Wisconsin Public Television University Place Space Place  Robert Benjamin A Visitor's Guide to the Milky Way Galaxy  October 11, 2011 UP635    cc >> Kay Kriewald: This evening's speaker is Bob Benjamin. Bob has been with the Physics Department here at the UW-Madison and is currently with the Physics Department at UW-Whitewater, and he's going to talk to us about a reintroduction to the Milky Way Galaxy. [APPLAUSE] >> Robert Benjamin: So, I don't know if my title is appropriate for everyone. Maybe some of you have never met the Milky Way Galaxy before. [LAUGHTER] In which case this will be your first introduction to the Milky Way. But in case you've learned something about the Milky Way in the past, or you've heard me speak about it in the past, I wanted to reintroduce you to the Milky Way. What we know about our own galaxy, what's changed, what's changing, and the parts, the history of it and the future of it. So I've started with a picture by a Belgian painter, Rene Magritte. Rene Magritte was a surrealist, and the title, the words that were actually written on the painting above this apple, it says "Ceci n'est pas une pomme," which means "This is not an apple." Which seems like a strange thing to put right over a picture of an apple. And of course, the thinking that Magritte was trying to emphasize is this is, in fact, not an apple, it's a representation of an apple. 

And what he wanted to draw your attention to is not oh, what a pretty apple, but look at the lighting, look at the shading, look at the way the composition of this representation of an apple. So another image that he showed was this. It says ceci n'est pas une pipe. This is not a pipe. And he very famously said about this image, "Of course, it's not a pipe, you can't put tobacco in it." I love the title for this painting, the title for this painting is called "the Treachery of Images." Images can be misleading. And that's something to bear in mind as we get to the next image that I wanted to show you, which is this. And actually, this is not the Milky Way. And one way you know this is not the Milky Way is because you're not inside this picture. If you were inside this picture, you would be located about right here, and this is, of course, not the Milky Way. This is our best representation of the Milky Way or one that I was involved with in 2008 trying to synthesize the work of decades of astronomers with some new results, some very old results, but just an attempt to put it together as one sort of encapsulating image. 

Some people look at this and say, oh, well that's what the Milky Way looks like. What I want to tell you during the course of this lecture is definitely you can expect changes, some in the short-term future and even in the long-term. I would hope that this is not the final picture of the Milky Way. So before I talk about the Milky Way itself, I'm just going to remind you what a galaxy is. Then I'll do a little brief overview of the Milky Way, some of the basic facts about that. And then I'll talk about the history of mapping the Milky Way. It's a long and tortuous tale. And then talk about current and future discoveries. And so that's what I'm going to do. So here's a set of images, a rogues gallery of spiral galaxies. 

If you were trying to identify which galaxy was yours and you had looked, you can tell these look different. These galaxies all have different structures. Although, one thing that unites them all is they are, in fact, spiral galaxies. Galaxies are, by mass, about 9% of their mass is in the form of stars. Most people when they think about galaxies they just think it's a bunch of stars. But actually 9% of the mass of a galaxy is stars. About 1% is in the form of gas and dust in between the stars, and the remaining 90%, roughly, is this form that we call dark matter, which physicists have yet to figure out what that is, and I'll explain why we think that 90% of the galaxy is in a form yet unseen shortly. But by function, the way you can think of these is basically stars grouped together in large collections called galaxies with huge gaps between them. So the size of a galaxy is tiny compared to the vastness of intergalactic space. And for some reason nature has seen fit to organize our universe in such a way that we have these islands that we call galaxies in a sea of intergalactic space. 

Understanding how these galaxies gradually convert their gas into stars over time is really one of the frontiers of this decade, this century of astrophysics. People are trying to get a sense of what determines what a galaxy will look like, what determines how it evolves. So the sun is just one of 200 to 400 billion stars in the Milky Way Galaxy. Sorry I can't be more exact than that. But most of them turn circular orbits around the center of the galaxy. So the total mass in the form of stars is about 45 billion times the mass of the sun. So if you added up the total number of stars, we have 45 billion times more than just our single sun. Interstellar gas, which is only 1% of the total, is still five billion times the mass of the sun when you add up all the gas throughout the galaxy. And so that's, five billion may not be as much as 45 but it's a lot of stuff. And actually, this gas is the source of all the new stars that form in our galaxy. 

