Image credit: NASA/JPL
What do brown dwarfs teach about stars? What do they teach us about planets? What keeps them warm, and how long do they live? And are they really brown? I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
So I was putting together my notes today for this episode, which is all about brown dwarfs, by the way. And I was reading journal articles and news stories, all the usual stuff to make an episode, and one word kept popping into my mind over and over and over and over again. That word is weird. Brown dwarfs are just weird. The short version of brown dwarfs and we're about to get into the long version.
Trust me. The short version is they're too big to be a planet, but too small to be a star. And you might think that such a thing wouldn't even exist. That's like the first weird thing is that brown dwarfs are a category unto themselves. You might think you have planets over here and you have stars over there.
And if you start making a planet bigger, you would just have a large planet. And at some point, surprise, instead of a planet, you have a star. You just have a small star and then you go up to start building big stars. But, no. Sadly, nature is much more complicated than we would prefer or maybe we do prefer because then we get to keep having jobs as astrophysicist.
But either way, brown dwarfs are indeed a thing all on their own. And not only are they their own identity, their own class of astronomical object, they have their own subclassifications because, man, just one category just isn't confusing enough. So we need to admit it. We get need to get out front. We might argue about the nature of brown dwarfs.
We might try to lump them in as planets or lump them in as stars, but they're not. They're neither. They aren't going away and they are weird. In this weirdness, this weird nature of brown dwarfs comes from a question, a couple questions. We have these things, these objects that sit in between the size range of planets and stars.
They're bigger than planets and or and also smaller than stars. Is it best to think of a brown dwarf as a failed star, as a wannabe star that couldn't quite make it? Or is it best to look at brown dwarfs as super planets, as really gigantic, highly involved, meta consciousness, you know, the the the best kind of planet there is? Or maybe both. Maybe neither.
Let's spend the rest of the episode looking upon the weirdness that is Brown Dwarfs. And as they say, if you stare into the weird, the weird stares back at you. First, a bit about the weird name. Kind of an awkward name, brown dwarfs. The the word dwarf itself is tossed around a lot in astronomy.
You have dwarf planets and dwarf stars. You have white dwarfs, and now we're talking about brown dwarfs. Heck, even our own sun is classified as a dwarf. Even though the last time I checked, it is somewhat large. As stars go, it's classified as a dwarf because it's not as big as the biggest stars, which are called giants.
In astronomy, if you're not a giant, you're a dwarf. Once again, I don't make the rules here. I just report them to you. But labeling brown dwarfs as a dwarf already sets us down the road of thinking of them as stars or as failed stars when we just as easily could have called them giants. Giant planets.
Oh, wait. Giant planet is already taken. That's what we call the gas giants. Shoot. How about how about how about supermassive planet?
That sounds kinda cool. Supermassive planet. Let's skip brown dwarf. Let's call them supermassive planets. But but they can't be planets because they're free floating.
They're they're not orbiting other stars. Planets orbit other stars. Oh, wait. Wait. Wait.
Wait. But but some planets are free floating. We call these the rogue planets. So they could be rogue super massive planets. But and and some brown dwarfs do orbit stars like planets do.
But, confused? Yeah. Same air. They're weird. They're weird.
In the end, dwarf, quote, unquote, dwarf is just a label that we're, a, going to have to live with and, b, move past. This is going to sound strange, but I don't want us to let the definition define the object. I I feel like there's some sort of self help motivational quote somewhere in there, but I'll let you dig it out and post it on the website. So we're just it's dwarf. It's just a court we'll we'll just forget any kind of connotations or denotations, definitions.
It's just it's just a name. It's just name. We could just call them Steve or Jessica for for all that matters. That's the dwarf part of the name. What about the brown?
Well, as we'll see in a bit, they are very much not brown. In fact, they're basically everything but brown. But when they are first theorized to exist in the nineteen sixties, they were actually originally conceived of as black dwarfs, as stars that are too small to glow. Just chunks of gas that are hanging out really cold, really dark. But black dwarf, that term was already taken.
