars using the Harvard plus Yerkes notation for that little V, the roman numeral 5, and if you see that you know it is a regular old hydrogen burner in the prime of its life, and the letter than tells you its mass based off the temperature or color. If you don’t see that V, or 5, at the end, you will be safe assuming it is either dying, huge, or weird, or some combination of all 3. But we talked about dying, and we’ll talk about it more in a bit, let’s talk about star formation for a bit. Stars tend to form out of nebulae, big dense pockets of gas that generally spawn a whole bunch of stars when something comes by and pushes on that gas or otherwise sets off a wave of star formation. These stars won’t all form exactly simultaneously but on astronomical timelines they more or less do, and they generally will stay together, occasionally having one meander off or a migrant join for a while or even permanently. These clusters and groups of stars tend to be about the same age but not the same mass. Stars tend to form in clumps and be of the same age, and we often refer to these ages as populations. Another weird bit of terminology, we have population 1 and 2 stars and even added 3 in recent years, and this actually refers to their metal content, but its called a population because the stars in a group tend to be of the same age and so you’d be looking at a big group, or population, of stars and trying to determine their age by how much metal they had in them. Metallicity in astronomy refers to any element besides hydrogen and helium in a star, not just things like iron or copper but stuff like carbon and oxygen too. As you know the heavier elements mostly come from stars that died making them and sprayed them out into the cosmos, so the oldest stars, ones that formed long ago, have a lot less of these metals in them while the ones that formed more recently have a lot more. Population 1 stars, or metal rich stars, are younger stars, and our sun is one. Population 2 are metal poor stars, meaning they are old, and population 3 is ultra-poor, and these are the very oldest stars, ones that formed near the beginning of things. Now metal content doesn’t just tell us about age, higher metallicity star are cooler than equal mass stars with lower metallicity, because it’s also got those metals down in its hydrogen burning core interfering with fusion. Which also make them live longer though as I said in the starlifitng episode if you want to extend a star’s lifetime you’d just remove mass from it, even those metals, since they extend lifetime but their sheer presence still adds to the mass and gravity of that star to increase fusion, just not as much as hydrogen would, and it would only matter if the star was on the convective side. Okay so we have our picture now of the main types of stars, so let’s talk about some of the abnormal ones. On the smaller side we have type-L, in this case because it is the letter right before M. These are hot brown dwarf stars, and they would give off a little bit of light, brown dwarfs are not brown, but would look a dim orange-red or magenta. Next down are T-type, and I’ve never found out the origin of that one, but these are Methane Dwarfs. Not because they are made of all Methane, but because they have some. You do not get a lot of molecules forming on the surfaces of stars, from the heat, but T-type are quite cool. Compared to other stars anyway, they are still warm enough to melt lead. The last new type, Y-class, are called Ultra-Cool Brown Dwarfs. They are not hot enough to melt lead, though still quite hot. I have no idea where the Y comes from either, but these are cool enough that some might even have water clouds in their atmospheres. Brown dwarf stars, the smallest stars and arguably not stars at all, can be anywhere between 13 and 80 Jupiter Masses, or about 4000 to 27,000 times more massive than Earth or about 0.13% to 0.8% of the Sun’s mass. In spite of this almost all of them are about the same size as Jupiter. They just keep getting denser and denser as they get more massive until they are massive enough for fusion. Once they get over 80 Jupiter Masses they become the dimmest of red dwarf stars, or M9’s. Hypothetically they can burn deuterium for a few million years, as it takes less effort to fuse, but there wouldn’t be a lot of it. If you’re in the star making business you could probably make rather smalls stars made only out of it deuterium but in nature it occurs at part per million. We don’t know just yet how common brown dwarf stars are, they definitely are not uncommon but they could outnumber all other stars. People wonder about living around a brown dwarf, like if a large moon could be habitable. The answer is probably yes since you could have a lot of heating from both tidal forces on the moon, something we discussed back in the Habitable Planets series, and from some light off the brown dwarf, even if virtually all of it was in the infrared range. We also have, back on the other side of the size scale, Class W, for Wolf-Rayet Stars, these are so big and huge that you can see them thousands of light years away with your naked eye. We used to think no star could form above a certain mass, because we didn’t observe any beyond a certain luminosity, known as the Eddington Limit. But we noticed some of the brightest acted a bit weird. Their spectra wasn’t right, too much helium and nitrogen and carbon. We realized these were a lot more massive than their luminosity indicated, and we have known of them for a long time, they are very uncommon but so bright we can actually see two of them with the naked eye. One of them, Gamma Velorum, is one of the brightest stars in the night sky, but