University Place

The Milky Way Galaxy: 1951 and 2018

– Thanks for coming to UW Space Place tonight. We have a guest speaker tonight, and that speaker is Professor Bob Benjamin from the UW Whitewater. Bob and I have a common interest in the history of astronomy as you can probably tell already. And so he’s linked together an interesting mix here of the history of our knowledge about the Milky Way galaxy. So please join me in welcoming Bob Benjamin. (audience applauding)

– Thank you. So, yeah, so what I’d like to do is tell you a little bit of a story about something that’s near and dear to my heart, which is the structure of the Milky Way galaxy. And I’m going to talk about– I’m going to try to focus on two years. I’ll have some things to fill in in the middle. You know, most of the time science is sort of incremental. We sort of gradually learn things, test things, move on. But then there are those years where everything breaks all of the sudden because of some advance in technology or some insight that allows people to move forward very rapidly. And I’m going to describe two years where that happened in the study of the Milky Way galaxy. The first year was in 1951, and the second year is this year, 2018. So I’m going to try to go through 1951 fairly fast so that I can get to 2018 because that’s all the cool stuff that’s happened in the last few months. So I’m starting here with a picture that’s actually not from 1951. It’s from 1950. This was a meeting held at the University of Michigan. It was a dedication of a telescope in Ann Arbor where many of the leading lights of galactic astronomy gathered to discuss problems of galactic structure. At the time of this meeting we knew certain things about the Milky Way.

For one thing, we knew that the Milky Way had different types of stars. Basically, Walter Baade, who was a German astronomer who did his work at Mount Wilson Observatory, he worked outside Los Angeles during World War II, and he had sort of the combination of being German, he wasn’t allowed to participate in the war effort, and being in Los Angeles during World War II meant that Los Angeles was blacked out. So he could use his telescope near Los Angeles to get really great images of the sky, and he discovered, among other things, that the Milky Way consisted of a different chemical, stars of different chemistry, indicating there was a young population of stars and an old population of stars. Another thing that we knew is that we did know that the Milky Way galaxy rotated, the stars in the galaxy were all rotating around a common center. They had measured that. And another thing we knew, thanks to Harlow Shapley, was roughly the size of our Milky Way. By looking at what are called globular clusters that are spread out spherically around the Milky Way, we had a sense of how big the Milky Way galaxy was. Now, lurking in the back row in this picture is Joel Stebbins. Joel Stebbins was a professor here at the University of Wisconsin. He was actually responsible for correcting Harlow Shapley’s estimate of the size of the galaxy.

So he’s most famous for basically shrinking, well, at the time, the universe. He shrank the galaxy back down a little bit by doing precision measurements of stars. And also from Wisconsin, although working at the University of Chicago, was Bill Morgan, okay? So they were all gathered together with many astronomers to discuss the topics of galactic structure. What was the Milky Way’s structure like? What would it look like if we were outside? And the big unsolved, still to be resolved question was, by looking at other galaxies we knew that flat galaxies tended to be spiral galaxies, but in 1950 there was no evidence for spiral structure. So, in other words, if we looked at the stars around our sun, we didn’t see very clearly that they outlined any spiral pattern. And so that was sort of the question of the day. And so before I get to what happened in 1951, though, I’m going to just, my overview of the talk here. First I’m going to give you a little bit of information about galaxies in general, and the Milky Way in particular. Then I’ll talk about this 20th century effort to map the galactic structure starting in 1951. And then I’ll describe to you what’s been happening over the past, well, the discoveries that have happened over the last past few months, although the effort leading up to that started in 2000.

And hopefully, by the end of this talk, you’ll be completely up to date on the structure of the Milky Way galaxy. So, what is the Milky Way galaxy? If you’ve ever been a dark site, you’ll know by looking up in the sky that you can see this band of diffuse light. That’s actually the light of a bunch of stars all blurring together. Galileo saw that when he pointed his telescope at the Milky Way. So here’s an image of the Milky Way. It’s that band across the sky. This is a picture taken from Oregon, so it’s fairly high latitude. One of the particularly interesting things is off to the right you’ll notice that it gets brighter, but it also seems to get somewhat dustier too. That dust, by the way, is dust clouds blocking our view into the center of the galaxy. And as you go further south on the Earth, that section of the galaxy, which is the direction toward the center of the Milky Way, will get higher and higher above the horizon.

So here’s the same sort of picture but taken from Thailand. And you can see that from the center of the galaxy, again where that band is the brightest, although, like I said, there’s dust clouds blocking parts of it, but you can see it’s further away from the horizon. And one of the best places to go is south of the equator. Here’s a view of the Milky Way galaxy from Chile where the Milky Way arches overhead, and the center of the Milky Way is pretty close to directly up. Okay? So if you really want a beautiful view of the Milky Way galaxy, and, actually, as it turns out, some companion galaxies, go south and you’ll really appreciate it. So, anyway, when you look at this picture of the Milky Way, it really looks like other galaxies that we see out in space that are tipped on their edges because galaxies can be in all different orientations. They can be face down or they could be oriented from where we are, edge-on. And so you look at this picture of the Milky Way galaxy from inside and you can compare it to this picture of a galaxy, of the galaxy NGC 891, and you see a lot of similarities. Essentially, what we see when we look in the sky, is we see an edge-on galaxy, our own, right? We’re in it and so we see it from an edge-on perspective. And so what we would love to have is the face-on perspective.