That's what happens when a white dwarf cools down in the far distant future. We're talking tens of billions of years from now. And even though no black dwarfs exist in the universe today because the universe hasn't been around long enough, the white dwarfs claim dibs on the name black dwarf and so after a couple decades, the astronomical community settled on brown dwarf because it's kinda like black, only less so. So brown dwarfs are not hot enough to be white dwarfs. White dwarfs are really glowing and they're really intense.
And they're definitely warmer than black, so maybe we should call them gray dwarfs. No. No. We're just gonna call them brown dwarfs. Why?
Because we said so. That's why. They also have a weird size. Size. I should probably mention that these things, these brown dwarfs, have the mass of a few tens of Jupiters.
And I'll get to the exact limits later. But I want you to think about, say, twenty, thirty, 40, 50 times the mass of Jupiter. So pretty, pretty heavy. And you would think that something this massive would be significantly larger than Jupiter. If you start piling junk onto Jupiter, it's should get big.
It should get fat. It shouldn't just be massive. It should be large. And you might be tempted to think that the most massive brown dwarfs, the biggest of the brown dwarf category would only be slightly smaller than the least massive of the actual stars. And if you notice stars, even the smaller ones are much much larger than Jupiter, much wider, much more volumous.
And you would think the biggest brown dwarfs would be, like, right up you'd you'd expect there to be, like, a continuum of just big, big, big planets, then transitioning to small brown dwarfs, then big brown dwarfs, and then small stars, and then big stars. But they're not. They're weird. Think of an object 50 times the mass of Jupiter. 50 times the mass of Jupiter and barely bigger than that planet at all.
Imagine all that stuff crammed into the volume of Jupiter. Brown dwarfs aren't much larger than planets. And the reason for this is their formation of how they got started and why they get the nickname failed star. I haven't done a full episode on star formation. Feel free to ask.
But in general, broad brush, stars form from the collapse of of big clouds of gas and dust. You take a cloud of gas and dust. It's hanging out. Some piece of it, some chunk of it starts collapsing under its own gravity and squishes down into a smaller and smaller volume. And, eventually, out pops a star.
But collapsing, as that cloud of gas and dust collapses, it releases energy in the form of heat. And you need to release that heat to shrink further. Otherwise, it'll just stay hot and being large, if you wanna shrink further, you have to cool off. You have to condense. Stars that are forming do it by pumping out lots of light.
They they get rid of their excess heat, their excess energy through the form of radiation, through the form of light, and this allows them to shrink down into a smaller volume. But eventually, their cores get so dense that it's too thick for the radiation to to get out, so it gets trapped. And as the further layers keep piling and piling and piling on around that core, the core's temperature and pressures rise up to a critical threshold to to trigger nuclear fusion, to start shoving together hydrogen and turning it into helium. And once the nuclear fusion party starts, it really is party time inside the star. They actually inflate because of all the incredible energies being released in the core of a star.
This pushes out. It wants to blow up a star. It is a nuclear bomb going on off inside of a star constantly. It wants to blow it up, but its gravity tries to squeeze it back together. It so it puffs up into an equilibrium.
Brown dwarfs are too small. They don't get that hydrogen to helium fusion party, so they just keep collapsing. They just keep shrinking. There's nothing to fight it. All the way down, all the way down, they keep compressing layer upon layer of material until they hit a wall.
They hit a wall provided by electron degeneracy pressure. I've did a whole episode on it. Feel free to listen to that. But in in general, again, broad brush, it's this weird quantum mechanical quirk of nature that says you can only fit so many electrons into a box, which at first seems obvious, but it's actually really deep. So listen to that old episode to get all the all the juicy gory details on it.
You can only squeeze so many electrons in a box. So if you have a big cloud of gas and dust, which contains a lot of electrons, you can only squeeze it down so much until the electrons just say, no. That's enough. Too much pressure. Back off, man.
Jupiter, our planet Jupiter, is also partially supported by electron pressure in its atmosphere, but it also has a solid rocky core, possibly a thick mantle. There's other layers inside of Jupiter. Brown dwarfs are different. They are entirely supported by degeneracy pressure. Entirely.
