Lecture: Brown dwarfsAuthor: QQ-464094252: Narrator: Listen to part of a lecture in an astronomy class. Professor: Over the past decade, we've discovered hundreds of celestial objects we call brown dwarfs. Actually, they are more reddish than brown. Theories about them have been around for decades, but, it's only recently that we've been able to find and observe them. Brown dwarfs are challenging for astronomers because they're tough to classify – they have masses too large for a planet but too small for a star. And they share some characteristics with planets than others with stars. For example, they seem more like planets and many of them orbit around stars but they apparently form not like planets but in much the same way stars do it, at least initially. Remember, stars originate in huge clouds of dust and gas, thousands of light-years across, um, molecular clouds, each with enough material to make dozens of stars. Young stars forming in the denser regions of molecular clouds, known as cores, which eventually collapse due to their own gravity. Now, within any given molecular cloud, there can be several cores, and when they collapse, the inner portions break up into trunks, which are stellar-embryos. Stars in the process of forming. So, a collapsing core can contain several stellar embryos, several of which can become stars. The usual path to star formation is that the gravity of the stellar embryo pulls in materials to add to its mass, and at some point, this embryo becomes so massive and dense that its material begins to fuse, to undergo nuclear fusion. Essentially, it ignites and becomes a star that will burn for billions of years. Brown dwarfs start out like stars we think. As stellar embryos collecting dust and gas in the cores of molecular clouds and as they gather mass, they're heated by all the material rushing in and begin to emit some infrared light. Certain molecules may even undergo a particular kind of low level fusion. But, if a stellar embryo fails to pull in enough molecules of dust and gas, it will never grow massive enough to ignite the powerful more typical shorter fusion that turns it into a full-fledged star. But, what prevents that? Why does it just stop growing? So that after several million years, a fairly short time in astronomical terms, this failed star that we now call a brown dwarf, just begins to cool again, and fade. Two theories. First, one called the ejection theory. Okay, well, according to this ejection theory, the smaller stellar embryos inside the collapsing core, the embryos that haven't competed so successfully for material to feed their growth, they're more likely to get tossed around by or evenly ejected by gravitational forces. So, right out of the core BEFORE they can collect enough material to become stars. So, what might have become a star gets ejected and ends up nothing more than a brown dwarf. That's the ejection theory. Then, there's the turbulence theory. The turbulence theory says that dust and gas are swirling around inside a molecular cloud and it's this turbulence that compresses some areas of the cloud into cores. But not every core has enough dust and gas to form into stars, so instead of stars some cores can only form brown dwarfs because they never had enough material to form stars in the first place. New born stars are typically surrounded by bits of leftover dust and gas called disks. Over millions of years, the disc material drains into the stars, and, some of it may go into forming planets, asteroids or comets. Now, if the turbulence theory is correct, brown dwarfs like many low mass stars should have stellar disks. But, if the ejection theory is correct, computer simulations have shown that any surrounding material would get stripped away mostly when the embryo is ejected from the core. So, do brown dwarfs have stellar disks? Huh, it turns out that many do. And the disks actually help us find brown dwarfs. See, like I said, brown dwarfs aren't bright but do give off infra-red radiation and stellar disks reflect this infra-red radiation and make it appear brighter. So, astronomers look for that infra-red excess when they're searching for brown dwarfs, and hopefully, as they observe the disks more closely, they'll be able to get more clues about the formation of brown dwarfs. I mean, we can't say for sure that the ejection theory is incorrect – maybe brown dwarfs form in different ways! Only if our space telescopes are able to catch them in the act of forming will we know for sure.