Planetary Radio: Space Exploration, Astronomy and Science - The Milky Way: A New Galactic Self-Portrait

Episode Date: June 23, 2008

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Smile, it's a new galactic self-portrait, this week on Planetary Radio. Hi everyone, welcome to Public Radio's travel show that takes you to the final frontier. I'm Matt Kaplan. So you think you know what the Milky Way looks like? Maybe not. Astronomers Bob Benjamin and Thomas Dame will share new revelations about the shape and structure of the galaxy we call home.

We are also just moments away from a Phoenix mission update. Emily Lakdawalla will help us celebrate confirmation that the white stuff exposed by Phoenix really is water ice. Later we'll join Bruce Betts for another What's Up look at the night sky. And Bill Nye the Science Guy is back with his own enthusiastic take on Martian water. And if that's not enough, we've got more space news at Planetary.org,

including a story about three super-Earths found circling a star 42 light-years away. Water on Mars. You can go all the way back to the first sightings of the polar ice caps for evidence. But we've never actually dug in and found any. Not till now, anyway. Emily, they did it. Yes, they did. They found the ice that they were looking for.

And with remarkable ease. Yeah, well, you know, they found that bright white smooth material just by landing on it because the thrusters blew soil away. But it took them actually a few weeks to be able to confirm that, yes, it's ice. But they finally got the evidence they needed this week. That evidence was that they'd been digging around in this site called Dodo Goldilocks. And while digging, they'd pulled out some of these chunks of white material from the ground, and there were a few candidates for what that white material could have been, and the two chief candidates were either ice or some kind of salt deposit that had been left behind as groundwater evaporated.

Well, those chunks disappeared after four days of being exposed to the Martian air, and salt just doesn't do that, so it must have been ice. And we know that it's water ice, not CO2 or dry ice, because it would have disappeared but a lot faster? That's right. Carbon dioxide ice is really very unstable at the temperatures, the relatively warm temperatures of the Phoenix landing site during the summer. During the winter, the carbon dioxide ice would actually sit around on the surface for a long time. But the imaging team likened it to regular water ice on Earth and its behavior at 140 degrees Fahrenheit, which, you know, wouldn't last very long.

So the carbon dioxide ice would have gone away very quickly. If people go to planetary.org, either taking a look at your blog or the very nice feature done by our colleague Sally Rail, AJS Rail, they're going to see this little sequence that shows you the ice disappearing, sublimating. And really, that's what it took. It wasn't any fancier analysis from the TIGA experiment or anything like that? Nope, it's really as simple as that. There you see it, there you don't. It's Sol 20 and-24. Ice chunks were there on SAL-20. They were gone on SAL-24. It's that simple. All right. So where are we now? The latest that I have seen is that things are going

pretty well. This data problem, I guess they are beginning to get a handle on it. But you get something as simple as a couple of doors, spring-loaded doors to an oven. That's right. The TIGA team is not getting any love from fate. They've had just one little problem after another with their instrument. To be fair to them, their instrument is one of the most complex I've ever heard of being sent on a space mission. So it's no surprise that they're running into some problems. But, gosh, it would be nice for them to get some luck now. Right now the problem is that they've tried to open a second set of doors on their instrument, and those doors, they barely popped open. There's just a little slit open over their oven.

And I haven't heard anything from the team about whether they think they're going to be able to get a sample in through those doors or not. But they don't have any options for trying to get the doors open any further. They're just spring-loaded. It's like a jack-in-the-box. You release the latch, and the door should pop right open, and there's nothing more you can do. Well, still plenty of reason to congratulate the entire Phoenix team. Many more details at planetary.org where you can continue to follow Emily's following of this mission in the blog right there at the homepage. Emily, thanks again very much.

You're quite welcome. Emily Lakdawalla is the Science and Technology coordinator for the Planetary Society. She's usually heard here doing our Q&A segment, but is giving us these regular updates on the Phoenix mission as well. No one is more excited than Bill Nye about water on Mars. He recorded this commentary just before the new photographic proof arrived. I'll be back in a minute with galactic astronomers Bob Benjamin and Thomas Dame. Hey, hey, Bill Nye the Planetary Guy here, Vice President of the Planetary Society.