By the way, you know, if evolution had proceeded at a different pace, the sun actually bobs up and down through the galactic plane. And we’re right almost smack dab in the middle of the plane right now. If we had come, if our society had developed, you know, a few, I guess several million years later, we would actually be high enough out of the plane to get a little bit of a top-down view. It’d still be a long sort of path length. But we’ll never get this kind of view. These are other galaxies. And one of the questions is will the real galaxy that the Milky Way is stand up? This is the goal to understand, because once we understand what our Milky Way looks like, we can make sense of all the details we see in our own galaxy to understand other galaxies. That’s really the basic idea. So galaxies by mass, it turns out, are 9% stars, 1% interstellar matter, that’s the gas, the nebulae you see in a lot of the pretty pictures of Milky Way, and, embarrassingly, 90% dark matter. So dark matter is simply a placeholder.

We see stars orbiting the galaxy, we see stars and gas orbiting other galaxies as if there’s more mass than we can detect. So if you look at the amount of light those galaxies are emitting, it indicates more mass than we can actually detect. And that mass has been called dark matter and people have searched for that dark matter so far with no success. But galaxies by function are something just– It’s an island, okay? Galaxies are an island of stars and gas surrounded by empty intergalactic space, okay? So where these little islands of light scatter throughout the universe. And understanding this function, how galaxies form and change and convert their gas into stars, is one of the big goals of astrophysics now. So to understand what I’m going to say later, I should also provide you a little bit of information about how galaxies form. This picture shows what’s called a space-time diagram. The time axis is the right. So as you move from the left to the right, you’re going from earlier times in the history of the universe to later times. And then the picture shows what did the universe looked like at different times.

And so you can see on the left, the left is the Big Bang. And the Big Bang, about 13. 7 billion years ago, there, all of the matter in the universe and all of the space in the universe was very small and compressed to an almost infinitesimal size. At some point, that started to expand. Initially it expanded very rapidly, the period we call of inflation. And then the gas started to cool, eventually cooling enough so that the electrons and the protons which had always been bouncing around without being attached to each other combined into the first hydrogen atoms. And the hydrogen atoms and the helium atoms eventually started to collapse into structures that formed the first stars about 400 million years after the Big Bang. From that point on, as gas started to fall into these structures, these protogalaxies, and galaxies started to merge with each other, the current structure of the universe took shape. So I’m going to show you a computer simulation. And the reason I wanted you to see this is because it’s very important to remember when we study our Milky Way is that our Milky Way has a history to it.

It’s probably eaten other galaxies. It’s probably been interacted with other galaxies as well. So what you’re seeing here is the yellow spots indicate light and the blue or the red indicates– Sorry, the blue indicates cold gas. And so after the Big Bang the universe sort of turned into this network of gaseous filaments, and along these filaments you started to condense galaxies. And when enough gas collected, stars started to form. And then those collections of stars started to interact with each other gravitationally and merge. And out of all that process, somehow we got our own Milky Way galaxy. Now this is a picture of a galaxy that we’ll never have. So how did we get it? I’ll tell you a little bit about the story later, but this is an artist’s picture of the Milky Way by Robert Hurt. I contributed to putting this picture together.

I sort of served as a consultant. And so it’s trying to take data from various sources and synthesize it into some picture, some schematic of what the Milky Way might look like. The order you see here, the beautiful spiral pattern, is probably a tendency to impart too much order to the galaxy than it actually has, okay? So I actually do believe that the galaxy would look a little messier than this picture. But many of the things that you see in this picture I can link back to data. In other words, I can say, okay, when we look at data we see some structure that we put into this picture in a certain way, so in this picture, the sun– Oh, the first and most important thing you’ll notice about this picture is that there’s a bar, a band in the diagonal band in the center of the galaxy. That’s called a galactic bar, and the reason it happens is because stars in the very center of a galaxy, and this is true of many spiral galaxies, have started to move in elliptical orbits rather than circular orbits. So the combination of all the stars in the inner parts of galaxies moving in elliptical orbits means that you get a band of starlight as opposed to a sphere of starlight. But out where we live, which in this picture would be where this yellow dot is, out where we live, which is 27,000 light years from the center of the galaxy, we’re going in roughly a circular orbit. Okay? So somewhere in between where we are and then the center stars tend to switch from circular orbits to elliptical ones, okay? And, again, that’s a process we are trying to understand in astronomy. Now, something I’d like to do with this picture I’ve never done before, some of you may have heard me show this picture and sort of give the overview of the Milky Way, but one thing you can think about in this picture is that the sun is going around the center of the galaxy roughly once every 250 million years.

That’s how long it takes to complete one orbit. And if you think of that as a galactic year, okay, since the sun is about 4. 5 billion years old, that means the sun is about 18 galactic years old, right? So the sun or our solar system is just ready to vote in galactic elections, okay? And so, you know, for many of the times around the center of the galaxy, our Earth was a cooling, you know, hot, magma-filled, cooling object. But about the next to last orbit around the sun, okay, so orbit 17 out of 18, was what was called the Cambrian explosion. So at the beginning of our 17th orbit was when a lot of the life on Earth came into being, okay? Now, I won’t, but I just want to start then with the last orbit around the sun. The last orbit around the sun actually started off with not a good note because this was actually the largest extinction that we think of in the history of the Earth, okay? So the Permian, let’s see, I think it’s called the Permian-Triassic extinction event, happened just as the sun was starting on its last, its most recent orbit around the center of the galaxy. So that’s the Triassic period. After the Triassic period, you had another extinction event where nearly 80% of life on Earth or 80% of the species were eradicated for some reason. That left the room clear for the dinosaurs to flourish. And the dinosaurs then lived during the Jurassic period and then the Cretaceous period.

