Date: April 17, 2009
Title: Giant Molecular Clouds
Podcaster: Sue Ann Heatherly
Organization: NRAO: http://www.gb.nrao.edu/
Description: Can you say diazenylium three times fast? Dr. Yancy Shirley of the University of Arizona studies this strange little molecule and others in Giant Molecular Clouds. In this podcast we chat with Yancy on matters of star formation, intersteller chemistry, and some great astronomical questions to be answered, perhaps, in the next few years.
Bio: Sue Ann Heatherly is the Education Officer at the NRAO Green Bank WV site. She comes to astronomy by way of biology (BA in 1981), and science education (MA in 1985) She visited the Observatory as a teacher in 1987 and knew she’d found Camelot. She has been employed with the NRAO since 1989.
Today’s Sponsor: This episode of “365 Days of Astronomy” is sponsored by NRAO.
Transcript:
SUE ANN HEATHERLY: Welcome to this addition of 365 Days of Astronomy Podcasts. I’m Sue Ann Heatherly, from Green Bank, West Virginia, at the National Radio Astronomy Observatory. And this month I am with Yancy Shirley, who is a Professor of Astronomy at the University of Arizona. And thank you very much for being with us this time on 365 Days of Astronomy.
YANCY SHIRLEY: Thank you very much.
SUE ANN HEATHERLY: So, looking on your web site – you have a very lovely web site, actually; it’s great – pictures, a little bit of text – I love it – uh, you’re very interested in many things, but one of them is how stars form. And I thought we could start out by, uh, you could give us a little five-cent tour of how stars form, and then we’ll talk more about how you figure all that stuff out.
YANCY SHIRLEY: I like to study the earliest phases of how stars and planets form. And, pretty much, you can see this process on a clear night. If you go out to a really dark site, especially in the summertime – if anyone has seen the Milky Way streaming up overhead – you’ll notice from a really dark site that it’s not a continuous stream of stars; but instead, there are all these little dark patches in there. And those dark patches are clouds of material, and it’s the raw material out of which today, in the Milky Way, stars and planets are forming. And so, we study those regions. Those regions are called Giant Molecular Clouds, and within them are where stars are actually forming. And what has to happen is, those clouds of material need to fragment, and at some point gravity needs to take over and allow that material to collapse down to actually form a star. And we can study these regions within the Milky Way. We can’t actually sit there and watch a star forming, because it’s a process that takes over a million years, but we can see objects that are in different stages of formation. And so, by studying those objects in the different stages, we can piece together the picture of how a star actually forms. And there’s all kinds of wild phenomena that are associated with this process. It’s really quite amazing. The cloud just doesn’t collapse down spherically to form a star, it has to conserve angular momentum. So, just like a figure skater that is spinning on her skates, if she pulls her arms in tighter, she’ll start spinning faster. So as this material collapses down it will actually start spinning faster and faster. And instead of collapsing spherically, it will actually form into the shape of a disk. And so around these protostars you have a disk of material that is going around. And also, some of the material misses the protostar, and it gets shot out in these jets that go off in each direction – it’s what’s called a bipolar jet, and it blows material both above and below the protostar out – and you can actually observe that phenomena as well.
And part of what I also study is, I’m really interested in chemistry and how chemical processes occur in space. And in these dense clouds of material that the stars are forming out of, there is a very, very rich chemistry that occurs, and all kinds of molecules form. And it turns out that molecules tend to radiate in the radio part of the spectrum. And we can take a radio telescope, like the Green Bank Telescope, and we can tune it up to a very specific frequency, and we can observe a molecule like, say, ammonia. And every molecule basically has a unique finger print, a set of frequencies that – it emits that – and we can tell exactly what type of molecule we’re looking at by tuning up to those specific frequencies. And so, I’ve been studying most recently how we can use that chemistry as sort of a tool in these star-forming regions. It turns out that there are several chemical processes that evolve with time, and so we can use those processes to determine sort of age-date some of these regions to see how long they have been forming.
SUE ANN HEATHERLY: So, you mentioned a couple of neat, neat things, and one of them is that if you look out into the Milky Way – go to a dark place – everybody listening to this podcast, please find a dark place and look at the Milky Way; it will take your breath away – that there are dark places there. And these are dusty places. And yet, you say that this is where all these, these processes occur, where disks form and bipolar jets and all those things. How do you know if they’re in those dark places?