It's actually thought, though also, and this is another area of my research, that some gas continues to rain in from intergalactic space. So even though some of the gas gets used up, there's a steady supply of new stuff. And then finally dark matter. As its name indicates, dark matter has yet to be detected, but its presence is inferred by looking at the rotation of the stars and gas around the center of galaxies. Actually, this is better done in other galaxies but you can see evidence for it in the Milky Way too. If you look at the rotation, the velocity at which things are going around the center of the galaxy as a function of the distance from the center, if you tried to look at all the stars and add up their gravitational effects, you would get a curve that goes down like this. But, in fact, in most galaxies you have a curve that flattens out. Indicating if we tried to add up the gravitational effect of all the stars and all the gas, it's not enough to explain how things orbit around. And so astronomers gave this deficit, they gave it the name dark matter. We assume that there's matter there but it's not emitting light of any type that can be seen by astronomers. 

So what is this dark matter is, again, one of the challenges of modern astrophysics. It's still not solved. So another thing to know about galaxies in general, and spiral galaxies in particular, is they're very thin. You look at those beautiful spiral patterns but if you tip them on their edge, they would nearly disappear. They have an aspect ratio of a DVD. So if you look at the thickness of a galaxy compared so its radius, it's about the same dimensions in terms of the ratios as a DVD. So here's an edge-on galaxy. This is NGC 4565 and you can see it's fairly thin. It's not quite tipped exactly edge on. You can see that the center part, the bulger or bar of this galaxy pokes above and below, but most of the rest of the galaxy is actually very incredibly flat. 

So that's really important because when we live in the Milky Way, some of you have been at a dark site and seen the Milky Way crossing across the sky. Going out in July or August in a very dark site, just look to the south and you'll see a band arcing from the south above you. Around midnight in July is about the best time to look. And you'll see this band of light sort of diffused light, in addition to all the stars in the night sky, and that band is the Milky Way. And what you're seeing there is the fact that we are in a galaxy but we're right in the middle of it, and so when we look along the plane of the galaxy we see the Milky Way edge on. So essentially the Milky Way is an edge-on galaxy because we're tipped on the side. So this is a picture, an optical picture of the inner, plus or minus 30 degrees, 60 degrees, of sky. So if you were to just take your hands and hold them out like this, that would be about 60 degrees from this direction to this direction. So if you were to just look at the sky and hold your hands and look at a strip, this is what you would see if you had really good eyes. This is a photographic image in different colors, so it's better than what you could do with your eyes. 

But you'll notice there's all sorts of things. If you're an amateur astronomer, you'll actually recognize things in this. This over here is the Eagle Nebula which actually has a very, very famous Hubble Space Telescope picture. This thing right above the center is actually called the Pipe Nebula, and it's a dark cloud. It's actually a very dense cloud of gas nearby. And so we view the Milky Way galaxy from edge-on perspective. We're in the middle of it. We're looking along the galaxy rather than from face down. And this means we can't see reliably across the galaxy using optical light. This cloud, for instance, is only about 300 light years away so it blocks out everything beyond that distance. 

And 300 light years, as you'll find out, is nothing. So, of course, what you do to get your bearings in the picture of the sky is some people will draw in constellation boundaries. So I've drawn the constellations. Here's the constellation Scorpio and Ophiuchus and Scutum and Sagittarius. And an astronomer would prefer a more regular grid, so we use a rectangular grid. So this is the galactic plane and these are angles away from the galactic center. And, Kay, actually my laser pointer is dying. If you have an extra one that would be great. So the image I showed you, as I said, comes from the inner 30 degrees of the Milky Way. And so the inner 30 degrees on this artist's picture would be this section of the galaxy here. 

So the galaxy, now that I've shown you the night sky, if we try to go to this face-on view, which no one could ever truly have, the sun is located here, a distance of about 28,000 light years from the center of the galaxy. The sun is one of 200 to 400 billion stars. It goes around the center of the galaxy every 250 million years, approximately. And since the sun is four billion years old, approximately, it's made around 16 trips around the center of the galaxy since it was first formed. So we've had 16 galactic years, if you want to think of it that way. And so this is what one would see in this section of the sky right along the plane of the galaxy. This is our view in the optical. If you want to blow this picture up, and here I'm just giving you a few basic parameters about our galaxy, our Milky Way is what we call a barred spiral galaxy. And so as it turns out the stars within 14,000 light years of the center, which is out to about here, they don't orbit in circles. They actually orbit in long skinny ellipses for reasons that are still debated among astronomers. 