They don't have layers. They don't have cores. They don't have mantles. They just have degeneracy pressure. And this is what allows them to compact so much stuff into so little volume.
At the end of the day, brown dwarfs are both heavyweights, but paradoxically tiny. Weird. I mentioned way earlier that there are different masses of brown dwarfs. There are ranges of brown dwarf mass. Yeah.
10 times, 20 times, 30 times, 50 times, etcetera, the mass of Jupiter, which means there are different kinds. And, of course, these different kinds, like I said, have a weird classification scheme. There are four classifications of brown dwarfs. And are you ready for them? Probably not, but here we go.
Class m, class l, class t, and class y. I'll repeat that. Four different varieties, four flavors of brown dwarfs, m, l, t, and y. Listen. I need to do an entire episode on how astronomers classify things and why we end up with horrible names for everything.
Feel free to ask. Here's the short version. It's mostly because classifications are based on how they look, as in through a telescope. And in the early twentieth century, when giant telescopes were really popular, our out our observational capabilities far outpaced our theoretical understanding. We were bagging and tagging all this cool stuff in the sky without really understanding what was going on.
And so astronomers started classifying things before having an understanding. And when you do that, you end up with schemes that have to be rearranged and modified to fit the new understanding that you eventually have, and then history and his is history and you're stuck. When we look at stars, there's a variety of star colors you might see, a bluish, whitish, reddish. The reddest ones are called class m because, like I said, reasons. Some brown dwarfs are actually so red.
They're red enough to compete with the color of the reddest stars, so they glow enough in the red to compete with the redness of the dimmest stars, those red very, very red stars. And so they also get the label m. So if you look at an astronomical object and it is classified as m class, you don't quite know if you're looking at a small red star or a large brown dwarf. But some brown dwarfs are less red or, in America speaking, more red. What's wetter than red?
It's it's infrared. It's beyond the visible. And some brown dwarfs are so deeply red that they are infrared, and so we give them a new classification. And the letter l is next to m, and so there's your second category. But some brown dwarfs are redder still so deeply in the infrared that they hardly even glow at all in the visible wavelengths.
They are not glowing at all invisible, but if you could look at them with infrared goggles, they would glow like embers, but we can't see them very well visible light. We're gonna call these very red, deeply red, deeply infrared characters class t because we said so. And then there's one last category, the even redder. The so deep into the infrared, you they hardly even count as visible light objects. We're gonna call these.
Why? Because shut up. In stars and in red dwarfs, there is a connection between color and temperature. The color of a star indicates the temperature of the star. And usually, there's a connection between the temperature and the size.
There are, of course, exceptions. The biggest stars in our universe are super hot, so they look very blue. The medium stars, blue being the the most energetic kind of light in the visible wavelengths, the medium stars are just kind of hot, say 10,000 degrees Fahrenheit, 6,000 Kelvin, and those look white, which is a balance of all the colors being emitted equally. The smallest stars are not very hot at all, and so they look red. The brown dwarfs are less hot still, so they're down there at low energy infrared.
How, not hot are we talking about here? Well, you. You. That's right. You.
I'm talking to you. No. No. No. Not not not you.
Next next next there there. You. You are emitting infrared radiation right now. That's how night vision goggles work. They look for the glow that we all naturally give off.
And we have a temperature of, what, a hundred degrees Fahrenheit, three eight Celsius rounding. So if a star is glowing with the same infrared colors as a human being, you can go swimming in it. It's room temperature. It's comfortable. It's a nice day at the pool.
I mean, yeah, the gravity is crushing strong and you can't breathe the air, but at least it would feel nice as you asphyxiated and fell into the core and crushed got crushed to your death. But it would feel nice for a little bit because the temperatures aren't so bad. Most brown dwarfs are a bit hotter than that. We're talking hundreds of degrees, but as space like objects go, that's not very impressive at all. They're not very bright at all because, one, they they don't have a big temperature.
They're not glowing a lot, and they're they don't have a big surface area, so there's not a lot of stuff to give off light. So they have something like one one hundred thousandth the brightness of the sun. Dim. They are dim. They weren't even first observed until 1994.
