If you get enough hydrogen together in one place, gravity pulls it together to the point that the temperature and pressures are enough for fusion to occur. This is a star. But what happens when you don’t have quite enough hydrogen? Then you get a failed star, like a gas giant planet or a brown dwarf.
Astronomy Cast episode 290 for Monday, January 21, 2013 – Failed Stars
Welcome to Astronomy Cast, our weekly facts based journey through the cosmos. Where we help you understand not only what we know, but how we know what we know. My name is Fraser Cain, I’m the publisher of Universe Today. With me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville.
Hi Pamela, how are you doing?
Pamela: I’m doing well, how are you doing Fraser?
Fraser: Good, did you notice I added that director of CosmoQuest?
Pamela: I did! That’s very exciting.
Fraser: Well we’ve been doing CosmoQuest for so long and we keep forgetting to include it in all the things that we do so for anyone that has never heard of CosmoQuest, what is it?
Pamela: It is an online research facility designed for the public so we work to bring anyone out there who is interested in becoming part of solar system and space exploration an opportunity to engage in the same ways that scientist do. We do science activities, we have weekly seminars and we have a whole range of different ways including forums. We wove in the 1:18… into CosmoQuest. We have a whole variety of ways for you to get involved and I hope you’ll take the time to check it out at www.cosmoquest.org.
Fraser: Yeah you can classify craters on the moon or search for icy objects in the solar system; really our goal is to try and help regular folk combine with scientist to do real science and this is what we’re doing.
Pamela: And we’re succeeding
Fraser: Absolutely, it’s awesome.
Pamela: I have one quick announcement, so sorry. We are in the process of phasing out the Astrogear store because while we love all of you, you don’t buy a lot of things. As we’re working to change out our staff, our wonderful Joe Ray has gone onto wonderful and better things than us and although we’re sad, we’re proud of him. He was the person running our store so we’ll continue to offer our t-shirts in the future but everything else we have is on close-out. If you want to buy things, now is when you should buy things. That’s www.astrogear.com.
Fraser: Alright… buy things… BUUUUUUY THIIIIIINGS. Now can we start the show?
Pamela: Yes, now we can start.
Fraser: If you get enough hydrogen together in one place, gravity pulls it together to the point that the temperature and pressures are enough for fusion to occur. This is a star. But what happens when you don’t have quite enough hydrogen? Then you get a failed star, like a gas giant planet or a brown dwarf. Today we’re going to talk about failed stars. You know failed stars are actually super common, maybe more common than regular stars? There are a lot of them out there.
Pamela: Yeah I don’t think we have enough statistics yet; that’s the crazy thing that we’ve only been finding these things since the 1980’s really and its only been with the 2MASS survey and a few others that we’ve really started to be able to find them in a meaningful way. We’re only finding them by the hundreds but we find red dwarfs by the bazillions basically (laughs).
Fraser: So let’s talk about the process of what it takes to make a star and that will sort of help us understand why things fail.
Pamela: We have entire shows on this, go back and listen to one of the shows on this. In short what happens is you have a giant molecular cloud of gas, dust and all of this material as the cloud gets shocked by something or gravitationally compressed by something. All of this gas begins to collapse and fragment and the individual fragments will begin spinning or sometimes it will split into multiple pieces and this is where binary stars come from. Some of those pieces just aren’t quite big enough to fuse hydrogen and that’s where we end up with failed stars. Where things get messy is where do baby planets come from? In this case you have a fragmenting, spinning chunk of molecular cloud and in its core you end up with a star forming and around that star will be a disk of material. That disk fragments into pieces that are orbiting around the primary star. Now when you have binary stars you end up with two collapsing, spinning bits and the non-disky bit is the star and you can actually end up with disks around both of those fragments that are forming the binary star. This can all get very complicated but the key component here is planets form in a disk of material through an accretion process whereas stars form via the fragmentation of molecular clouds and the collapse of those fragments into things that hopefully burn hydrogen.