How do you form, how to you get a whole bunch of stars to all simultaneously start to change their orbits to produce these long skinny ellipses? But when you have many, many stars doing that, what you have is you produce an over-density of stars that produces a diagonal along the ellipse and so you get a barred galaxy. So the Milky Way, it turns out, depending on what wavelength of light you use, somewhere between 30% and 60% of galaxies have bars, these long sort of groups of stars crossing through the center of the galaxy. So there was actually indirect evidence for this back in the 1950s, but it wasn't until people could actually see evidence for the stars using infrared observations in 1991 that people said, okay, yeah, the Milky Way is definitely a barred galaxy. And here's actually some of that evidence. Here's the night sky again as seen in the optical in the inner part of our Milky Way galaxy. And I can actually over-plot. These color are basically lines of constant infrared brightness. So you can see if I go back, there's lots of dust. It's all sort of patchy. It does seem like there's a bright region here, but there's also a bright region here, so it's hard to really tell where there really are the stars because of all that dust in the way. 

But in the infrared, each one of these contours is a factor of two fainter than the next one. So basically from here to here is a factor of two, then a factor of two and so on. So you can actually see that most of the stars in the center of the galaxy are, in fact, here. That there's this concentration of stars in this area, and that is what is referred to as the galactic bar. Any questions at this point? I'm happy to entertain questions if people want to jump in. And so one of the things we've done with the Spitzer Space Telescope is we've surveyed this, again in the infrared, but even higher resolution, meaning crisper, sharper images. So this red boundary indicates the section of the galaxy that we've re-observed with the Spitzer Space Telescope. And I'll talk about that later. So the galaxy is barred. As it turns out the galaxy is also warped. 

I said it's like a CD. For those of you who remember vinyl records, since they warp a lot easier, leave a record out in the car in the sun and it can get warped. It turns out the Milky Way does, in fact, have a warp. If you get far enough out from the center, if you get out to a distance of about 42,000 light years, which puts you about here from the center of the galaxy, you can actually see that the gas starts to warp up or warp down below sort of the midplane defined by the center of the Milky Way Galaxy. So this is actually going to show, I'm going to show a little movie here. You're going to see a little region here of the sky and it's going to rotate around this image of the galaxy so you get a sense of where you're looking, and you're going to see a layer, the estimates of the layer of gas, the thickness and the location of the midplane. So you're going to see, there's two different versions, a white version and a red version based on different estimates, but you're going to see if the galaxy were completely flat, this line would just lie right here in the midplane. And then as we go around the Milky Way, you'll see the lines fluctuate. So now we're scanning across the galaxy. And you'll see that the layer of gas is actually going up above the midplane. 

So the galaxy is warping up out of the picture in this section. And then as we move towards looking at exactly the opposite direction of the galaxy, we'll have problems using this technique, I'll explain why shortly. So we lose it for a little bit, but then as we come out the other side, you'll see that this layer starts to warp down a little bit, especially in this region. And it doesn't warp down on this side as much as it warps up on the other side. So the warp is very asymmetric. And again, I'm sorry to say, just like the bar, the origin of the warp is still debated. What causes the Milky Way and other galaxies to have warp is still not clear. But the best guess is that it has something to do with the fact that the Milky Way suffers bombardment. Other small galaxies come near the Milky Way, and that tends to sort of unbalance the outer parts of the galaxy and they sort of flop up and down sort of like a sail. And I have seen some really nifty simulations of this process. 

So, that's sort of the overview of the Milky Way. If you remember nothing else, I hope you'll remember the Milky Way is a barred, warped, spiral galaxy. Okay? And so let's talk now about some of the history of the attempts to map it because as it turns out that spiral thing, that's sort of uncertain. Not that it's uncertain that it's a spiral galaxy because all flat galaxies are spirals, we observed that in other galaxies, but to map out the spiral structure has been a challenge. And it's sort of bedeviled astronomers for decades. So the early attempts to map out the galaxy, before we had the opportunity of seeing the gas, was done with counting stars, which actually, oddly enough, is a technique that I'm using now. But it works a lot better when you're not using optical light. Actually, I'll have a reading list at the end of this talk if anyone is sort of interested. But you can read about the story of William and Caroline Herschel. William Herschel was better known for discovering Uranus, the planet Uranus, which made him the preeminent astronomer of all times. 

He was the first sort of person to discover a planet in our solar system since the ancients. And so what he did is he would climb up a ladder, he developed a telescope, he'd look through the eye piece and he'd count of number of stars in a certain direction. He'd call out to his sister Caroline. She would mark it down carefully in the notebook. Then William would reset the telescope, point it another direction, count the stars, Caroline would record it, and then the two of them together tried to create a picture of the numbers of stars as a function of direction to get a sense of what, at the time, they thought the universe was because at that point they didn't know about the existence of other galaxies. All you could see were the stars. So this was the picture that actually in their 1765 paper they came out with. The galaxy was a big splotch where the sun was located here. So there were certain directions where you saw a lot more stars and other directions where you didn't see so many stars. And as it turns out, we now know that was entirely due to the dust. 