In order to find them, we have to use the exact same techniques that we use to find planets. That's how dim and dark these things are. The color they have is closer to magenta. It's It's closer to magenta. It's it's reddish.
It's reddish because it's mostly in the infrared, so you get some reds and but a little bit of other light. And, of course, that color can change depending on if there's any interesting chemical characters in the atmosphere of those brown dwarfs. But magenta is pretty a pretty weird color among space like objects. You know, it's not it's not something you see every day. The biggest brown dwarfs, by the way, are also the hottest brown dwarfs and the most bright invisible red.
And then the smaller ones are cooler, and they glow more in the infrared. But, of course, brown dwarfs can be born with all sorts of masses. Right? You might just pop out of your interstellar gas cloud as a 20 Jupiter mass object or a 60 Jupiter mass object. It depends on all sorts of cool factors.
So a brown dwarf might be born as a really hot visible m class or might be born as a dinky T class. But they all have a weird life cycle. They just after they're born, they just cool down and chill out ever so slowly over billions of years. That's that's it. They don't get to blow up.
They don't get to change different kinds to different kinds of nuclear reactions. They don't evolve. They don't have plate tectonics because there's no plates. They just hang out being brown dwarfs, not bothering nobody, not being bothered by nobody. As they cool off, as they age remember, they don't have sources of heat like a star does.
They were born with some amount of heat depending on their mass, and they slowly leak that heat out into the rest of the universe. And as they cool, they shrink and and become slightly more dense. So this life cycle that brown dwarfs have of starting relatively hot and then cooling off into not so hot brown dwarfs, How quickly that happens is depends on the mass of your birth. If you were born relatively large, then you'll take a long time to go through this cycle. You've got a lot of heat to dump off.
But if you're have a small birth mass, if if you're a little bit underweight, a little bit lightweight brown dwarf, then you will, emit radiation that much faster and you and you'll cool off much faster. The youngest, largest brown dwarfs that we have examples of, the biggest ones, the brightest ones, are barely distinguishable from actual stars, from the smallest stars. And I'll talk later about how we can make that actually distinguish between the two. But if you just took a picture, if you just took the picture, you may not be able to tell the difference between the biggest brown dwarfs and the smallest stars. And the oldest brown dwarfs, the smallest brown dwarfs are barely distinguishable from cold planets.
They're barely distinguishable. Like, you take a picture of a really, really, really old star or really small brown dwarf. Is that a brown dwarf, or is that just a planet? Hard to tell until we get a mass estimate. But just from a picture, you can't tell.
Just by looking at a brown dwarf, you don't know its mass. It could be born that way, baby. It could just be old. It might be confused with a small dim star. It might be confused with a very massive planet.
But either way, that's the life cycle of brown dwarf. It it's born, and it cools off. Just chilling. Kinda weird. Their outsides are weird.
Like, if you take a picture of brown dwarf, that's kind of a weird thing. Their insides are weird too. There's a fun jargon term here that I'm gonna use. A jargon term called fully convective. Fully convective.
Their their cores are pretty hot. There's lots of trapped heat. But their surfaces are cool. They're hot on one side and cold on the other. Well, they're hot on the inside and cold on the outside.
If you said, say, had a pot of water on the stove, it's hot on one side and cold on the other side. It's hot on the bottom, cold on the top. You get convection. You get pockets, parcels of fluid that heat up at the bottom. This causes them to expand and become buoyant.
They rise to the surface. They touch the top surface and they touch that cool air above it and that causes them to cool off and they condense more and they shrink back down. And this cycles up and down, up and down in your pot of water. This cycles in and out, in and out for a brown dwarf. It's almost like like a series of elevators inside of the of the star, inside of the brown dwarf.
You can land on the surface. You can't land, but but go with me here. You can land on the surface and you can ride one of these convection cells down to the core. Or and once you're in the core, you can hang out for a bit and then you can find a rising pocket of of fluid of gas that will take you back up to the surface. It is fully convective through its entire layer.