Fraser: So really we define that star as that ability that enough mass has come together, enough is going on, that you have that fusion and the star ignites. Our sun, obviously, is one of these stars but they get a lot smaller right?
Pamela: They do
Fraser: So how small can you get when you still have a star? You still get a success ribbon?
Pamela: The cutoff, as near as we can tell, and we haven’t actually found the smallest possible star that you can have yet, but as near as we can tell from theory is between 80 and 85 times the mass of Jupiter. So at a certain point you stop using the sun as your point of comparison and start using Jupiter. Take Jupiter, multiply it by somewhere between 80 and 85 and (6:33) start fusing.
Fraser: If you were going to go the other way and look at, say, the sun, what percentage of the sun would it be… around 10%?
Pamela: So compared to the sun these are tiny objects. These are about 71/2 to 8% of the sun so tiny, tiny, tiny, stars.
Fraser: I always find the process of these red dwarfs really fascinating because they have no radiative zone, it’s all convective zone and the whole things is just churning it’s material and they last a really long time. They can keep the stellar fusion going.
Pamela: This is the red dwarfs that we’re talking about. They’re fully convective so just like with your lava lamp you see the blobs going toward the surface then going all the way back to the bottom. In red dwarfs you have the same process going on where there is nuclear fusion going on in the core, but then the hot material rises up to the surface fully circulating. When a red dwarf finally finishes the hydrogen process it’s pretty much used up everything that can be used up in the star.
Fraser: But it will last, even the small one, will last trillions of years.
Pamela: Yeah, these are the longest lived things in our galaxy.
Fraser: Totally. So that is sort of where we set our limits; anything above 7 1/2% of the sun. There’s really no 100 times the mass of the sun? That’s a big range. Obviously you’re going to end up with clumps of hydrogen coming together at smaller amounts than this 7 ½% of the sun so what do we call these?
Pamela: Those are where you start to get into the brown dwarf stars. These are objects that we define not just how they form but also how they sort of, kind of, but not very successfully for very long do have a fusion process in their core. Brown dwarf stars are objects that are 13 to 80-85 times the mass of Jupiter. At that cutoff they’re able to very briefly burn tritium and deuterium in their cores. These are heavy forms of hydrogen that have extra neutrons in their centers.
Fraser: Where does the extra hydrogen, the heavy forms of hydrogen come from?
Pamela: It’s just one of the components of the universe. If you look around the universe you’re going to find heavy hydrogen.
Fraser: Oh I see, so there is a certain percentage of just a blob hydrogen that’s going to have those elements in them.
Pamela: Yes, just like water. There is heavy water and we can find it in the ocean. It’s just part of the ocean where part of the H2O formed with a deuterium atom and instead of just straight hydrogen.
Fraser: So does this stuff just fall inside the star and clump together, or is it just a percentage of it that’s able to use?
Pamela: It’s just a percentage of it that is easy for it to use. Hydrogen doesn’t burn when it’s missing those extra neutrons nearly as easily as the heavier forms with the extra neutrons in it. Physics in play lets these stars more readily burn and doesn’t allow it to burn hydrogen that is missing these extra neutrons and unfortunately the heavier forms of hydrogen are much more rare.
Fraser: And so because it’s rare, it only a small percentage of the overall object that’s made up from this stuff, how much energy, how much heat, how much can it do?
Pamela: Well at the end of the day it’s only able to burn for only a few hundred million years. You have this fully convective little star that depending on how big it is, in some cases it can actually burn some lithium as well because lithium burns very easily. It’s only for a few hundred million years and once they’re done they’re done.
Fraser: How hot do they get?
Pamela: That’s the awesome thing is that these things are, during their normal observed state in some cases, actually human body temperature on their surface
Pamela: Yeah. We’re looking at stars that, in general, are less than 1000 degrees Kelvin.
Fraser: But way hotter the deeper you go? Jupiter is hotter right?