And I think most of you are aware that the dinosaurs are not with us except in bird form, okay? And so the dinosaurs were wiped out by another extinction event that was basically three-quarters of the way through the most recent galactic year. And then, after that, we have various epics where mammals started to take over. And all of human history is in the last fraction of a second of that galactic year around this orbit. Now, as you look at this picture and think about the sun going around the center of the galaxy, something that’s important to remember is everything else is going around the center of the galaxy too. In fact, the bar that you see here would have rotated. It would have actually rotated slightly slower than the sun. So the sun would overtake the bar and come around. So everything is rotating, I would love to actually take this picture and put it into motion with our best understanding of galactic dynamics. So that takes us to 1951, okay? Now the dinosaurs are dead. We’re here.

We developed civilization. 1951. We don’t know where spiral structures are but we’re about to find out. 540 days was when that meeting in Michigan occurred. 540 days before the announcement of spiral structure in the Milky Way. At that meeting, Walter Baade, who I pointed out, described how you might map spiral structure in the Milky Way, and he was motivated by a picture similar to the one in the background. This picture actually shows M31, the Andromeda galaxy. And if you look at this picture, you’ll notice that there are little red spots along it, okay, that trace out a band. Those red spots are H2 regions. And so what Walter Baade suggested to a couple astronomers, one of whom took him up on it, was why don’t we try to find those H2 regions in the Milky Way, those ionized nebulae, try to get their distances by measuring the distance to the stars that are heating the gas, and then make a map.

And you should see that those H2 regions lie in some sort of band, which would evidence for a spiral arm. And so it turned out that suggestion from Baade– Oh, I should point out, even at that point Walter Baade realized that this was going to be a limited picture of the Milky Way because of the dust that blocks our view very far in different directions in the Milky Way. So at this meeting in 1950, he says of one thing we can be certain, on account of the heavy obscuration of the plane of the Milky Way, we will best get only a glimpse of short pieces of spiral arms. And one guess that we can probably safely make is that our sun is located in a spiral arm because the brightest B stars, our sort of hottest stars, and dust around our sun in all direction. I know this argument will not overly impress you and that you would like to see the arm or a piece of it demonstrated at oculus. So would I. At oculus means obvious to the eye. That’s what people wanted. They wanted a plot or a diagram or some sort of measurements that made it obvious, oh, yeah, there’s a spiral arm. And that’s what appeared for the first time in 1951.

24 days before spiral structure was announced one of the most famous astronomers of his time, Jan Oort, for whom the Oort Cloud is named, the comets at the edge of our solar system, he, who’s shown on the right here, visited Yerkes Observatory, which is in Williams Bay, Wisconsin. You can tell it’s Williams Bay, Wisconsin, on December because of the snow, okay? And so Oort was in the United States to give a lecture at the American Astronomical Society meeting. He was receiving an award, and he came to Yerkes Observatory, which was one of the premiere observatories in astrophysics at the time here in Wisconsin. He talked with several of the people there. The people, seen from left to right, is Subrahmanyan Chandrasekhar, who was famed for figuring out the maximum mass of a star before it would collapse, the so-called white dwarf, the Chandrasekhar limit; Bill Morgan, who I’ll say more about in a second, was the person who found the first evidence for spiral structure; Gerard Kuiper, who studied the solar system and other things, the Kuiper belt of our solar system is named for him; Bengt Stromgren, a professor at the University of Chicago who understood the theory, the physics behind these ionized nebulae. So this is a picture of them sitting on the steps 24 days before Morgan announced his discovery of spiral structure, which he shared with Oort. At this time Oort had a discovery of his own to share, which is the Dutch had just barely lost to the Americans but then proceeded to outpace the Americans in detecting radio waves from hydrogen gas. And so Oort was primed to start to study the structure of the Milky Way galaxy using radio waves from radio telescopes to look at the distribution of gas in the Milky Way. Both the first detection of the 21-centimeter line and the discovery of the spiral arms via these H2 regions were presented at the same meeting, but Oort and Morgan were discussing it a month in advance. Okay? So then, on December 25th, because apparently astronomers would travel to meetings on Christmas day back then, on December 25th Morgan arrived in Cleveland ready to present his discovery of spiral structure, which I’ll show you in a second.

He met with a pair of astronomers who are famous for their work on the structure of the Milky Way galaxy, Priscilla and Bart Bok. This is a picture shown of them later on in 1958. Actually, Priscilla Bok was a professor of astronomy at Smith College when she went to Leiden for an astronomical meeting. Bart Bok was a graduate student who was assigned to be her guide to Leiden, to just show her around. At the end of the one-week long meeting he proposed to her. The graduate student proposed to the professor. And she said, “I don’t think so. ” And so she, but they continued to write and correspond and eventually they did decide to get married. And they worked as a team in studying the Milky Way. And they actually wrote a popular science book starting in 1941 on the structure of the Milky Way galaxy.