William and Caroline Herschel did not know that there was dust in space blocking our view. Now Jacobus Kapteyn, who was a Dutch astronomer, did this in the early 20th century, it's sort of a sadder story because he, in fact, new about dust, but when he did a very similar project as William and Caroline Herschel except rather than counting stars by eye he got photographic plates, the new technology of the day. By the way, Leiden is a terrible place to observe. So he would have his photographic plates shipped from nicer places with better telescopes. And then he would just sit there and count the stars, he and his assistants. And he tried to work out the shape of the galaxy. And, again, he got sort of an oval shape in 1922, but it turns out it was again the wrong picture because he didn't correct for the dust. Even though he was aware that there was dust out there, he didn't try to correct his data for dust distinction, the fact that dust obscures your view. Although, as it turns out, doing that correction would have been nearly impossible with the data at the time. So the first discovery of spiral structure, or the claims of spiral structures in the Milky Way, was actually a Wisconsin discovery. 

I just want to point that out. Of course, it was made by a professor from the University of Chicago, but he was using Yerkes Telescope here in Wisconsin in Williams Bay. And what he did is he measured the distance, he didn't count all stars, he just focused on these very bright, hot stars, in astronomy we call O stars, and he estimated the distance of those, tried to put a face-on map and he found that his O stars lay in three bands. So here's a picture of Yerkes Observatory in the winter. That's fun science in Wisconsin. And when Morgan, basically what Morgan did is he used this telescope in Yerkes, the refractor to estimate the distances so these hot stars. Now, I don't actually have the original data, though I'm looking to get that, but this is a blow up of the small section of the galaxy where the sun is, what he found is there's three groups, the stars lay in three bands which he named the Perseus, Orion and Sagittarius arms. The moment he discovered this, and he presented it in the 1951 meeting of the America Astronomical Society, he was a hero. It was the very first evidence that anyone had ever had for a spiral structure in the Milky Way. We knew the Milky Way was a spiral galaxy, but he had found the very first evidence for it. 

So that's why he got a standing ovation. He actually shortly thereafter had a nervous break down, and it took him years to actually publish the results that he presented in Cleveland. But any rate, that was a start. And it turns out he scooped a group that was about to map the Milky Way using a completely different technique. And that technique and that was group was based in the Netherlands. During World War II, Jan Oort, a very famous Dutch astronomer, he's responsible for many important discoveries in astronomy, but one of the things he's known for in particular is the pioneer of what we call 21-centimeter radio astronomy. A student of Oort's realized that, I said that there's all this gas between the stars and mostly it's hydrogen gas. And hydrogen being the most common element in the universe by 90% of the atoms are hydrogen atoms. And it turns out that hydrogen atoms emit radio waves. There's a particular, what happens is a hydrogen atom consists of a proton and an electron, and the electron actually can go from one state to a different state, we call it the spin states of hydrogen, so it can go from spinning this way to flipping over and spinning this way, and when it does that it loses a little bit of energy and emits it as light, and that light has a particular wavelength, a particular frequency, and the wavelength is 21 centimeters which put it into radio waves. 

So it's a radio emission. And because it's a particular line, you can actually measure the Doppler shift. Many of you know that if something comes towards you, like an ambulance, the pitch is shifted to a higher pitch, and if it goes away, it drops to a lower pitch. That's because the wavelength of the sound wave is changing as the source comes towards you or goes away from you. The same is true with astronomical sources. We have red shifts where the object is going away from you and the wavelengths are shifted to longer wavelengths. And we have blue shifts when an object is coming towards you and the wavelength is shifted to a shorter wavelength. And by looking at the wavelength that you actually observe in space, if it's not exactly 21 centimeters, it's either gas coming towards you or gas coming away. And so Oort had the idea that since you could have this line, you could look at the hydrogen emissions throughout the galaxy, measure the Doppler shifts and use the Doppler shift, the velocity to estimate the distance to the gas. I'll explain how that works in a second. 

So they originally, to do this project, they actually recommissioned a German radar dish that had been left in the Netherlands. So eventually Oort immediately started work on building his own radio telescope, but it turned out they thought, well, maybe we can use the Wurzburg dish. So they repurposed this radar dish and started to use it to search for 21-centimeter radio emission from space. And, in fact, it's one of the things I've learned over the past year because I've been trying to educate myself more about the history of this field, most of the major discoveries in galactic structure were all in place by 1958 with this technique. There was still a lot of fighting to come, but what we learned about mapping, all of the hints of galactic structure that were later found out were in place even before 1960. So, as I said, basically these works traced the interstellar gas, not the stars, but they didn't suffer from dust distinction. They did, however, have, as I mentioned, one fatal flaw. And the flaw is related to the following. I'm not going to go through the vector diagram, but basically here's the artist picture of the Milky Way, and what these lines show is at a given place in the galaxy we would observe a Doppler shift corresponding to the numbers, these are contours. So as an example, actually, so basically at any given point the rotation of the sun combined with the rotation of the gas means that the gas is either tends to be moving towards you or away from you with some speed. 