Pamela: Yeah, totally true but the fact that on their surface they get to be human temperature. Trying to figure out what to do to these has forced us to expand the way we look at stars. We normally have the O’s as the hottest, B, A, F, G. We’re one of those normal G type stars, K, M, red dwarfs and as we start adding new types they had to add an L class which start to have hydride bands and they start to have alkali metal bands and they had to go on to add T class stars. These are stars where we actually start to see carbon monoxide in the atmospheres of the stars. There is a handful of what we call Y type stars and these are stars where we start seeing things like absorption lines from ammonia. This actually made a much more polite and disturbing mnemonic for how we think of all of this
Fraser: Oh Be A Fine Girl Kissed Me
Pamela: Oh be a fine girl kissed me is the normal one that we’re used to but now we’ve added an L a T and a Y so it’s become Oh Be A Fine Girl Kiss Me Later, Thank You.
Fraser: Right so then is there this distinction between these brown dwarfs that are actively consuming and burning these heavier forms of hydrogen and the ones that have run out of fuel. Do astronomers make some kind of distinction between them?
Pamela: No and I honestly don’t know if we’ve observed any that we can specifically say “This one is currently undergoing nuclear reactions.” These are extremely rare objects in our current observational sets. I can’t tell you how rare or not rare they are in the sky but because we’re only starting to observe them we only have so many data points. They burn for such a short period of time that trying to catch one in our few hundred observations as actively burning, I don’t know if statistically we can say we should have done that with certainty yet.
Fraser: Is it one of those situations where it gets to its temperature and then it just takes a really long time to cool down? I know that we talk about stars that turn into white dwarfs then the white dwarfs will eventually turn into black dwarfs but that process is going to take billions and trillions of years for these stars to reach the background temperature of the universe.
Pamela: At the end of the day, these just don’t get that hot. They just don’t get that hot.
Fraser: But they’re still cooling down over long periods of time.
Pamela: They are but it’s not the same way you think of white dwarfs cooling off. With a white dwarf you’re starting out with something that is tens of thousands degrees Kelvin. When they cool off to a few hundred degrees Kelvin and become what we call black dwarfs, that’s a massive change. These guys start out at about a thousand degrees Kelvin and cool off to a few hundred degrees Kelvin so when you’re looking at something like that it’s a very different situation. These are stars that don’t work in the ways that we think of. The smallest of them like Jupiter are supported through normal gas pressure but the largest of them are supported just like white dwarfs through electron degeneracy pressure. Here you have something extremely small and fairly dense but not white dwarf dense. All of them are within 10-15% the same radius. Take Jupiter and add stuff to it and it doesn’t get bigger, it just gets denser. Keep adding stuff and it changes how it supports itself from gas pressure to electron degeneracy pressure. Their temperature doesn’t vary much across the entire range. These things just don’t behave in the way we’re normally used to thinking of stars because they’re not normal stars. They’re this weird transitional object.
Fraser: Okay so I guess the question that I wanted to ask next then was what is the method that astronomers use to find these objects because they aren’t bright and they aren’t shining.
Pamela: Infrared. It’s not just that they’re not bright; it’s that they’re not bright and they’re not really giving off light in useful wavelengths. It’s perfectly possible to detect a very very faint blue object or red object with a normal telescope. Big deal. They’re faint, they’re annoying… we can do it! Now brown dwarfs pose an entirely new challenge because they are so extraordinarily red that the bulk of their light is given off in wavelengths that aren’t readily observed with your normal optical telescope. You have to get above the earth’s atmosphere and you have to start using things like the WISE telescope; that’s one of the instruments that have been used. They are found ground based on digital sky surveys that have done a lot of work to find them. The easiest way to find them is to start looking in the IR. The other problem that you run into in trying to find these suckers is they like to cuddle up next to nice bright stars and so now you have to start doing things like using what are called chronographs which is where you essentially put a disk in front of your stellar disk on the sky, block out its light and look to see if there is anything faint near that bright star. It gets kind of tedious to use a chronograph to look at every bright star in the sky to try and find brown dwarfs that are binary systems. It’s the isolated ones that are easier to find.