This is unlike most stars. Stars always have a core. Some stars have what's called a radiative layer and then a convective layer. Some stars just have a core and a convective layer. But either way, there is a difference.
You can't go all the way to the center of a star. You can't ride these elevators, and they don't go all the way down. But in a brown dwarf, they go all the way down. And it is unlike most planets. There is convection happening inside the mantle of the Earth.
It's a very slow process. But again, we have layers. Both stars and planets have layers. Brown dwarfs don't have layers. And so the surface of these brown dwarfs can be can be mottled with massive convection cells, almost like a like a gassy honeycomb of plumes of material rising to the surface and then slinking back.
And these plumes might take up a good fraction of the surface of this brown dwarf. So that that'll look weird. That'll look weird. Surface is weird. Interior is weird.
And the innermost portion of a brown dwarf is weirder still. Brown dwarfs are not stars because stars are defined as the things in space that fuse hydrogen as a part of their life cycle. If you want to be called a star, you must fuse hydrogen. Brown dwarfs are too small. They never get the temperatures and pressures up high enough to ignite hydrogen, but they're weird.
They can fuse, but they're not going to fuse hydrogen. They're going to fuse deuterium, which is just a single proton and a single neutron glued together In sufficiently high densities and temperatures, that deuterium can get smacked with an extra proton flying around. They get glued together forming something called helium three, and that process releases a little bit of energy. And the threshold needed to join the deuterium burning club, the deuterium fusion club, is less than the threshold needed to join the hydrogen burning club. You know, the entrance fees are a little bit more reasonable.
So brown dwarfs sometimes get to play the fusion game but just with deuterium. The smallest ones will only do it briefly, and then they'll quickly cool down. They won't have the critical temperatures and densities needed. And the biggest ones will do it briefly. Because they are fully convective, because there is constant recycling throughout their entire bodies of material, Any deuterium in a big enough brown dwarf where it can do this deuterium fusion thing will use up all of its deuterium in, like, ten million years, and then it's just back to being blase.
So some of them get to do something cool. Brown dwarfs might seem all quiet and boring, and they kinda are, but they can do some pretty weird and interesting stuff, like contribute to Patreon. That's right. Patreon.com/pmsutter is how you keep this show going. Thank you so much to all my loyal contributors.
If you have a few bucks to spare, I appreciate it. To keep all my education outreach activities going, that's patreon.com/pmsudder. To date, no brown dwarfs have ever contributed to my Patreon, but that's okay because they leave iTunes reviews and I appreciate that. Sometimes brown dwarfs can give off random blasts of x rays or power strong radio pulses. That's pretty weird.
How? Well, as long as they're rotating. And as long as they maintain at least a little bit of heat in the center where you can develop these convection cells and you can spin these convection cells around, You can use what we call dynamo mechanisms to build up strong magnetic fields. That's right. Brown dwarfs can carry magnetic fields, pretty decent ones too.
The exact same way our sun generates a magnetic field well, not the exact same way that the detailed physics are different, but the principles are the same. Our Earth can generate a magnetic field. Jupiter can generate a magnetic field, and a brown dwarf can generate a magnetic field. And if you twist up a lot of magnetic fields, if you take a bunch of magnetic fields, start wrapping them around each other, bundling them up tighter and tighter and tighter, you can build up a lot of tension. And this tension can be released like a snap, like an overstretched rubber band, and that release sends out tremendous amount of energies in the form of, say, x rays.
This is how solar flares happen on our sun. Too much magnetic field is being twisted into too tight of a configuration. They can't stand it and they they snap and release a bunch of energy. This can happen on brown dwarfs. So imagine encountering one of these objects in space alone, freely floating, an object not much bigger than Jupiter but many times heavier, its color a deep but dim red, its surface broken up into massive cells with ferocious arcs of lightning reaching across the surface, powerful enough to release x-ray flares.
That is weird. The brown dwarfs aren't stars, but they sure are born like them. Similar histories. They're they're formed from the collapse of bigger clouds. But why do they fail?