And by assuming that the gas rotates in a circular motion, so everything is assuming everything rotates in a circle, you can make an estimate you can convert velocity to distance with one major problem. And that major problem is if you look in this picture and imagine you look in a certain direction. So let's say that we look in this direction. 30 degrees to the right of galactic center. So again, if you're starting in the center of the galaxy, let's look 30 degrees. And we will see that gas here would be at minus 20, gas here would be at minus 40 kilometers a second, minus 60 kilometers a second. This is all do to the relative motion of the gas compared to the sun. But if we look in a given direction, we see a problem. And the problem is the following. Here, this point, gas would appear to be rotating at minus 40 kilometers a second. 

That is, it would be coming at you at 40 kilometers a second. But, along the same direction, at this point gas here also would appear to be coming at you at 40 kilometers a second. That is, blue shifted at 40 kilometers a second. So we had a fundamental unresolvable problem using this technique. And the problem was is that the only thing we can measure is the Doppler shift, the velocity. But if you want to convert the velocity into distance, you have a problem here. And the problem is that one velocity could be at two different places. And without any additional information, you don't know when you see an object or some gas moving at one velocity, you don't know where to put it. It sounds like a detail but it was absolutely, it was a show stopper. And the reason is because you had to make assumptions to convert your velocity to distance. 

Now actually there's some other problems with this too. I'm sure some of you have already picked up on it. The assumption that went into this was that everything rotates into nice circles. If that's not the case, you also have problems. And of course, there's certain directions you may notice where if you look you can't use this method at all. Like if you look at directly towards galactic center, the sun is moving this way, everything else is moving this way, so you are neither moving towards the material or away from it. You're basically moving in parallel tracks. So everything along the line connecting us to the center of the galaxy will have no Doppler shift at all. So there's no way to place that material. So you can start to get a picture of where things might be in certain sections of the galaxy but in other sections of the galaxy you are out of luck. 

Which didn't mean that people didn't try for the next 30 years. So center part of the Milky Way, basically notice that this only affects in this circle where you have this double value problem, and that circle is defined by the location of the sun. So everything from the sun's orbit inward becomes hard to map. Bummer. On the other hand, anything outside the sun's orbit, you can look in a certain direction and you can say, okay, say here or here or here there's a nice mapping, a one to one mapping of the velocity, the Doppler shift, to the distance. So you can map the outer part of the Milky Way with some confidence without having to make lot of assumptions. So that's what the regular astronomers did. They mapped the outer part of the galaxy first. They got some results. They were happy. 

Decades later people had forgotten some of their results and were still rediscovering them. But in the inner part it became a slug fest because astronomers had to debate each other on the quality of deciding between near and far among other things. And it all basically boiled over in 1985. In 1985, this is actually, I don't know how influential this paper really was at the moment. I don't know that the moment Harvey Liszt showed this figure in his talk, everyone threw up their hands, stormed out of the meeting and said no, we're not going to work on this problem anymore. But in retrospect, it was sort of the moment where people said, you know what, we're just not getting anywhere here. This actually shows several different astronomers' models of a spiral structure of the Milky Way. And the amazing thing about this picture, so Harvey basically collected all of these pictures from people's different papers, tried to put them on the same scale so that you could get a sense of how easy it was to compare them, and all the models except this one are based on the same data, okay? In other words, all of this group, and this group, and this group, and this group, and this group, and this group, all were looking at the same data and trying to convert it into a map of the Milky Way. And so you can see. That's a really frustrating moment. 

And in astronomy when you read the textbooks you always read about the victories, you very rarely read about the failures like this really sucked, and we just moved on. [LAUGHTER] But this is one of the failures of galactic structure. And what made it worse was the personalities. This was the frontier and there were some bitter, angry arguments over this. I'm told people would yell at each other and call each other names at the meetings. And so eventually people just gave up. I would never want to go to a meeting where people yelled at me and called me names, especially when it would be hard to actually uniquely convert the data. So we sort of limped along from 1985 to 2003. People collected more information. They would try to ask that we find more and more regions where stars are forming. 