Fraser: Right so the point being that if get a situation where the star is in a binary companion with a brighter star, this gives you away to know where to look because they’re so hard to see. I know people are also looking for them in these stellar nurseries right? They’re looking for places where brighter stars are likely to be.
Pamela: Right. We look for them all the places we look for normal stars but they’re annoying to find. We really have to be looking in the IR and in the near IR.
Fraser: Now we’ve got the James Woods space telescope coming out in the next… 5 years? Will that be able to help the search for brown dwarfs?
Pamela: I think that would be a strange use of such a powerful telescope to use it to survey for new brown dwarfs but what it can do and what I expect it will be doing is imaging not just brown dwarfs but also giant Jupiters. We’re now at the point that we are starting to be able to individually look at some extra solar planets; Bitzer has done this in a few cases. They’ve also looked at a few brown dwarfs this way and to individualized meaningful studies of things that are already discovered. It really takes a whole family of different types of telescopes to first survey the sky and catalog what’s there and follow up in detail and understand what those objects are.
Fraser: So it might not be the tool for surveying but it definitely will be the tool for doing full on observations. It’s going to be an enormous telescope. Hubble is like 1.6 meters and this is a 6.5 meter telescope. It’s enormous. That makes sense; it might be a waste of time to be surveying for them. So you actually led into this right? We’ve got this situation where we have these brown dwarfs, the high end of the failed star, but it’s really a spectrum. Wherever you get hydrogen clumping together all the way down to nothing, you’re going to have some situation. Let’s go the other way. As we get smaller and smaller and smaller, less mass, I guess smaller isn’t a good way to put it right because as you said they kind of stay the same size they just get more dense. How does that work on the lower end?
Pamela: On the lower end is where things start to get messy and people start to argue because we can’t, basically, stick a probe inside one of these extra solar planets or brown dwarfs and figure out “Did it ever do any burning?” So what we start doing is start looking if there is lithium in the atmosphere. If there is lithium in the atmosphere it means it didn’t burn lithium so that puts one level of constraint on this system. As we go down people just start arguing. We know that below 10 masses is not a planet. We’re pretty sure that above 13 masses is a failed star; it did have some temporary nuclear burning. In that middle range you have these weird objects snuggled up against stars that we call brown dwarfs but they’re at the 10 Jupiter mass level. It’s thought that there is either some sort of mass loss or something else happened. It’s unclear what to call some of these objects. Are they failed stars, or are they below planets and that’s one where I think a lot of work on the definition still needs to happen and we need better models.
Fraser: Part of it is: “Is it orbiting a star?” But I guess that’s the distinction? “Is it a binary companion or is it a planet going around a star?”
Pamela: If we didn’t watch it form and we don’t see a protoplanetary disk that it’s a part of, we have no way of know if the object we’re looking at formed via an accretion process like a planet or a collapse process like a star.
Fraser: You mentioned earlier on that things like Jupiter, for example, if you add mass to Jupiter or clouded two Jupiters together, you wouldn’t necessarily get a much larger would you?
Pamela: No, you’d get an object the exact same size, more or less, within a few percents. That’s one of the awesome things. It’s one of those cases where the density just keeps going up. The way the pressure and gravity balance, the radius stays very similar as you go from roughly Jupiter sized to one of these 80 Jupiter mass, not-quite-yet-a-star objects.
Pamela: Yeah it’s really awesome, physics just kind of balances out this way.
Fraser: Now if you could look at a brown dwarf what would you see?
Pamela: You’d see a magenta object that has convected cells on the surface. When you look at the sun through a really good hydrogen alpha filter and you magnify it sufficiently you can see these boiling cells on the surface. You actually have convective cells driving brown dwarfs as well. Brown is really a misnomer. Brown isn’t something you get through additive light processes generally. Rather they’re this deep, deep magenta. I hate to say this but they are the color of my hair currently. They’re magenta objects but brown is just easier to say and spell.