they’re both in the southern hemisphere unlike 90% of my audience. What happens with these stars is that they are too big to be stable. They have very high solar winds and regularly belch off swathes of matter. That’s why they seem to have too much helium, carbon, and nitrogen, their outer layers get flung off. You will also hear a lot about Variable Stars, particularly Cepheid Variables. A variable star is just one whose brightness varies, and there are tons of types. These come in two major categories though, intrinsic and extrinsic, extrinsic variable stars have something causing us to see its luminosity change. Intrinsically variable stars have something about the star actually altering its luminosity, which is true of every star but we use the term for big changes, like the star swells in size then shrinks. There are dozens of classes of variables but the Cepheid, named for the first we noticed this with, the star Delta Cephei, is historically significant. Cepheids expand and contract in a very regular way and are big bright stars, so they were instrumental for two big things. First, Edwin Hubble used them to show that distant nebulae were actually galaxies, something that was already proposed but they settled the matter for good, and second they were key to the original determination that the Universe was expanding, again something Hubble found. There are too many of these to list right now and most are important just for astronomy so I’ll bypass the others, but that’s what variable stars are and why they are so important. Time to move onto dead stars, the three popularly known ones are white dwarfs, neutron stars, and black holes but we have quite a few other including some hypothetical ones like Iron stars, so this will spill over into our final category. White Dwarfs are the most common fate of a star, anything less massive than 8 times our sun’s mass end up as one, which means virtually all stars. Not only are stars bigger than that less common in the sky because they are short-lived, they also don’t form as often as smaller ones. Also, as mentioned earlier, the only white dwarfs around are those from stars as massive as our own or bigger so exact behavior of the ones that will come as less massive stars die in the future is still a bit speculative. In the future we would probably have white dwarfs composed mostly of helium from stars that were not massive enough to fuse helium into carbon and oxygen and other elements. Your average white Dwarf is about the size of Earth because it is a leftover stellar core of something that blew all it outer shell away when it went into final helium burning modes. We wouldn’t expect ones actually composed of just helium to be quite the same. Now when a red giant occurs you basically get a crunch in the center as the core starts burning helium or the inner shell burns hydrogen, when that is all done that big thin cloud that makes up virtually all the volume and a fair chunk of the mass turns into a Planetary Nebula. The rest settles down as a dead but super dense and super hot planet sized white dwarf that is slowly cooling. Eventually it would become a red dwarf again for instance, in terms of being tiny and red, and eventually a black dwarf, one too cold to radiate light. The universe isn’t old enough for either of those yet either. In the meantime it is a super-hot super-dense thing you wouldn't want to dump hydrogen on because it can cause a nova. Just a regular old nova not the super kind. Nova being latin for new, a nova star, or new star, sometimes pops up in the night sky from a place where we couldn’t see a star before because it was too dim. Novae are quite bright for a little while and a lot more common than a supernova. Unlike supernovae that happen maybe a couple times a century in our galaxy, regular novae happen a couple times a month, but they are not so bright they you can see them across the galaxy. Still one happens close enough to make a new star visible to the naked eye every few years. Novae can vary a lot in brightness, since it just depends on how much hydrogen lands on them, and they can happen multiple times to the same star. Now a supernova is a more extreme event and can happen two ways, the first you know, it is when a really big star dies, but the more common kind is when a white dwarf absorbs enough mass to cross a threshold, about half again what the Sun masses, and it is too much for even the electron degenerate matter to handle and the white dwarf simply explodes. We are not quite sure as to the exact process but we think that when it crosses that mass boundary it’s enough to ignite carbon and oxygen fusion and get some convection going on to stir the star, and that after about a thousand years that results in a detonation, ripping the white dwarf apart rather than just causing it to collapse into a neutron star. In any event that is a Type 1A Supernova. Now not all Type 1a Supernovae tear the star apart, sometimes, we think, they leave behind a remnant called a Zombie Star, one that might have about half the mass left over. This would be a lot like a white dwarf, just a less massive one, but still basically a white dwarf. A Superhot super-dense planet-sized object that is going to slowly cool off since it no longer has any fusion going on. You could conceivably live around one of these but they are a lot dimmer than normal stars and keep getting dimmer and dimmer. Incidentally, Supernova Types 1b and 1c involve very large stars too, like the Type 2’s, the Wolf-Rayet stars I mentioned earlier, so type 1a, the most common type, are the only ones involving white dwarf stars. There used to just be types 1 and 2, and type 1 meant we didn’t see any