YANCY SHIRLEY: Yeah, that’s a great question. Cause if you look with an optical telescope, you really can’t see that anything is going on. These regions where the stars are forming, if they’re forming deep down in one of these clouds, any light that’s being emitted from the protostar that’s forming doesn’t get out of the cloud; it’s absorbed, and it’s scattered by all that raw material that’s around the forming star. And so, in order to probe down in those regions, there is a couple different techniques you can use. Uh, one is to go to longer wavelengths. As it turns out, if you go to longer wavelengths, that light has a better chance of getting out of the cloud. So, first people started going to infrared wavelengths to try to study these regions. Some regions where the stars are forming are so deeply embedded, there is so much material you can’t even see them at near infrared wavelengths, and so you have to go to even longer wavelengths. The radio is particularly useful for this. The same material that does a really good job of absorbing and scattering that light from the young star that’s forming also absorbs that energy. And when it does that, it heats up a little bit. And anything that has any kind of temperature to it will emit radiation – will give off light. And it just depends on what it’s temperature is as to what wavelength range that it emits at. So, for instance, our Sun is very hot. And so, the surface of the Sun is so hot that it’s emitting light in the visible part of the spectrum. Just like a glowing piece of metal – you know, it glows yellow or red – you can see emitting at the visible part of the spectrum. So, something that’s thousands of degrees Kelvin will do that.
The human body is about 300 degrees Kelvin, and if you had eyes that were sensitive to light that is about 20 times longer than yellow light – visible light – you would see our bodies all glowing at 10 microns, roughly, in wavelength. Well, it turns out these little dust grains in the cloud – and that’s what actually absorbs the light is all these dust grains in one of these big clouds – they absorb a little bit of energy. It’s not very much, but they absorb a little bit, and they can heat up, and they can maybe be 10 or 20 degrees above absolute zero, only 10 or 20 Kelvin. And as a result, they do emit radiation, and it comes out in the radio part of the spectrum. So, for instance, wavelengths that we can observe from the ground, say, in the sub-millimeter. So, wavelengths that are slightly less than a millimeter in wavelength, and even longer, these dust grains emit radiation. And so, you can actually observe with a radio telescope and map out that radiation. And because it’s at such a long wavelength, it has no problem getting out of the cloud, and so we can actually peer all the way through these clouds at these very, very long wavelengths.
SUE ANN HEATHERLY: What lets you know, when you do that, that you are looking at a. . .a place where a star is actually forming?
YANCY SHIRLEY: Yeah. So, what’ll happen is, where a region or a star is forming, there will be a lot of energy there. there will be a lot of heat. So when the gas is collapsing down to form the protostar, the protostar itself heats up. And also the region where the gas is hitting the protostar, that accretion region also can get very, very hot: tens of thousands of degrees. And so all that energy then gets absorbed by the surrounding dust and that dust heats up. So within this larger cloud we can see the regions that appear brighter at these radio wavelengths, and we know those are hotter regions where the dust is being heated up by forming stars.
SUE ANN HEATHERLY: Tell me how the study of different molecules – you mentioned that molecules give off specific radio waves – what does that tell you about stars in formation?
YANCY SHIRLEY: Yeah. So there is all kinds of chemical processes that actually happen. And basically, if you look at the abundance of a particular molecule versus time, these molecules will evolve. Their abundances will change over time. So, for instance, in a region where stars, let’s say, like our Sun, are forming – that are about the same size of our Sun – there is a set of molecules that form very, very quickly in the gas. And things that tend to be formed along carbon chains, things like that will actually form very, very quickly in the gas. And so, we can look for molecules that have these carbon chains, and we know we might be looking at a region that’s very young. There are other molecules. A lot of them tend to be nitrogen-bearing molecules, things like ammonia that tend to take a little bit longer to form in the gas phase. And so one idea, and something that’s. . .that we’ve been studying quite a bit at Arizona, is using ratios of these kinds of molecules. So, for instance, taking a molecule like ammonia which takes a little bit of time to build up an abundance, and comparing it to, say, the abundance of a carbon-chain molecule which builds up an abundance very quickly, and the ratio of that abundance will tell you something about the evolutionary state of the cloud.
We can also observe all kinds of weird molecules in space, things that wouldn’t exist in the Earth’s atmosphere. So, for instance, one of the molecules I like to study is this little ion. It’s called N2H+. It has two nitrogen atoms and a hydrogen atom, and one electron stripped off. And in space this is a very stable molecule. It can hang around and build up an abundance, and. . .and it’s easy to actually observe at radio wavelengths. Whereas, if it was in the Earth’s atmosphere, it wouldn’t last any time at all, it would react. It would react right away and fall apart into something else.
SUE ANN HEATHERLY: And why is uh. . .what did you say it was? N2H+.
YANCY SHIRLEY: N2H+. Yeah.
SUE ANN HEATHERLY: It probably doesn’t have a name.