So if you want to learn the history of the structure of the Milky Way galaxy, reading the successive editions of this textbook, or not really a textbook, more of a popular science book from 1941, 47, 57, 74, and 81, sort of you can see the progression of the efforts to map out the Milky Way galaxy. So the night before the meeting Morgan met with them. They were in a dorm room at Case University. And Morgan went through his entire presentation with them while they were sitting on his dorm room bed. And Bart said, you know, at the time he said, “I was really grateful that he showed this to me in advance. ” He says, “I had been trying to do a similar thing, “but I was using a different kind of star. “I was barking up the wrong tree. ” So Morgan had beat him to it, but Bart Bok was magnanimous in this victory of Morgan’s. So at the meeting, on December 26, 1951, Bill Morgan presented a synthesis of several different things that all tied together to show the first map of the Milky Way spiral structure. And almost all of this data came from Wisconsin.

So the first thing was that they used a camera to find the H2 regions on the sky. Like, where were those regions of ionized gas on our sky? This is a wide field camera that they had available at Yerkes. It was eventually, that data was published in 1952, but it was using a green sign handy camera at Williams Bay. Then Morgan identified the stars, the hot stars associated with those H2 regions, those nebulae. And then, using a spectra, basically taking spectra, looking at the light from the stars and examining the light as a function of wavelength to tell you what kinds of stars you had. So most of that was done at Yerkes, although some of it was also done at Warner Swasey Observatory in Ohio. But then the last thing you had to do to do this process to get the distance to these stars is you had to make some estimate of how much dust there was between us and the star because a star could be dim because it was far away, or it could be dim because there was dust, okay? And you had to know which reason, why was the star dim. Was it dust or distance? And so with the dust estimates that came from Washburn Observatory here in Madison, he was, Morgan was able to correct for the dust absorption and estimate the true distance to the stars. So he presented a synthesis of these results showing the two bands of H2 regions, the B stars, the first evidence for spiral structure. And this is actually the way Morgan remembered it.

He said, “Oort had introduced me, and when he sat down to listen, he sat down in my seat. It was one of those steeply sloping classrooms at Case with all the seats way up high. Well, when I got through, the first thing was I had no place to sit down. And the second thing was that people started to applaud by clapping their hands, but then they started stamping their feet. It was quite an experience.” Bart Bok proclaimed this a triumph on Morgan’s behalf and astronomers were really excited. It was the first evidence, the first map of spiral structure in the Milky Way. The map that was presented at the meeting was actually never published in its original form. The original map that was presented in Cleveland survives as a board with nails pounded in it, okay? And then little Styrofoam balls stuck on the nails. And that board is hanging in the Adler Planetarium.

If you ever go to Chicago, you can see the original map of spiral arms from Morgan as a board with nails, okay? A section of that board was photographed for the April 1952 edition of Sky and Telescope. So those little cotton balls that you see there is a band of H2 regions. That band is one of the spiral arms of the Milky Way, the local arm. And then the Sky and Telescope magazine put a picture of Walter Baade’s spiral arm section from M31 to say, “Look, they’re just the same.” Now, this is actually a picture of the board I got from Adler. This is what it looks like now. And eventually Morgan did publish this in 1953 with two colleagues, Albert Whitford and Art Code, who were both professors here in UW Madison. So you’ll notice the original publication was actually a combination of Yerkes and Washburn Observatory. So that was the first evidence. And this picture here, if you look, you’ll see that the H2 regions, the nebulae sort of trace three bands in that picture.

I hope everyone can see that. So if you look, I didn’t add an annotation, but if you see, there’s a lower band, a middle band, and an upper band. And that was actually the first evidence for spiral structure. As it turns out, I just wanted to say as a little historical footnote, this is actually a picture from that camera. So the picture on the left shows the all-sky view of our own Milky Way galaxy. The picture on the right shows a picture at the time of NGC 891. But that camera sort of got misplaced or lost. Yerkes didn’t have it. They didn’t seem to be aware they didn’t have it anymore. But it resurfaced here in Madison.

(laughing) And the reason was, the picture here on the left shows the picture of the camera taken at Yerkes shortly after its construction. But a professor here in Madison, Art Code, borrowed the camera to go down to the southern hemisphere and take pictures of the sky from the south. And then after he came back from South Africa, he kept it. And they used it for lots of different reasons, including the site Pine Bluff Observatory. And so it was then put in a closet in graduate student offices and over time it just sort of got misplaced and no one knew what it was. You know, people sort of knew Art Code used that camera, but it was not realized until Jim Lattis and the former director of Yerkes and I put it together. It was like, “Oh, my gosh, we have the camera that made “the H2 regions for the first map of spiral structure. ” So that camera has now been returned to Yerkes Observatory where it’s in the closet again. (laughing) But hopefully it won’t disappear again. So, you know, Morgan got his discovery in just in time because he actually was quickly outstripped in this attempt to look at the structure of the galaxy by the Dutch effort to measure the radio waves.

So the picture, the little table on the left here, of this diagram, shows the timeline. The Americans, Purcell and Ewing at Harvard College Observatory, detected a 21-centimeter radio wavelength line in March 1951. The Dutch detected theirs in May 1951 after they consulted with the Americans. The Australians did a crash course and made their own detection in August 1951, and all three groups published the first detection of these radio waves of hydrogen gas in September of ’51. The spiral arms were announced in December 1951. And then it was off to the races. Basically a whole frontier of mapping the Milky Way was opening up before everyone’s eyes. The first effort to map out the hydrogen gas was done in the Netherlands using not actually an official radio telescope. They were using a radar dish leftover from the German military on the beaches of the Netherlands. So they retrofitted this German radar to turn it into a radio telescope while they were building a proper radio telescope.

