Somewhere between a planet and a star
Keivan Stassun discovered key properties of brown dwarfs.
Keivan Stassun is assistant professor of astronomy at Vanderbilt University and an adjunct professor of astronomy at Fisk University.
Dr. Stassun spoke with Earth & Sky’s Jorge Salazar in March of 2006 about the first–ever measurement of key physical properties of brown dwarfs, stellar objects with characteristics somewhere between a planet and a star.
Salazar: Dr. Stassun, thanks for talking with me today. So, tell me about this discovery of eclipsing brown dwarfs announced recently in the journal Nature.
Stassun: What we’ve discovered is the first known system of two brown dwarfs that orbit one another, and specifically, as they orbit one another, they periodically block one another, they periodically eclipse one another, as seen by us on Earth. The reason why this is so important is that such a system, what we call an eclipsing binary system, is very rare in nature. And when we do find such a system, we’re able to directly and accurately measure all of the fundamental physical properties of the two objects that make up that system, in this case, two brown dwarfs.
Brown dwarfs are what astronomers call failed stars. They are star–like objects, in that they are very large balls of gas, but what makes them fundamentally different from stars is that they weigh so little that they are not able to initiate the fires of nuclear reactions in their cores, the way that stars do, to generate heat and light. Instead, these brown dwarf objects are something in between a star and a planet. And so the reason that understanding these brown dwarfs, and finding a system where we?re able to directly and accurately measure their physical properties is so important is that ultimately, if we want to understand how legitimate stars like our own sun and our own solar system come to be, if we want to understand the physical processes that govern the birth of stars like our own sun, we also need to understand when, and under what conditions a star fails to be born.
And so, these brown dwarfs tell us, if you will, why a star is sometimes still–born. The other reason these brown dwarfs are important is because, as I said, they’re something between and betwixt a star and a planet. And so they give us an opportunity to study the process simultaneously of stellar birth, but also the process by which planets are formed. And so there’s a real richness of information to be gained by studying these brown dwarfs. And this is the first time that we’re able to measure directly, all of their fundamental physical properties.
Salazar: How did you find this binary system?
Stassun: The technique that we use is twofold. First, we need to make measurements, over a long period of time, of how the overall brightness, the total amount of light, that we receive at Earth from this eclipsing binary system, changes over time. The way that we found it is that we went and made measurements over the past twelve years in the changes in brightness of thousands and thousands of young stars that are forming in one of the stellar nurseries nearby to Earth in the Orion Nebula. And, by looking for stars that most of the time are constant in brightness, but that periodically become very dim, and then quickly go back to their regular brightness, we sought to find this rare class of eclipsing binary systems.
Basically, we were looking for the momentary winking out that an eclipse will cause. And once we found this system, we then, just using a regular telescope, in visible light, and a digital camera to record the images, we measured over the past twelve years the brightness of this system and how it changes over time so that we could monitor, very, very accurately and carefully how those eclipses are behaving, how long do the eclipses last, how much light do we lose during the eclipse, etc… So that’s the first thing, basically monitoring the changes in brightness of the system in order to measure in detail the eclipses that occur as they block one another.
The second thing is to use a very different kind of telescope, that has affixed to it, instead of a digital camera, a device that we call a spectrograph. That allows us to spread the light out that we receive into its different colors. And one of the things that we can do with such an instrument is to, using a phenomenon called the doppler effect, we can measure how fast an object is moving based on subtle changes in the color of the light that we receive. And so using that technique, we were able to measure, very precisely, how fast the two brown dwarfs move in their mutual orbit about one another. And so together with the information about how the eclipses behave as the two objects pass in front of one another, and with the information about how rapidly they move in their orbits, we’re then able to simply apply the basic laws of physics, Newtonian gravity, Newton’s laws of motion, and from that we can measure how much each of the brown dwarfs weighs, how big across they are, what their diameters are, how hot they are, their surfaces, how much light they put out, each one of them independently. Those are all of the basic physical properties that astronomers need in order to characterize these objects. And we’re able to make those measurements directly using those techniques.
Salazar: One of the interesting things about the binary system, I think, is that one of the pair of brown dwarfs is considerably hotter than the other one.