What are we looking at this week? Well, take a look on Mars. The Phoenix Lander has scraped away the surface just a few centimeters, the width of your thumb deep, and there's this white material. White, stark white, like maybe it's chalk. Maybe it's water ice. Now, if you were a geologist and this were on Earth, it would take you a moment to figure it out. You'd just feel it with your fingers. Oh, it's cold. It melted. I've seen many geologists just touch it to their tongue. And then you'd know just right away. But no, we are hundreds of millions of kilometers away. And figuring out

this fundamental stuff is not so easy. It is rocket science. But we have the people to do it. If we find water, my friends, it will mean there are hectares and hectares, acres and acres of water on Mars. And maybe once there were living things there. If we discover them or evidence of them, it will, dare I say it, change the world from hundreds of millions of kilometers away. Space exploration is as exciting as it gets right now, my friends. Stay tuned to Planetary.org. I'll talk to you next week on Planetary Radio. I've got to fly.

Bill Nye, the Planetary Radio. I've got to fly. Bill Nye the Planetary Guy. Bob Benjamin is at the University of Wisconsin-Whitewater. He is part of a team that announced a discovery of galactic proportions at the 212th meeting of the American Astronomical Society. Bob's colleague Thomas Dame is a radio astronomer with the Smithsonian Astrophysical Observatory and Harvard University. Dame's team found the first strong evidence for what is essentially an arm of stars on the other side of the Milky Way. Taken together, their work has given us a new and probably much more accurate rendering of the swirling expanse of gas, dust,

and hundreds of billions of stars that comprise our home galaxy. Thank you so much for joining us here on Planetary Radio. And I feel especially fortunate that we're able to get the two of you together because it was basically your work being unveiled to the public on June 3rd, which has changed our view of the Milky Way galaxy, although you tell me perhaps not in quite as radical a fashion as I first thought. Bob, why don't you get us started? Talk about how it is that we came to, for so many years, decades upon decades,

had this image of the Milky Way that turns out to have not been quite right. The study of the galaxy is sort of an evolutionary process. It's a long-term effort by astronomers over decades that has yielded the picture that we refined and discussed at this June 3rd meeting. The study of spiral structure dates back to 1951 when an astronomer from Yerkes Observatory first announced the detection of three spiral arms in the Milky Way. From that point on, astronomers pieced together a picture, mostly from looking at the results of radio telescopes serving the molecular and neutral

interstellar gas to form a picture of a spiral galaxy in which outside the central region of the galaxy we had four major arms that also seemed to be traced by regions of star formation. What prompted the meeting just a few weeks ago was the unveiling of a new survey of mid-infrared data taken from the Spitzer Space Telescope that was a survey of the galactic plane where we got basically a completely clear view at these wavelengths of all the star formation and all the stars in the galactic disk. And so we decided with this sort of new observational view of the Milky Way that it was a good time to get people together to talk about

sort of the overall structure, what all of these things that we were seeing would look like from above. Bob, I discovered this in a press release from NASA and got a hold of you and wanted to get you on the show. But you said, we got to get Tom Dame on as well, because while you have sort of, your work has demoted a couple of these arms, if that's fair. Tom, you actually showed us that there is this second arm out there, which is pretty impressive. That's right.

It's quite near the galactic center, near the bar, which Bob and his group has shown very convincingly exists now in the galactic center. An interesting point is that, as Bob pointed out, the spiral arms were first discovered optically and reported in that 1951 meeting. But from then on, starting in the 50s, the optical astronomers couldn't go much further because of the dust right in the plane of the Milky Way. And so the radio astronomers sort of took over for decades. One of the most significant things about this meeting and Bob's announcement

is that now at last we're going back, and because he's gone to the near-infrared part of the spectrum where the dust is much less important, we can again finally study the stars throughout the galaxy. That's where he's coming from. My arm was discovered in the radio waves. In fact, in the 1950s, the very first radio surveys, that was our real first look at the entire galaxy, was in these radio surveys, mainly of the emission from atomic hydrogen. And of course,

most of the universe is hydrogen, so it emits very copiously in the radio regime and pretty much fills the galaxy. So we got our first look at the entire galaxy with these radio studies. And when we did, one of the things we learned was that the galaxy was quite an orderly place, that most of the stars and the gas orbit the galaxy in circular orbits. So it was very nice. It was sort of easy to understand what was happening, and you could sort of relate distances to velocities that you measured.