And so the original survey of hydrogen gas in the Milky Way was done using a leftover German radar dish from World War II. And so the Morgan, Whitford, and Code paper that I showed you came out in May 1953. The first gathering of astronomers to start to coordinate their efforts on galactic astronomy happened in June 1953. That was IAU symposium number one. And then the famous paper that provided the very first map of the spiral arms of the gas of the Milky Way in hydrogen gas, neutral hydrogen gas, in 21 centimeters was presented in ’54. And that was the paper in which the first spiral arms that Morgan presented were named. So Morgan didn’t name them in 1951. But once they thought they were seeing the corresponding things in the radio waves, they consulted and so I’ll just read from the paper that Van de Hulst, Muller, and Oort wrote on their radio surveys. “After consultation with Morgan, the following designations are proposed: for the arm passing through the sun, the Orion arm, for that passing through H and Psi Persei, the Perseus arm, and for the first arm encountered when proceeding in the direction of the center, the Sagittarius arm, being inside the sun’s distance from the center of the latter does not appear in our present article.” They stuck in their initial analysis to the gas outside the sun’s orbit because inside the sun’s orbit there is complications.

And so this was their attempt to match their data with the stars, the dots that Morgan had measured the distances to. So I’m going to have to speed up a little bit, otherwise I’m not going to get through the thing. So let me just say a couple things about mapping the Milky Way. There are several different techniques to mapping things with distance. Okay? And the most fundamental and the most reliable and astronomers won’t argue once you have this measurement done right is what’s called parallax. Okay? Since this being videotaped, I normally have people demonstrate parallax. I’ll show you a video of this later, how it works. But, basically, it just uses the idea that if you have two different lines of sight on the same object, like if you put your finger in front of your face and look at, line the finger up with something in the distance and look at it with one eye and then switch to the other eye, you will see the position of your finger shift against the background wall. And then if you move the finger closer to your face, you’ll see it shift more. So, basically, the amount of shift is related to the distance to your finger, and you can use that to work out the distance to the source, finger in this case, using geometry.

Now, it turns out it’s very hard to make those measurements, and the best attempt until just recently was a European mission called Hipparcos. But that had very, very limited range. The white circle indicates roughly how far could you map with parallaxes before 2018. Then you could what’s called the standard candle, and that’s what Morgan used. The idea that if you have a source of known luminosity and it appears to be a certain brightness, you can relate the known luminosity to the brightness by assuming some distance. In other words, if you have headlights very close, they’ll be very bright. If you have headlights very far away, they’ll be very faint. And by measuring the brightness or the faintness of the headlights one can infer the distance of the car if you know the luminosity of the headlights. You do have to correct for dust. And because of the dust, you’re limited to how far out you can see because the Milky Way does have a lot of dust in the disc of the galaxy.

And then the last method is using what’s called the kinematic method. What you’re doing is using a Doppler shift of something. So you see some object moving and if you assume it’s moving in a circular orbit, and you can probably already see where this is going to be a problem, if you assume it’s moving in a circular orbit, you can turn the Doppler shift into an estimate of distance. So I’ll just demonstrate this with this little picture. This shows the artist picture with a contour map of how you would convert a velocity to a distance. So, for instance, if you saw a cloud in 30 degrees to the left of galactic center, that’s along that orange line, and it was observed to have a velocity of plus 40 kilometers a second, that would come from measuring the Doppler shift, it would have to be where I’ve put it in this diagram because you can see that’s where the 40 line intersects the orange line. However, the big problem of this method is that you can see– And so the idea here is the cloud is moving in a certain direction around the center of the galaxy, the sun is moving in a certain direction around the galaxy, and that means the sun is moving partially towards the cloud and partially to the left, the cloud is moving away from the sun and partially to the left, but the net effect is that we are not moving towards the cloud as fast as the cloud is moving away from us. So this cloud would appear to be moving away from us just due to galactic rotation, and it would be red shifted, it would be Doppler shifted to the red. And so that’s how this method works. The problem of course is that there’s another place in the galaxy where you get the same Doppler shift.

So basically one measurement of velocity could put the cloud at two different places. And that made this method for mapping the galaxy incredibly complicated. Not to mention there are certain directions where it won’t work at all. If you’re looking towards the center of the galaxy, everything is moving parallel to you because all the orbits are parallel, like in this picture they’d all be traveling to the left, and so there’s no Doppler shift from anything towards the center of the galaxy. And so it really makes it hard to map the Milky Way reliably. Nevertheless, astronomers made pictures like this. This just shows the direction, the galactic longitude, which is just the direction around the galactic plane, you’re looking, and the Doppler shift velocity on the Y axis. The color bands or the color represents the brightness of the emission. I’m not going to describe the details here but if you look at this plot, you might say that I see patterns. I see bands.

I see hooks. I see little segments. Okay? And each of those bands or hooks or little segments is what astronomers interpreted as sections of spiral arms. And from the sort of painstaking analysis of diagrams like this, astronomers came up with a map of galactic structure, which I’ll share in a second. But the most important thing to know, I think, about the Milky Way is that the Milky Way does have this bar, and that was actually not really firmly confirmed until the 1990s when we could use infrared astronomy to see the stars in the center of the Milky Way for the first time. So this actually shows a computer simulation of what that bar would look like viewed from different angles. You can see as seen on edge-on the bar actually has a thick part and a thin part. This is, again, a simulation, not the actual data. And now the rest of the galaxy has been removed and just the part, the stars that would be participating in those long, skinny, elliptical orbits are shown. So you can see what does the structure of the bar looks like in a computer simulation of a barred galaxy.