hydrogen in the explosion, it all got fused or scattered long before. Type 2 does have hydrogen in it. These are the classic supernova you are most familiar with, big stars that explode and leave behind a neutron star or a black hole. These occur in stars heavy enough to ignite more fusion, and the highest element you can form this way is iron so the very biggest stars end as supernova when their core turns into iron, these then get imploded by the supernova into neutrons stars. We estimate there are 100 million to one billion neutron stars in our galaxy out of many hundreds of billion of stars, but many would be old and have cooled down. They’re quite small, no bigger than a small moon or large mountain, so they only emit a lot of light when they are very young and hot. They also tend to rotate very quickly, having kept most of the angular momentum of the parent star but squeezed in tight like when an ice skater twirling around bring their arms in and spins faster. So they rotate several times a second, or even hundreds of times a second, unlike most stars which rotate in days or months. This causes beams of electromagnetic radiation to come off them which we can see if we’re at the right angle and when they are young and hot. Those ones are called Pulsars, we’ve found about 2000 in our galaxy, and there are presumably at least ten times as many that are just at the wrong angle for us to see, but a Pulsar is just a young neutron star titled at the right angle relative to us. Now as you know a black hole is hypothesized to be what you get when an even bigger star goes off and we have talked about them a lot before in other videos so we’ll skim it for now and slide into hypothetical stars because many of them are thought to be intermediary stages between neutron stars and black holes. Neutron stars get their name because they are made up of nothing but neutrons. White dwarfs are under so much gravity and pressure, without any fusion to push back, that you can’t really have atoms anymore, just a sea of atomic nuclei and electrons, electron-degenerate matter, it’s too crunched up for electrons to wrap around nuclei and space atoms out. Neutrons stars are so dense you can't even have protons and electrons anymore, just neutrons. A neutron left to itself usually decays into a proton and electron, or their antiparticles, this is essentially the reverse process. But protons and neutrons are both made up of quarks, and under enough pressure it might be that even neutrons can’t exist anymore and you get a big mess of quarks. Or a quark star. This might be a short phase right before a black hole forms or maybe a stable one where inside a neutron star is a core of raw quarks. Probably Strange Quarks too, rather than the up and down quarks that make up most matter. We do have a few neutron stars that seem a bit too dense, and it’s been suggested they might be quark stars, or have a quark core at least. They don’t necessarily drop into black holes after that either. We still have four more possible steps. One option is the Electroweak Star, named for the unified electromagnetic and weak nuclear forces that occur at high enough energies. Here the collapsing neutron or quark star would have a super-dense core about the size of your fist with a couple planet’s worth of mass jammed in, and the rest all around it still at the lower but still absurdly high densities of neutron or quark stars. Gravity is too strong for even quarks to exist at that core and those quarks are being converted into leptons… electrons are a type of lepton, they have no quarks. This energy keeps the star from collapsing further and it’s been suggested this could go on for millions of years before the jig is up. From the outside it would still look like a neutron or quark star presumably. Another option is a Boson Star. Bosons are particles that can be in the same place as other particles. Fermions, what regular matter is made out of, cannot be in the exact same place as another fermion. Quarks, protons, neutrons, and even leptons like the electron are fermions and cannot occupy the exact same place as another fermion. A boson can, and photons, the particle of light, are an example of a boson. Considering a radio wave, or radio photon, can be bigger than your house, it would be kind of hard to listen to the radio otherwise. One of the candidates for Dark Matter, which we discussed in its own episode, is the Axion, and it has been suggested it could be a type of boson that could form this sort of star or even start a star off this way if it were made back in the Big Bang. Boson Stars are also a candidate for Dark Matter, though I skipped them in that episode for here. We also have other options for Dark Matter stars, or Dark Stars, which we’ll get to in a moment. Another option is a Preon Star, Preons are another hypothetical particle, originally suggested to be what quarks and leptons were composed of. There’s no evidence they exist but there’s always an understandable desire for some most basic particle all the others are built out of and Preons were one of the more popular ones, much like Strings from String Theory. Like Boson stars they are also a candidate for dark matter and could have been formed in the Big Bang, or in a supernova. Both of these are much denser than even a neutron or quark star. The densest though is the Planck Star, and these are not a last step before a black hole but thought to be what might be at the center of a black hole. We don’t know that a black hole is actually point-like, and I mean the mass at the middle, the actual black hole is just the region around it where gravity is too high for light to escape, that rises linearly with mass, double