But there was this one arm which stood out very clearly as being unusual, and that was called the expanding 3 kilopasic arm. It was close to the center, very striking in the early surveys, found right away in 1957. It was very puzzling because it didn't follow the usual rules of circular orbit. It was expanding towards us at 50 kilometers per second. Almost all of your listeners could guess the first theory that emerged to explain that, and that was an explosion at the galactic center. But there were problems with that because there was a lot of gas moving really fast,

and there was a question of where that much energy could come from. It was sort of ad hoc. There was no other evidence for an explosion. But soon after that, in like 64, there was another suggestion that maybe the galaxy was not just a regular spiral, but was a so-called barred spiral. And one of the strongest evidence put forward for that was the fact that there was this 3-kiloparsec expanding arm

which had these very large motions away from the center, and that's what you would expect from a bar. But, of course, that suggestion raised the issue of where was the other side of the bar because, you know, the bar has two ends and would tend to drive very symmetric two-fold structure. And so what we found at last, after 51 years, is that two-fold symmetry. We found the arm which emerges from the other side of this bar. The most significant result of that, I think, is just that we can at last see,

unambiguously and basically raw data, this symmetry that people have long hoped and suspected the galaxy had. But now we know for sure that there are these two beautiful symmetric arms in close, basically trailing off the end of the bar, at least most likely that's the situation. So these two rather prominent arms. But Bob, your research, as you've looked at the distribution of stars across the galaxy has told us that the other two arms that were found in 1951, I guess, are a little more wispy than we thought. Well, yeah, basically the arms that Tom is describing sort of surround the bar.

They trace sort of, we think, along the front side and the back side of the bar as we look through the galaxy. the front side and the back side of the bar as we look through the galaxy. But sort of out much further, extending out to much further out in radius from beyond the ends of the bar were these other four structures. So actually we're up to six arms now, two surrounding the bar and four beyond the bar. Of those four, whose directions were traced out by radio astronomy, as Tom was saying, we looked in the directions of what are called tangency directions. So where your line of sight, you're looking out in a certain direction, and your line of sight goes for a fairly long length along the arm.

And so we knew which directions to expect those tangencies. And when we looked at the number of stars that we saw using these mid-infrared wavelengths where we didn't have to worry about dust, we saw that in one direction, sure enough, there was this huge enhancement of stars. And it turns out it's about an increase in the density of the old stars, red giant stars, by about a factor of 30%, which is pretty significant. And so we said, oh, well, that's exactly what we expect. People have said that that's a spiral arm tangency, and there is an enhancement of stars right in that direction. But then when we looked for a different tangency, which seemed to be just as well settled by radio

astronomers as the previous one, we saw nothing. We saw no evidence for any enhancement in the number of the old red giants. And it turns out that we were not the first people to see this. Earlier sort of generations of infrared telescopes where they couldn't count the individual stars, but they could get a sense of just how bright it was, they had seen this before. An experiment done with what was called the COBE satellite, the Derby instrument, saw the same effect, that in certain directions where they expected tangencies, they saw a lot of emission, and other directions they didn't. What we presented at the AAS meeting was this result and how to interpret this result,

which was the idea that perhaps there's something fundamentally different about these arms, that two of them, we suspect, are associated not just with an overdensity in the old stars, but overdensity in gas and star formation. And then the other two are just compressions in the gas, so that the expectation would be, if you were to look at the Milky Way from the outside, that of these four arms outside the bar, two of them would be stronger and two would be weaker. More about the Milky Way from Bob Benjamin and Tom Dane in a minute. This is Planetary Radio.

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The Planetary Society, exploring new worlds. Welcome back to Planetary Radio. I'm Matt Kaplan. My guests are Bob Benjamin of the University of Wisconsin-Whitewater and Tom Dame of the Smithsonian Astrophysical Observatory and Harvard. Their just-announced findings have reshaped our image of the Milky Way galaxy. What strikes me as I listen to the two of you is how very much your work complements each other, though you have taken very different approaches

and used very different instruments. That's exactly right. And not just the 3 kiloparsec expanding arm work, but of course, as a radio astronomer, we are complementing the work of Bob. In fact, to support what Bob said about this tangent direction, which showed no enhancement in the old stars, that arm, which is called the Sagittarius Carina arm, is to radio astronomers the most well-defined, prominent, grand-designed spiral arm in the galaxy. You know, if you ask a radio astronomer,

what's your best example of a grand-designed, beautiful, logarithmic spiral arm that goes almost all the way around the galaxy, Sag Carina was it. And Bob's telling us that, you know, maybe it's one of these secondary gas arms. So it's quite an interesting result. There are still many, many books out there. When they show you a picture of the Milky Way galaxy, they basically show you something that looks like Andromeda.