Why don't all objects make it as stars? Maybe there's just not enough stuff there in the first place. Maybe they get ejected from a forming stellar cluster and through complex gravitational interactions, they get kicked out before they could suck up enough gas to make a star. Maybe they get blasted by nearby young stars and get stripped of material. Kinda hard to tell.
We don't know a lot about how brown dwarfs form. There's a lot of open questions. Sometimes brown dwarfs get planets in orbit around them. Sometimes they are in orbit around real stars. We honestly don't know.
We don't know a lot about the formation history brown dwarf? Is there anything sitting between a planet and a brown dwarf? Is there anything sitting between a brown dwarf and a star? Or is it all three things, planet, brown dwarf, star? Is there anything else?
The smallest stars possible are around, 7% the mass of the sun, around. And you can use what's called the lithium test to pick out the brown dwarfs because some brown dwarfs might be bigger than that. Some stars might be smaller than that. And if you just have the picture, if you just see a red glowing object, you're not gonna know right away if it's a brown dwarf or a star. Stars, in addition to fusing hydrogen, also fuse lithium.
They they're sprinkled with just a little bit of lithium, and they can burn off this lithium. And as the star forms, it ends up using all its lithium. So a brown dwarf, if you're able to capture the light cap coming from a brown dwarf, you can look for the fingerprint of lithium in its atmosphere, in its body. If you spot lithium, then in that is probably a brown dwarf because it never had the chance to burn it off. What's the lower boundary?
So we have an upper boundary, like 7% the mass of the sun. We can use this lithium test to some most of the time pick out brown dwarfs from stars. But what's the bottom limit? Where's the transition line between planets and brown dwarfs? Currently, the definition is around 12 times the mass of Jupiter because we think that's the minimum threshold necessary to start deuterium fusion, which is that's a kind of loose definition because not all brown dwarfs fuse deuterium at all stages in their life cycle.
What if you encounter something like a 10 Jupiter mass object? Is that a brown dwarf? Is that a planet? There are some suggested names. I'll give you some of these suggestions just to see how deep this rabbit hole goes.
Sometimes they're called cluster planets. Sometimes they're called directly imaged gas giant planets, free floating planets, free floating planetary mass brown dwarf, interstellar planet, isolated extrasolar giant planet, isolated planetary mass object, nomad planet, orphan planet, playmo, a contraction of planetary mass object, planamo, same deal, planetar, which was originally coined for the name brown dwarf, but is now no one uses it, rogue planet, starless planet, sub brown dwarf, sunless planet, super Jupiter, wandering planet. In some cases, there are flowcharts involved, folks. The upper boundary of brown dwarf between stars seems it's a little bit fuzzy, but it's pretty much there. But the bottom boundary between the biggest planets and the smallest brown dwarfs is definitely fuzzy, and we don't know a lot about it.
So what are brown dwarfs? What box do we put them in in our insect display case of the universe? Are they stars that didn't quite make it? A d list celebrity, a rock band that never broke out from the bar scene in their hometown? Something that tried for greater things but didn't have what it takes?
Or are they super planets? Did they climb the corporate ladder and make fat paychecks as a high powered consultant? Maybe an artist that worked hard over decades and built an international reputation. Not a flashy flashy, glitzy, glamorous life, but doing way better than the rest of us. They're neither.
They're their own thing. They buck the trends. They reject social norms. They're not celebrities. They don't have a traditional career either.
They're the invisible kids in high school that you never paid attention to. And then twenty years later, you find out they're, I don't know, they're they're taking a sailboat around the world. You start to ask question, why are they doing that? How long have they been doing that? How do they even get a sailboat in the first place?
More questions than answers. Because brown dwarfs are their own thing. They are weird, and they are proud of it. Thank you so much to Ted w on email, Giselle s on email, and Ardent Defender on YouTube for asking the questions that led to today's episodes. Hey.
If you haven't heard of a movie called UFO, go to your favorite digital platform. It is a movie that I played a science consultant. I didn't play. I acted as the science consultant for this movie and it was great, a really fun experience. I do appear in one scene with David Straytherin.
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