We get their velocities, we try to plot them on to a map using this kinematic distance technique, but if I were to take away these lines, I think you would have a hard time placing lines on to this picture. The lines are really guiding your eye. So take away this line and would you really put an arm here? So this is sort of an attempt to compare the distribution of star-forming regions throughout the Milky Way with this picture. By the way, this is an interesting sociological problem. Is a bad map worse than no map at all? [LAUGHTER] Which would you rather be, trapped in a forest with no map or trapped in the forest with a really cruddy map? Well, I guess with a really cruddy map because what happened is over time for some reason people gravitated to this one picture because it was the one picture that was not based on the neutral hydrogen gas. It was based on trying to estimate the distance to star-forming regions. So people thought, well, maybe that's a better one to work with although there was no real good justification for that, but it's what we did. And you'll notice in the Artist's Conception that those arms are still here. So this should not give you a warm feeling about this picture. [LAUGHTER] Essentially, this is the best attempt. 

And this is the artist's conception again, done by Robert Hurt. And Tom Dame, who I will mention in a second, said, okay, in this picture I have shaded in color things where I think I can point at data and arguably make the case that this data should be identified as that arm. Now, the conversion of the arm to an actual position on the map, that is very problematic. But there are patterns in the data. It's that last step of converting the Doppler shifts into a face-on the map that has stopped astronomers. So Tom basically did this, and if you divide this arm and this image, you can actually divide it into two zones. Everything sort of in here was discovered before 1970. Ask actually, really, I think, before 1966. This last one might have been a late comer. So that is not to say that, so there are patterns in data that you can make an argument that we saw in the data then, we see in the data now, we think they're spiral arms, we're not quite sure how to map them, but if we were to map them, this is approximately where they would go. 

But something interesting has happened in the last few years. First of all, there should be something fishy about this. Now that I'm sort of pulling. First of all, notice anything that doesn't have color is imaginary. It was filled in by the artist. Except, as it turns out, this image was made in 2008. In 2004, this segment of arm was found by Naomi McClure-Griffiths in Australia. She found it in H1. We have yet to see it in other things that tend to trace spiral arms like molecular gas, molecules in the bright star-forming regions. But I look at the data and I say, okay, I see something in the data. 

So it's probably a spiral arm, we think. So let's go ahead and try to put it on to an image. And then you'll notice at this point that there's something interesting about things that have been discovered. Here's a line through the center of the galaxy. Everything that we found is on the near side. Essentially, there's this daunting realization on astronomers that we've only actually mapped, or at least attempted to map, half the galaxy. So there's the other half, the half that's beyond galactic center. Now this thing is shaded but it was actually discovered in 2008 just in time to be put on the artist's conception by about two and a half weeks. So Tom Dame, who actually discovered this, Tom Dame and Pat Thaddeus, they contacted us because they knew we were putting together the artist's conception and all the speakers at this meeting had been asked to contribute, and Tom Dame said, by the way, how is that image coming along? And he says, well, because I have something new to add to it. The interesting thing about this discovery is it's exactly symmetric to this thing. 

This was discovered in 1957. This was discovered in 2008. In terms of the properties of the spiral arms, how much gas do they contain, they're almost identical. And the reason this was not discovered before is because it's further away, it's smaller and it's fainter. And it fell below the sensitivity of many surveys of the Milky Way Galaxy. And then finally, this arm right here was discovered in 2011. And this is sort of an interesting case because you'll notice that the artist's picture was made in 2008 and this was discovered in 2011. And it turns out if you actually try to take the data and map it on to this galaxy, this is approximately where it goes. So this is a case where the art seemed to precede the science. And it may indicate something profound about the galaxy. 

There's always some fighting to do, but if astronomers settle down and think, okay, this really is a spiral arm, Tom Dame and Pat Thaddeus, once again they found it in neutral hydrogen gas, they actually found some molecular clouds along it as well, which is almost a lock for being a spiral arm in the Milky Way, I think. But the interesting thing about it is it turns out that if you say why did the artist do that? Why did the artist put something in there? All they did is they took this side of the galaxy and flipped it over. So what it seems to indicate is there's some hope, and, by the way, spiral structure is a field of broken dreams. So maybe 5 to 10 years from now I give this talk and I say, well, here's where we were wrong. But there is some hope now that the Milky Way has, basically, some underlying symmetry that things we have on the near side, we have things on the far side. And there's more to test. So for instance, if this is a real segment, this is still imaginary, but there are three groups now searching to see if they can see evidence. It would be hard because of the Doppler shifts that you would expect, but they're looking for evidence that maybe we can fill this arm in and connect it up with the arm that we believe is on the near side of the galaxy. Now, I know that this is a figure that would challenge many astronomers, but what this shows, this is two visions of that arm in terms of data. One thing it shows is the arm as a function of the longitude. 