With these revelations, is there another galaxy of all the millions out there that you would point to and say, yes, that one with that bar and those two very prominent arms, that's probably what we look like? Well, that's actually one of the goals of galactic astronomy, to get a sharper picture of what the Milky Way would look like from outside so that we can study Milky Way-like galaxies both from the inside and from the outside.

You know, one of the fun things about having been involved in this project is we wanted to provide a graphic that tried to capture what the current thinking, and of course it will evolve over time and maybe may change dramatically again, but the current thinking on the structure of the Milky Way. So it sort of serves as a police sketch. We have all of these different witnesses who describe different things. The artist's conception was sort of an attempt to blend all of these features together. And then we can chop it around to people who study galaxies from the outside saying, have you seen this galaxy? And I actually have gotten some suggestions from people who study other galaxies saying,

hey, check out this one. Maybe this one's similar to the Milky Way. Tom, is there either of you, actually? Can you share the names? Well, you know, Bob and I and others at the meeting worked a lot on this, especially Jay Gallagher, who's an expert on external bot spirals. And that was one of our goals was to find the best possible Milky Way analog. And I don't think we did settle on one.

One of the ones that I like, from my knowledge of the galaxy, when I saw it, I said, that looks like our galaxy, was M109, Messier 109. But, you know, other people have other favorites. But Messier 109 has quite beautiful, basically two large arms, as described by Bob, and then some intermediate arms, which potentially are gas arms. And it shows a beautiful bar and possibly even this sort of long bar, short bar, which we haven't actually talked about, but was another sort of finding of the Spitzer survey. We will, of course, post links to places online where people can learn more about your work,

and CM109 and perhaps that artist rendering, Bob, that you talked about. We're essentially out of time, but I've got to ask you, where your research is headed now, or do we know everything we want to know about the Milky Way? Somehow I think the answer is no. Bob? Oh, well, I think there's a whole different set of directions we can go. I intend to continue working on the stellar structure of the Milky Way, tracing out the arms, tracing out the bar, tracing out structures using the old red giants of the

Milky Way, and seeing if we can really pin down the hard numbers for the mass per unit area of the Milky Way's disk so that people can use that as a starting point for figuring out how gas flows in the gravitational potential provided by that mass. It's basically to provide the initial conditions for people who want to model spiral density waves in the galaxy. Tom, very quickly, where do you go from here? Well, we have a lot of work to do right away on this firearm, which we finally identified. An interesting thing about this arm is that it's actually not weak.

It's not, you know, most discoveries are discovered just above the noise level. But, in fact, this one was just hiding in a lot of stronger features in the galactic center. The center region is very complicated. But it's actually quite a strong feature, and so we're going to go after it with various other telescopes, the Green Bank Telescope to look at it more sensitively in the atomic gas. We're going to look for star formation in these arms.

We're going to try to follow them further out and see if they actually do connect to both ends of the bar. So we have a lot of work to do on this, and other people are joining the effort almost daily. Gentlemen, thanks so much for the galactic self-portrait. Sure. Thank you for having us on. Bob Benjamin is a professor of physics at the University of Wisconsin-Whitewater and has done this work using the Spitzer Space Telescope

and its amazing capabilities in the infrared. Thomas Dame is a radio astronomer with the Smithsonian Astrophysical Observatory and a lecturer on astronomy at Harvard University. Time for What's Up with Dr. Bruce Betts, the director of projects here at the planet. It is really hot. It's really hot. It's really hot. It's even hot back here. We should stop complaining, I suppose.

But yeah, it's really hot by most any standard these days. Well, he's the director of projects for the Planetary Society, nevertheless, in very hot Pasadena, California. Cool me off with talk of the night sky. Oh, it's going to be so cool. It's dark and soothing. Cool and relaxing. Ooh, ooh, ooh. It's like ice just a few inches under the surface.

Exactly. So, God, hey, they found ice. Isn't that cool? Isn't that cool? That is cool. It really is water ice. That is so great. And, you know, our own Emily Lakdawalla had found the images

and already had them in a little movie the day before the release. And so apparently they agreed when they did the calibrated images, the nice version, seeing ice sublimating in the trenches of Phoenix. She's good. She's just darn good. Spiffy Keene gets on those raw images right away. So, yeah, well, we should come back and talk more about ice because it's really a big deal. I'm just going to talk about it now for Phoenix because everyone expected the ice there.