YANCY SHIRLEY: It does. Actually, it’s called diazenylium. It’s not something you would learn in chemistry class.
SUE ANN HEATHERLY: It’s easier to say N2H+, I think.
YANCY SHIRLEY: That’s right. Yeah.
SUE ANN HEATHERLY: Why is it interesting?
YANCY SHIRLEY: Yeah. It’s one of these molecules that takes a little while to build up an abundance. And it turns out that once the regions get a bit more evolved, um, it’s abundance stays relatively constant. So it turns out that particular molecule can be a very good density tracer in these regions. So, we can learn something about the density structure of these regions by mapping that molecule and studying its distribution on the sky.
SUE ANN HEATHERLY: When I think about stars forming and how hot they are, and how they throw out all of this radiation, it makes me wonder if it doesn’t break apart these molecules. Do you see places where there are less around these star-forming areas?
YANCY SHIRLEY: Yeah. Absolutely. So, there is a couple of different effects that happened when the protostar starts to form and it really starts to heat up these clouds. In the very inner regions of the clouds, the regions can get quite hot actually and several things can happen. So, for instance, one of the processes that happens in these clouds, in the outer parts of the cloud where it is still cold, it’s actually so cold that a lot of molecules – like, for instance, water or carbon monoxide – will freeze out of the gas phase, and they’ll actually form ices on the dust screens. And we know those ices exist, because we can observe them at mid-infrared wavelengths. We can see absorption bands from background star light. And it’s very particular frequencies that ices will absorb at, and we know that those ices exist. Then when you get closer to the protostar, it gets hot enough again that the gas will sublimate – will come back into gas phase-off of those ices. And it turns out that, in that ice phase, there is a lot of chemistry that can actually happen in the solid state phase as well. And a lot of very complicated molecules can start to form in that phase, and then they can come off again when the protostar heats this material back up. So, for instance, one of those molecules is methanol. It’s a molecule that if you just used the molecules in the gas, it would be very hard to form it. But when you get it in the ice, it’s actually quite easy to form it in the ice. And then, when it gets heated back up, it comes back off into the gas and we can observe it in the gas around these protostars. And then that also does happen in the very, very inner regions. When you get this warm material, it actually will destroy certain molecules, and you will see those molecules start to go away in the very, very high inner regions. So, just like there are molecules that are very good tracers of the early phase, and other molecules that are very good tracers of the later phases; there is other molecules that are very good tracers of cold gas, and there are ones that are very good tracers of warmer gas.
SUE ANN HEATHERLY: It’s pretty interesting. I hadn’t thought about that intermediate step where you’re going from solid back into the gas and then you can see them again. That’s pretty cool. So in the last few minutes, Yancy, tell us, what are the mysteries that you’re hoping to answer in the next few years or so.
YANCY SHIRLEY: There’s lots of great mysteries. So, for instance, one of the things that we’ve been working on is trying to understand the complete stellar evolutionary sequence. So, right now if you take an observation of a star that’s already been born – that’s out there and you can see optically – you can do a set of observations and you tell where it is in its lifetime. But for the very earliest stages, the formation of the star, we’re still working on developing that evolutionary sequence and making that an accurate sequence, and so something that’s. . .I think there is going to be a lot of development in over the next decade.
There is a lot of very interesting questions, like, for instance, why does on average it seem like stars that are about the mass and the size of our Sun be the preferred size and mass that forms? That’s a very fundamental question that we’re still trying to answer.
SUE ANN HEATHERLY: I like that, though (inaudible).
YANCY SHIRLEY: And another really interesting one is, how do planetary systems form. We know from the detections over the last 10 years that there is well over 300 expo planets that have now been discovered around other stars. We’re slowly pushing with those techniques down to the sensitivity where we’re going to start detecting Earth mass planets around other stars. And so, an interesting question is going to be to see if we can see those kinds of systems evolving and forming in the very earliest stages. So, for instance, we have now identified a set of objects in the nearby clouds that are forming stars that are right on the cusp of forming protostars. And those are the regions where you want to look if you want to see the very initial conditions out of which a disk where planets would eventually form – what those initial conditions are actually in a disk. And then in the later stages we’re going to be able to use some of the new interferometers that are coming on line, like the Alma array in Chili is going to be a great instrument for actually probing the structure in these disks to actually see this very early phases of planet formation where you actually have a proto planet that is still accreting material in the disk. So there is a lot of exciting things on the horizon in star and planet formation.
SUE ANN HEATHERLY: I hope I’m around to find out what happens when you start to make these discoveries. Thanks a lot for joining us today, Yancy.
YANCY SHIRLEY: Thank you very much. I appreciate it.
End of podcast:
365 Days of Astronomy
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