So from 0 degrees to 70 degrees, so this section. And one of the things you notice is if you look at the arm and you say where is it above and below the midplane, you'll notice that this arm actually arcs upward as a function of longitude. What that means in terms of this picture is that this arm is actually coming out of the picture. It's actually coming towards you a little bit which is associated with that warp. Remember I said the galaxy is warped? This arm is actually in the warped part of the Milky Way galaxy. So that's part of the reason it was missed in the passed is it warps above the plane in the galaxy. And then this shows the direction you're looking compared to the velocity, the Doppler shift of the arm. And you can see it traces a nice pattern here. And the yellow dots are where Dame and Thaddeus have actually detected molecular clouds. Each molecular cloud, by definition, is what astronomers call a giant molecular cloud. 

To detect it on the far side of the galaxy there has to be a large concentration of molecules. So, big surprise. The Milky Way has new arms. I saw one of the headlines said Milky Way grows a new arm, or grows an arm. So this arm may have grown all the way out to here if we can start to fill in this center part. And so that's the hypothesis and we'll see where it goes. Another thing we're finding, and this is actually based on the GLIMPSE data, is that this is data from the Spitzer Space Telescope, and we have in our Spitzer Space Telescope, and you can go look on the wall on the way out there and see some of our data. If you find a tiny little fuzzy thing, you're seeing a star-forming region. And it turns out all of the tiny ones are completely uncatalogued. So Tom Bania, of Boston University, and Loren Anderson, his graduate student who's now at Marseille, they went to these objects, they used radio telescopes to try to measure their Doppler shift, and then they put their, they used the Doppler shift once again as best they could to map the distance. 

Interior to circle, this solar circle, you have the problem of not knowing whether to put it near or far. Although, they found a way to try to estimate whether an object is near to the sun, I apologize by the way, the sun is now at the top of this picture. So I flipped the galaxy over. There's a debate among us astronomers as to whether the sun is properly put at the top of the picture or at the bottom of the picture. Maybe after the end of this talk we can all vote and see. But these H2 regions, these objects are actually way far away from the sun, and, in fact, if you look at their positions, they actually are all above the plane. So once again, as you look in this section of the galaxy, the same section where Tom Dame and Pat Thaddeus discovered their new segment of spiral arm, the galaxy is warping up and the star formation regions are warping above the plane to the galaxy. So we're following up on this discovery with the Spitzer Space Telescope. The red shows, this is one of those "doh" moments because we actually, with the Spitzer Space Telescope, tried to cover, in infrared wavelengths, the midplane. So this is actually showing 360 degrees of sky and the red band shows the section that we observed. 

But we've neglected to take into account that when you get to-- we thought we were seeing, because the distant part of the galaxy is further away and will appear to be smaller an angle, we thought we were basically getting the whole galaxy in one shot. We neglected to take in account the fact that the galaxy curves up. And so these green areas are areas that we've been approved to now resurvey to try to find the star formation on the far side of the Milky Way. So this is an ongoing project that we're now doing. So I'm actually going to skip a little bit of this, but I do want to say the benefits of the infrared, since I have become lately an infrared astronomer, is that you can see through the dust. So the point is that radio waves pass through dust. On a foggy day your radio reception doesn't get worse. And on a foggy day you can still see the sun, although the sun tends to look a lot redder. And the reason sunsets, the sky is blue and sunsets are red is actually all because of the effect of dust and also molecules at some level scattering light. So when the sun is on the horizon or gets close to the horizon, you're looking through a lot more air. 

The blue light tends to get scattered away but the red light actually penetrates through. And so red light is absorbed less and scattered less than blue light. Infrared is absorbed even less than red light. So it turns out infrared light is also sometimes called heat radiation, but that radiation basically just goes right through dust. Firefighters actually use infrared goggles or infrared vision to see through smoke. And so that infrared light penetrates through. And infrared light also tracks things that are warm. So for instance your body, if we were to shut off all the lights here, your body would glow at about 10 microns, 10 micrometers, which is not visible unless you have thermal goggles. But you're glowing as bright as about a 100-watt light bulb. So if you can detect the infrared, you can see it. 

So the Spitzer Space Telescope is an infrared telescope, and this is just a fun picture, a fun NASA movie. It actually was put in orbit around the sun. So it was kicked out of Earth's orbit and in 2003, launched in August 2003, and so it's actually trailing behind the Earth, getting further and further away over time. In 2009, I guess, it ran out of coolants to operate some of the detectors, but other detectors, including the one that we've been using, continue to operate just fine. Ultimately, it will get far enough away from the Earth that it's too difficult to keep up radio contact to transmit the data, and at that point our mission is over. But then what it does is it will observe a section of the sky. This is supposed to be light coming in, not light going out. And then it aims its transmitter back to the Earth, transmits the data back to Earth, to NASA's Deep Space Network, gets a few instructions and then off observing again. We've been keeping this telescope very busy because we've been taking snapshot after snapshot after snapshot of the sky. Very fast cycling. 