Fraser: So they’re sort of a reddish color? They’re on the spectrum of a red dwarf but they are a deeper red? A darker red?
Pamela: Red dwarfs are much more Crayola in color. This is where you start to get that deep maroon color. The MIT “blood on concrete” is the joke they use. That deep maroonish, reddish color.
Fraser: But if you look at Jupiter you see it’s got these bands and storms on its surface and yet when you reach the brown dwarf size you’ve got convective cells blobbing up like a lava lamp. Where does that happen? Or do you go from one to the other?
Pamela: It’s all going to depend. We only have one example of Jupiter so it’s hard to say. What we’re seeing with Jupiter is these different cells where we did an entire episode on the weather of these planets where you end up with different atmospheric levels rotating the planet at different rates. This leads to bands of various colors going at different rates around the planet which causes some to appear to move backwards relative to others and you don’t see the active convection. Now we can’t actually image the detailed surface of a brown dwarf so we’re basing everything we know of what they look like off of models. Based on what we know from models you should end up with convective cells that are visible on the largest of these but at you get to smaller and smaller ones as you start to go from the “later” to the “thank you” part of our mnemonic out to the Y spectrum class stars. Now perhaps you’re going to begin getting that band similar to what we see on Jupiter. Until we have observations I can’t tell you exactly when these transitions take place or exactly when the convective cells begin to get hit by weather patterns in the atmosphere of these failed stars.
Fraser: I know there were some observations of some extra-solar planets where they were able to see that they were tidally locked to their star.
Pamela: Well they don’t see that they are tidally locked.
Fraser: No, they calculate that they were tidally locked and yet the heat was being distributed across the entire planet so there had to be ferocious storms that were transmitting so you would see these bands of these storms as they were swirling around the planet. If you got bigger and bigger eventually that convective process would take over. There is no clear line on where that happens yet, it’s real interesting.
Pamela: This is where we need orbital interferometry. We need the ultra high-resolution imaging capabilities from space where we can be above the atmosphere and hopefully, sometime in our lifetime, the money will be invested to make this possible but until then we have models in our computers and the models are getting better slowly.
Fraser: Now I think there was a great misnomer that flew around the internet a couple years ago and we’ve covered it a couple times in astronomy cast; this idea that the Galileo space craft, the nuclear powered Galileo space craft, if it was crashed into Jupiter that it would ignite and turn into a second star.
Pamela: No… (Laughs)
Fraser: Based on the conversation we’ve just had that that concept was deeply flawed.
Pamela: Deeply, deeply flawed. That’s like saying me squishing a mosquito on my skin will cause me to go thermonuclear. No, it’s not… even if it is a radioactive mosquito that will give me radioactive powers. Yes, Gallo was carrying nuclear fuel on it but that just means that it was giving off a lot of heat as those radio isotopes did their normal half-life thing and decayed and gave off energy and powered emission. It’s not like it was a nuclear bomb nor had that capacity to become one.
Fraser: And even if it was it wouldn’t matter.
Pamela: Right we could blow nuclear bombs up in the atmosphere of Jupiter and it would disrupt the weather patterns for a while but not for that long. We’ve dropped comets… well we haven’t personally…
Fraser: We’ve done that!
Pamela: The solar system has dropped comets in the atmosphere of Jupiter giving off the energy equivalent of nuclear weapons. In the process Jupiter took it on the chin and healed up rather quickly.
Fraser: So the only way that Galileo could do that is if it happened to have 79 times the mass of Jupiter somehow.
Pamela: Yeah and even that is questionable. To guarantee it you need at least 83 times, 84 times.
Fraser: Yeah 84 times the mass of Jupiter packed into that little spacecraft and then it smashes in and boom… it has to be hydrogen too.
Pamela: Maybe if it was that weird red stuff that was theorized in the recent star trek which doesn’t work…
Fraser: Yeah… alright… cool. Awesome, well thank you very much Pamela.
Pamela: Thank you very much.
This transcript is not an exact match to the audio file. It has been edited for clarity.