Stassun: That’s right. What was surprising is that most of the time, when we look at stars, what we find is that the more massive stars are hotter than the less massive stars. And theoretically, what we expected was to find a similar relationship with brown dwarfs, that the heavier ones would be hotter. What we found, to our surprise with this system is that the heavier of the two brown dwarfs in this binary pair is cooler than the one that weighs less. Or said the other way, the lighter one, the wimpier one, is actually hotter at its surface than the heavier one. And we don’t entirely understand what that means right now. One possibility is that these two brown dwarfs actually were formed separately rather than together, and then later came together to become a binary later in their lives. One way that that would explain this strange finding of ours is that brown dwarfs, when they’re very, very young, start out relatively warm and big. And over time, they continually shrink and cool down. So, if these two brown dwarfs are slightly different in age, if they didn’t actually form together, then that would be one way of understanding why one of them might appear cooler than the other, just because perhaps it had a little bit more time than the other to shrink and cool down. But it could also mean that our basic, theoretical understanding of the physical properties of these failed stars is not complete. And so I think that it’s safe to say that the theorists will be scratching their heads over this very curious finding.
Salazar: Let’s step back a second. Just what do scientists know about how brown dwarfs populate the cosmos?
Stassun: It’s very difficult to say right now, how common brown dwarfs are, relative to real stars. We believe, some astronomers believe that brown dwarfs actually outnumber the number of real stars in our galaxy. Or at the very least, brown dwarfs are very, very common. The reason that question, currently, more definitively, is actually related to the importance of this new discovery. And that is that for the vast majority of the brown dwarfs that we believe have been catalogued so far, our belief that they are brown dwarfs is based on the relationship that we think exists between, for example, how hot a brown dwarf is and how much it weighs. And so, if we point our our telescopes at a faint, dim, and cool object, and we measure its temperature and its brightness, we can say, well, we think that’s a brown dwarf based on its temperature and its brightness, but its only now that we have determined, directly, the actual relationship between how much a brown dwarf weighs, how hot it is, and how bright it is, that we’ll be able to go back and reassess how many of the objects that we’ve cataloged so far really are brown dwarfs. We now have the rosetta stone, if you will, that gives us the basic translation between all of the physical properties of brown dwarfs. So, we now have the ability to conduct an accurate census within our galaxy of how common brown dwarfs are.
So, the real utility of this discovery for moving forward with our understanding of stars and the stellar birth process is that, what these two brown dwarfs give us is, as I said, a kind of rosetta stone, a way for astronomers to translate the physical properties of other brown dwarfs that we find out there where we don’t have the luxury of weighing them directly, we now will be able too take the kinds of things that we can measure for those brown dwarfs, things like their temperatures and their brightness, and now be able to translate those measurements into a very good estimate of how much those objects weigh, and ultimately, that’s what we need in order to determine which objects out there are brown dwarfs versus stars, and therefore to answer one of the most important questions out standing right now, which is the one that you asked, how common is it that nature gives rise to a real star versus how often is the stellar birth process unsuccessful, if you will.
Salazar: Thanks for taking time out to speak with me. Is there anything else you’d like to share with the public today?
Stassun: I’ll just add one thing. One of the really unique kinds of measurements that we’ve been able to make with this newly discovered system is that by measuring exactly how long each of the eclipses last, we’re able to measure, directly, the diameters of these two brown dwarfs, and one of the things that your listeners might find interesting is that these brown dwarfs, even though they respectively weigh but a few percent of what our sun weighs, these two brown dwarfs, in terms of their diameters are almost as big as our sun. That is, they’re extremely light, but very large. That may sound a little funny, but that actually is in line with what theorists have been predicting for what the properties of these brown dwarfs will be like when they’re very, very newly formed. These brown dwarfs that we found in the Orion Nebula, we believe are extremely young, maybe only a million years old or so. And so what we have seen, basically, is that brown dwarfs, when they are very young, are very star–like in their properties. They’re very large, they’re warm, and these brown dwarfs that we found are in the very earliest stages of gravitational collapse. Ultimately, in a billion years or so, these two brown dwarfs will be very small and very dim, resembling our own Jupiter. But what we’re seeing now is these very newly–formed brown dwarfs are just in the earliest stages of this gravitational collapse process. So basically, these brown dwarfs start out looking very much like stars, and end up looking very much like planets.
For more about Keivan Stassun: Read the interview in Exploration magazine.