And the Milky Way bears a very strong resemblance to what computer models predict for this bar of the Milky Way. So I’m going to skip over the next section and jump to 2018, where we are right now. The effort to map out the Milky Way was something in 1951 people were incredibly optimistic and excited about. By the mid-1980s to late 1980s, people had given up. The data that we had at the time and the techniques that we had at the time were really showing their limitations, and people could start with the same data and end up with different maps of the Milky Way, which led to a lot of fights, a lot of bad blood, and eventually a lot of people saying I’m going to move on. In fact, during this time, Priscilla Bok said to her husband Bart, “Bart, get out of spiral structure. (laughing) “The future is in star formation. ” And Bart followed her advice and moved from studying the structure of the galaxy to studying how stars form in these gas clouds for most of the rest of his career. Only in the last year before his death did he return to spiral structure a little bit. So in 2018, we now have an opportunity to restart some of what we’re doing here using a satellite that was launched by the European Space Agency, called Gaia.

Gaia is providing precise measurements of parallaxes and proper motions, parallaxes I explained to you. Proper motion just means how do stars move across the sky due to the relative motion of the sun and those stars for 1. 5 billion stars. This is not a photo of the satellite, this is an artist representation of the satellite. But this satellite was launched in 2013, and it did something very unusual for astronomical missions, which is the team that was processing the data from the satellite was just working to create a data product. They wanted to make the data as good as possible. They didn’t skim anything off the top. They didn’t hold onto it until they milked it all dry of all the cool discoveries. They just released a data product to all the rest of us astronomers at one moment on April 25th, 2018. So they worked for years to get ready.

They worked from 2013 until this last April to get the data ready for a total of 22 months of observations. They were measuring the positions of billions of stars multiple times to see how they shifted back and forth due to parallax and how the stars drift across the sky due to their space motion. And so, again, the picture here, this little animation shows how parallax works. So, in the lower picture, you have something like the Earth would be the blue dot and the yellow thing would be the sun, and the red dot would be the object you’re looking at. And you can see as the Earth goes around the sun, the red line lines up with different points along the yellow screen, okay? And so what it would look like on the sky is shown on the upper panel. So you can see when the object is close, it shifts back and forth a lot; when the object is nearby, it shifts back and forth just a little. And so that’s how parallax works. The nearest star has a parallax of less than an arc second. But Gaia can measure parallaxes as small as five to 10 micro arc seconds, giving them the ability to estimate distances, with some bigger error bars at those things, but out to 60,000 light years, okay, which is across to the far side of the galaxy. And the angle they’re measuring is roughly the diameter of a single strand of human hair 600 miles away, okay? So you can make measurements that precise with this telescope.

And a talk on the technology of this telescope is fascinating. They have to do all sorts of amazing– Every time a dust grain hits the telescope they have to fire a little bit of fuel to stabilize the telescope against the momentum from the dust grain. Every time a piece of dust hits the telescope they have to correct for it. The other thing it can measure is space motions. So this is actually showing the stars of the Big Dipper. It shows how they, over time, going into the future, sorry, actually going back into the past and then going into the future, how the Big Dipper would warp over time, okay? Because all of those stars are all moving around the galaxy just like we are and our vantage points are going to change on those stars, and so we’re going to see them shift with respect to each other. And if you know the distance to the stars as well as their motion across the sky, the motion across the sky is just an angle change over time, but if you know the distance to the star, you can get the true space velocity to the star. And so that is actually what this satellite was measuring for 1. 5 billion stars, okay, which is a significant fraction of the total number of stars in the Milky Way. So I’m going to show you a Gaia representation of the same thing.

Although, they’ll show you the stellar positions moving but grossly exaggerated just so you can see what’s going on. So in a second you’re going to see all these stars start to move around in circles. And you’ll notice they’re all moving around in circles in the same way because what you’re really seeing is the Earth’s motion around the sun. And so the Earth’s motion around the sun is causing the nearby stars to move in sort of big circles and the distant stars to move in small circles. Now, if you attach the brightest stars to constellation, you can see how the constellations would wobble if they were all much, much, much closer to the sun than they actually are because, again, this is exaggerated for effect. By the way, at the very end of the talk I’m going to leave a link to, you can actually download a simulation to fly through the galaxy using the most modern data to see where the stars are and watch them go by using the modern precise positions. Now, this thing shows how the constellations would move over time. The proper motions of the stars going forward into the future. So, of course, this is grossly exaggerated but you get the idea here, that these motions allow you to measure the velocities of stars. So the data was released on April 25th, and I was forced for the first time in my life to start following things via Twitter.

(laughing) And so this is a list of all the papers that have come up using Gaia data since April 25th. It’s a gold rush, okay? And everyone is rushing to find their things. I had something I wanted to do with the Gaia data and I ran into problems and I found out eventually, just last week, that other people have run into the same problems I did. They ended up publishing a paper but they ran into the same issues. So what I’m going to do is describe some of, I think, the more interesting discoveries that have been made since April 25th. I’m going to give a little bit of an overview first and then talk about some of the more interesting ones. One thing that we can learn about is the evolution of stars themselves. One of the things we get out of Gaia is with precise positions we can measure precise luminosities because we know the true distance to the star now, and so if we know how bright it appears, we know how luminous it truly is. And so we can put lots and lots of stars with very, very well measured luminosities onto a plot and see what happens. So one of the things that we found is we have a complete census of white dwarfs now for the very first time.