But it was kind of one of their big goals was to study water ice. So it would have been kind of a bummer if it weren't. So the fact that they found it within a few centimeters of the surface and are already getting to it in no time. In no time at all. It's fabulous. Very exciting. And did you notice the picture of our DVD carrying a quarter million names and visions of Mars?

With ice on it now, or is it just dirt still? Dirt. They dropped dirt clods on it. Yeah, our dirty DVD. On Mars, as one of our listeners said. Leave it to the society. Well, we got some nice color pictures out there now of the dirt. Hey, you're going to run out of time to tell us about the night sky.

There's nothing really interesting. It's just the usual, huh? Yeah. Well, no, but let's tell you what the usual is. We've got in the early evening, we have Mars and Saturn over in the west growing closer together until their July 10th conjunction. And they get quite close. Right now you've got Saturn on one side of Leo's bright star, Regulus, and you've got Mars on the lower side, lower right, coming up. Mars kind of reddish, Saturn kind of yellowish.

It's a good time. Jupiter coming up in the mid-evening now in the east, brightest star-like object out there. It is high overhead in the pre-dawn. If it's the brightest star-like object and it's not moving rapidly that's jupiter on to this week in space history hey it's uh 150 years ago as i'm sure you've been waiting for the anniversary of the birth of george ellery hale famous astronomer guy who they named a 200 inch telescope after your telescope impressive yes my

my own personal telescope and uh and then uh sad note 1971 it was this week when the soys 11 crew died during re-entry remember that well yes on to random space fact touching isn't it it is i'll give you a quick one. And it's undoubtedly repetitive, but we're talking about water ice on Mars. And it's good for people to remember water ice on Mars acts like carbon dioxide ice on Earth, like dry ice. It goes straight from a solid to a gas. We say that it sublimates. And indeed, it was seen to be sublimating in those pictures from Phoenix.

So there's a quick random space fact. On to trivia question. We asked you, speaking of Phoenix, what's the height of the rocket that launched Phoenix? How'd we do? Kevin Hecht, he didn't win. But he wanted to know before or after fueling. To within the air accuracy I'm looking for, it doesn't matter. We have guys who, listeners, who really care about accuracy.

Because he was concerned the cryogenic fuel and oxidizer would shrink the rocket a little bit. Mark Smith went nuts because he couldn't find a schematic. So he gave three different versions. With fairing, sans fairing. But I'll tell you, Maurice Sluka, who was one of the first listeners, I think, to this show, hasn't won in a couple of years. Well, Maurice, you got it. He said that Delta II rocket that Phoenix sat on top of was 39.6 meters, or pretty much 130 feet tall. Maurice, up there in Prince George, British Columbia, our second Canuck in a row,

we're going to send you a Planetary Radio t-shirt. Congratulations. On to our next trivia contest question. I would like you to tell us the only astronaut or cosmonaut whose last name started with U. Of the more than 400 people who have flown in space, only one has a name that at least, you know, if it's an astronaut, then clearly it starts with U. If it's a cosmonaut, transliteration. But it should be pretty straightforward.

So go to planetary.org slash radio and get us that random trivia question answer. Got till the 30th, June 30th, Monday at 2pm Pacific time, of course, on the 30th. And you know what?

I just remembered that we said no more T-shirts for a while. What else can we give people? Should we just do a T-shirt and leave the alternative for next week? Let's do a T-shirt and leave the alternative for next week. We do have a lot of other cool stuff, but we failed our listeners by not digging into the inventory. You know, I think that because last week we said that, I think a whole lot of people said, oh my goodness, I've got to get in there

because we had a sudden rush of people to get in on the contest. All right, and definitely we'll do another Planetary Radio t-shirt for this contest. All right, there you go. All right, everybody go out there, look up at the night sky, and think about flossing. Boy, that's unpleasant. Thank you, and good night. You're just wishing you had been for the last six months,

aren't you? Yes. Guess where Bruce is going right after this. It's that chair you sit in and they stick things in your mouth and give you bad news. Well, we'll check in with him again next week to see if his mouth is still in working order.

I suspect it will be because he's the director of projects for the Planetary Society, and he joins us every week here for What's Up. Planetary Radio is produced by the Planetary Society in Pasadena, California. Have a great week. Thank you.