the mass, double the diameter of that event horizon, but we don’t know that matter keeps compacting after that until it simply becomes an tiny dot of zero width. A Planck Star is one option for the core of a black hole, and they are insanely dense. About 10^97 kilograms per cubic meter, which for scale, the whole observable universe is only about 10^52 kilograms so if squeezed down to Planck Density it would fit into single atomic nucleus, and remember an atomic nuclei is very small compared to an atom, like the Sun is to the solar system. A Planck Star would be similarly tiny compared to an atomic nuclei, but with a whole star’s mass crammed into them. Needless to say these are also a candidate for dark matter though again the key notion is that they might be the actual center of a black hole. Now the last two we are going to look at are not dead stars, although one is probably an extinct class that no longer exists and the other is a kind that won’t exist until everything else is dead. Dark Stars, stars made entirely of dark matter, are considered something that might have existed early on in the Universe when things were more packed together. Depending on how dark matter actually behaves you could get clumps of it mixed with normal matter that prevented that matter from achieving the density necessary for fusion. Such stars would radiate very little light even though they could be bigger than our solar system. We talked about these more in the Dark Matter episode but they are hypothetical and almost certainly no longer exist. Our last is Iron Stars, and they are the ones we’d probably focus on if I ever do a sequel to the Civilizations at the End of Time video that focused on Black Holes. The Star Forming era of the Universe will end in about a hundred trillions years, we’re about 1% of 1% of the way through it. We talked in that episode about how potentially that might be just the beginning of real civilization as expansion drives cooling that allows hyper-efficient computing. But that era lasted at most 10^100 years, just a trillion, trillion, trillion, trillion, trillion, trillion, trillion, times longer than the Stellar Phase of which we are again only a percent of a percent through. Some folks asked me if things could keep going after that and there are a few options. For matter that hasn’t been swallowed into a black hole during all that time, say an old white dwarf or neutron star left on its own, and assuming protons do not decay, given enough time, and we are talking timelines that make the entire black hole era seem as short to it as a second is to that era, you can have cold fusion occur. The insane improbability of a cold fusion event occurring by quantum tunneling is one of the reasons physicists roll their eyes at this or that cold fusion device. It’s easier at super-cold temperatures but it is still so improbable that to convert an entire stellar mass object into iron would take timelines only meaningful in scientific notation. 10^1500 years. That’s a ten followed by an entire page of zeroes. As mentioned earlier, iron is the heaviest element formed by fusion so eventually, given a long enough time, and assuming protons don’t decay, everything should end as iron. Of course that isn’t necessarily the end of the universe but we will save that for another time, I think. Iron stars are the last of our hypothetical stars both for the Universe and this video. Speaking of future episodes, this episode was originally planned for next week, as you may know we were going to do Colonizing the Outer Solar System this week, but Fraser Cain, the Publisher of Universe Today, offered to collaborate on a video. We both latched onto discussing colonizing the outer, and inner, solar system and while at first we thought we could get it done by the original planned air date we decided to give ourselves a bit more breathing room, so that is next week. And I suspect we will both be doing a lot of more in-depth looks at that topic so make sure to subscribe to my channel, and his as well, for updates when that and other videos come out. As mentioned this episode was also the third Patreon winner we had, the topic being selected by Neo Navras, our prior two being the Spaceship Propulsion Compendium selected by Drew McTygue and the second being Starlifting with Bill Mains. They were all great picks and I really enjoyed the process, Drew and I spent a couple hours on the phone bouncing ideas around for instance, so it is definitely something we will be continuing though I still haven't decided exactly how. Needless to say I am very grateful to the channels Patreon supporters for paying the bills for the channel and if you’d like to become one there’s a link to the channel’s patreon account in the episode description. Again next week is Colonizing the Solar System. In the meantime you can try out some of the other episodes on this channel. If you enjoyed this episode, don’t forget to hit the like button and share it with others, and until next time, thanks for watching, and have a great day!
Info
Channel: Isaac Arthur
Views: 218,772
Rating: 4.9286213 out of 5
Keywords: Star, Stellar, nova, neutron star, zombie star, supernova, white dwarf, preon star, quark star, dark star, iron star, pulsar
Id: o6EtvSvQp4s
Channel Id: undefined
Length: 40min 31sec (2431 seconds)
Published: Thu Nov 10 2016
Reddit Comments

If you're interested in this kind of stuff I would highly recommend checking out Artifexian on YouTube for world building videos. He did a couple recent ones on climate that were really helpful for both gaming and writing.

👍︎︎ 3 👤︎︎ u/thelastoneusaw 📅︎︎ Aug 31 2019 🗫︎ replies
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