We have evidence that white dwarfs have merged so that you have two types of white dwarfs. White dwarf stars are the leftover remnants of ordinary stars, like the sun. The sun will be a white dwarf remnant. But if the sun had a binary companion, you might end up with two white dwarfs, and you can actually see evidence that there are white dwarfs that have merged. So you can have, basically, single white dwarfs or merged white dwarfs at the end of the star’s life. Okay? For those of you who have had or remember introductory astronomy may remember something called the main sequence. If you haven’t, hold on. But it turns out there’s a gap in the main sequence. They’ve detected many, many hyper velocity stars. These are stars that are moving through the galaxy at such high speeds that they are on their way out of the Milky Way.

That is, their velocities are higher than the escape velocity for the gravitational pull for the Milky Way. So these stars are on their way out. How did they get going so fast and why? So we see white dwarfs being ejected after their companions go to supernovae. We see regular stars also traveling at hyper velocities. That’s harder to explain. There’s all sorts of stellar clusters that have been identified. There has also been some clusters that have been de-identified. There were some clusters of stars that people have been studying that turned out to be just chance projections of stars on the sky. So there have been lots– Luckily there’s been more clusters discovered than demoted. But so we’re still going upward on the cluster count in the Milky Way.

But the study of clusters in the Milky Way is really undergoing a revolution. You can look at the stellar kinematics of the galactic disc. You can look at how stars orbit around the center of the galaxy and see, are they moving in circular orbits or are they not? And the answer is, at least for the sun, by and large they’re moving in circular orbits but there are groups of stars that are deviating in systematic ways from those circular orbits. There are streams, lots and lots of star streams in the Milky Way, and we don’t know why yet. There’s also one big merger, which I’ll get to in a second. We can measure, by measuring the orbits of stars around the Milky Way galaxy, we can constrain the dark matter because those stars are basically orbiting under the influence of gravity, and so we have new constraints on how the dark matter is distributed around the Milky Way. Spoiler: so one thing that people have always wondered about the dark is, is it round around the Milky Way or is it flattened like the disc of the Milky Way? And it seems to be, as best you can tell, pretty round. So the dark matter is distributed in a round sphere around the galaxy. There’s been lots of satellites and streams found around the galaxy. My favorite one is called Phlegethon.

It’s a stream of stars 60 degrees long across the sky. That would be about this long, if you just hold your arms out. And it’s probably a disrupting globular cluster. If you don’t know what Phlegethon is, it’s actually the river in the underworld in Greek mythology. And the kinematics of stars in the local galaxies, you can actually see evidence for the stars in other galaxies rotating for the first time. Well, not for the first time. Actually, there was some evidence from Hubble before that. And then I’ll talk about spiral structure in a second. So we’re going to rush through a few of these. So for those of you who know introductory astronomy know that stars are distributed across a diagram if you plot brightness on the vertical axis or temperature or spectral class on the horizontal axis.

This is called the– And the band of stars that goes through the middle is called the main sequence. That’s where stars sit when they’re burning hydrogen and helium during their normal main sequence lifetime. And so that sequence is now understood to be a sequence of mass. In other words, on that diagonal band, stars that are on the upper left part of the band are stars that are very massive, very luminous, very hot. They burn through their material very quickly so they’re very short-lived. Then if you go down to the bottom right along that band, you have stars that are faint, low mass, low temperature, and a star that was formed even at the beginning of the universe wouldn’t have finished burning up all its fuel. In fact, it would have billions of years to go. So one of the surprises is we thought that was a continuous sequence but if you actually use the precise positions and you zoom in on the section of this diagram shown with the arrows, the data that was used from Gaia, does everyone see the picture on the right? Do you see a little bit of a band cutting through that? There’s a gap in the main sequence. This was completely unexpected. Although some people said, oh, well maybe I thought of it.

It seems to indicate there’s a temperature in luminosity that stars just avoid. And it’s thought to be telling you something about the structure of these low mass stars and how they go from, how they carry their energy out to the surface. Basically, stars at that mass range should carry most of their energy from the center of the surface using convection, just like boiling and water. So it turns out there’s a moment where you go from partially convected to fully convected where the stars just might never be able to be at a combination of temperature. But in that band there’s a 24% deficit of stars, or that gap, and that’s probably an underestimate because every effect that you could think of that would affect this would tend to fill the band in, not empty it out. Okay? So that was a surprise on stellar evolution. Hyper velocity stars, I’m not going to tell you about all the discoveries of hyper velocity stars, but this one I found particularly interesting. There were three white dwarfs that were found to be traveling at somewhere between 1,000 and maybe 3,000 kilometers a second. For reference, it takes about 500 kilometers per second to escape the gravitational pull of the Milky Way. And so the curves here sort of just show the probability of a star having, of these three stars having a given speed because you can’t measure it exactly.

You have to make some assumptions. But one of the stars, shown in the picture on the right, appears to have come from a supernova or from the vicinity of a supernova remnant. By the way, I didn’t read this until today, and I was like, oh, my gosh, we know a lot about that supernova remnant, I should jump on that. So there’s a supernova remnant at a galactic longitude of 70 degrees and then below the plane of the galaxy. That’s shown with that green circle. And it looks like the star that they’re seeing that’s a hyper velocity may have been ejected in that supernova explosion. So that’s sort of an interesting find. Globular clusters, some of you may know in astronomy we have something known as globular clusters. These are dense balls of stars, super dense, and thought to have formed early on in the history of the Milky Way galaxy. Now you can measure the motions of all those stars across the sky and you can see evidence that they rotate.