And one of the things we've been able to do is count stars. Just like William and Caroline Herschel did in 1765. We actually, this is a picture, a section of the sky in the galactic plane in the mid-infrared in the optical. I never get tired of showing this image. I know some of you have seen this before but to me it's just so wonderful. This is actually about an hour or so of observing with an optical telescope down at -- done with some students that I had. And so now let me show the exact same section of the sky in longer wavelengths of light. What's called the near infrared. So it's just beyond the range of human vision. So now let me run this back. 

Pick one of the bright stars here that you see, particularly some of these redder bright stars, and let's run it backwards. And it should have gone away. So stars that were invisible in the optical start to become visible in the near infrared. And the wavelengths we're using are even further into the infrared. So now go ahead and pick some faint red star, and my favorite one, let's see, it's a little hard to see. Just pick a star, any star, and let's see what the sky looks like in the mid-infrared wavelength. So about the same wavelengths where your bodies are glowing the brightest. Now, I'd like to run it backwards. Pick one of the bright stars that you see here. These are among the more luminous stars in the Milky Way Galaxy. 

And now let's see what your star does if we go back to the optical view of it. Okay? So what the mid-infrared has allowed us to see is most of the stars in the galaxy. And what I am now doing and continue to do and have been doing for years now is dividing the sky up into bins, counting the stars as a function of brightness. And I don't expect you to understand or to explain this plot in detail. A friend of mine actually says it reminds him of a scan of his thyroid at the doctor. [LAUGHTER] But what this shows is, this the axis that tells you the direction you're looking, and this is the axis that tells you the brightness of stars that you're seeing. And the color basically tells you whether or not you're getting an excess bump of stars. And in particular, you'll notice there's a big blue band going through this image and this is sort of our newest discovery with this technique. This is actually evidence that the Milky Way, the disk of stars, you go out to a certain distance away from the galaxy, that the disk of stars suddenly comes to a fairly abrupt end. Effectively, an edge to the galaxy. 

Although the analysis of this was complicated by the fact that the galaxy is also warped. So sometimes you don't whether you're seeing the end of stars or that your line of sight just left a curved galaxy. But at any rate, this is actually not data, this is a simulation, a model I made. This is what that diagram would look like if there were no truncation, if the stars just sort of gradually died off as a function of distance from the center of the galaxy. And then this plot is what it would look like if you had some place at a certain distance out in the galaxy, in this case about 40,000 light years, where you just ran out of stars. And, in fact, this doesn't look, this looks fairly similar to the data. So the galaxy is filled with surprises. It's the only spiral galaxy where we can make out a wide variety of details. We can see the individual stars. We can look at how stars are forming. 

We can look at the vertical thickness of the bar and the spiral arms. We can't do that in any other galaxy. The Milky Way is unique in that regard. And for many years the attempts to work out this big picture have been stymied. First by dust and then by the Doppler shift problem for the gas. But we're starting to make progress again thanks to infrared telescopes that can see through dust and for people basically looking at the older radio data with new understanding like the Dame and Thaddeus results of the spiral arm on the far side of the galaxy. So this is the picture in 2008. And this is the picture in 2015. You might wonder why there's a Tyrannosaurus Rex here, and I have to thank Tom Dame for making this analogy. There's a question really how to think about this artist picture of the Milky Way. 

It has had an influence among astronomers. But on the other hand, we know it's partially made up and in many ways suspect. So Tom Dame said the way you should think about this is it's a model of the Milky Way in the same way that this is a model of the Tyrannosaurus Rex And you look at a model of the Milky Way and you think, wow, isn't it amazing that astronomers can figure that out. It's so cool that they can do that. The same way, you look at the Tyrannosaurus Rex and you think isn't it amazing that paleontologists can look at the bones and they can figure this out. And you'll say is that what you found, when you dug it up, did it look like that? And they'll say, well, no. And you'll say, really? What did you find? And they'll say, well, I found an arm. And they'll say, well, I assumed the dinosaur was symmetric, so I added the other arm. There's a lot of guess work and reconstruction. Just like you can only understand a Tyrannosaurus Rex in pieces and put it together, we're trying to understand the galaxy in pieces and put it together. 

And the data can change, the synthesis can change, so there could be changes ahead. I expect and I hope there will be. And I'll stop there. [APPLAUSE]