Okay, so you can actually see that ball of stars isn’t just sort of randomly, the stars aren’t randomly swinging around. There actually is a net sense of rotation for the stars in the globular cluster which gives you some sense of how that cluster might have formed in the first place. The kinematics of stars in the galaxy, this is an attempt to look at just what are the stars doing in terms of their vertical motion. And this is, actually, this was done over a weekend by a graduate student in Italy. She just took the data and said, I wonder what I can do with this, and out popped the paper the next week. And so this is an attempt to basically look at the distribution of young stars in the Milky Way. The sun is in the center of the picture, and the galactic center is at zero-zero. But it shows, the top two panels show the density of young stars, like the O and B stars, and then the density of the old stars, these are red giant stars. And then the bottom plot shows the vertical motions of those stars. And what you can see is that, as you go away from the center of the galaxy, which is to the right on these plots, that the color becomes increasingly red, and that means the stars are getting a net vertical velocity, meaning stars are not just orbiting in these nice, flat orbits.

There seems to be this vertical motion, and that is actually correlated to the fact that we know the Milky Way galaxy is warped. In this direction that they’re seeing that the stars are moving upward, we actually know that the mid-plane of the galaxy warps up. So we’re going to get a sense of why does the galaxy warp and how does the galaxy warp by studying this data. And this is probably to me the most interesting and surprising result. And this has been found by two groups. They found evidence that there are stars spread out through the inner part of the Milky Way whose orbits are all off. That is, they actually are not rotating in the same sense as all the other stars in the Milky Way. They’re actually going slightly the opposite way. Slightly what’s called retrograde. And so the plot here, the color, the orangish color shows the density of those stars plotted on the sky.

And then the blue stars show particularly useful stars, what are called (mumbling) stars, that are part of this weird kinematic motion. But what they think is happening here is they’re seeing a collection of stars by looking at their difference in these stars orbits compared to all the other ones, that we’re seeing the evidence of a galaxy that was swallowed by the Milky Way some billions of years ago. A galaxy roughly the mass of the small Magellanic cloud rotating a slightly retrograde to the Milky Way’s motion, and some of the globular clusters, which are indicated with numbers, may actually be part of this swallowed galaxy. When this object was first found it was called The Blob. Then another group found it and called The Sausage. And so now the first group has decided they wanted to call this Gaia-Enceladus, that this was the name for the galaxy that the Milky Way swallowed. And in case you’re wondering where that comes from, Enceladus was one of the offsprings in Greek mythology of Gaia, the Earth, and Uranus, the sky, okay? So Gaia-Enceladus. But that seems to be the biggest, in my opinion, the biggest discovery so far. But another neat one shows the motions of, the actual motions of stars in the Andromeda galaxy, M31, and in M33. So you can actually measure the motions of stars across the sky.

The blue lines here are the model predictions, and the black lines are what they’re measuring with the data. So you can see the agreement is not perfect and there’s some areas where you think, well, I’m not sure, but the data will only get better with time, and we’ll have more and more reliable maps of how rotation is actually occurring in other distant galaxies. And then, finally, the spiral structure of the Milky Way. One of the biggest things I wanted to do with this data I knew is I wanted test Morgan’s maps of the spiral arms. I was ready to jump on the O and B stars and measure their distances precisely and make sure Morgan was right back in 1951. As it turns out, Gaia has a problem in this area. The very brightest stars have some problems in the measurements of the parallax. They’re hoping to correct it for the next data release. But it means that the bright OB stars are not reliable, do not have reliable distances right now. And so there was a group here that started off with about 5,772 stars.

Those are the red dots. And they were comparing to the models of spiral arms, the Sagittarius, the local arm, and the Perseus arm that I showed you in the beginning. Although the data points shown here are now maser parallaxes testing the distances of these arms. But they wanted to do it with those stars, but they found that if they only used the O stars with 10% distance uncertainties, that left this– Sorry, this plot is after they do the first cut where they remove the most uncertain stars. So they cut their sample down from 5700 to 2800. But then if they only use stars with distance uncertainties that are, I think, and they remove all the stars that are moving fast, anomalously fast. By the way, I mentioned the hyper velocity stars, there are other stars that are basically called runaway stars. They’re not going to escape the galaxy, but they’re anomalously fast and they seem to have been kicked out of star formation regions. So you remove those and they are left with 241 stars. Here’s their map with 241 stars.

And they say, well, you know, even those have some problems so let’s go down to another cut, and that’s all we have left. So I’m not sure we can test Morgan’s picture of spiral arms quite yet. But hopefully by data release three we will. So that’s it. Things to remember from this talk: the Milky Way is a strongly barred, partially mapped spiral. It’s about 10 billion years old. I didn’t mention that but it’s true, roughly. The sun formed approximately 4. 5 billion years ago and it’s made 18 galactic orbits. You know, one thing I do hope you remember is the first evidence for spiral structure was here from Wisconsin and using distances to local star forming regions.

Much of the mapping of gas was done, the hydrogen and molecular gas, but the Gaia measurements are going to open up a new frontier to give us new tests, new challenges, new problems, and hopefully new maps. And then, if you are interested in downloading that Gaia sky thing where you can fly through the Gaia data, the modern data released in April, I’ve included the link down here at the bottom so you can download it and try to set it up on your computer and tour the galaxy yourself. Thank you. (audience applauding)