This is knowing the universe, the history and philosophy of astronomy. I'm Chris Impey, distinguished professor of astronomy at the University of Arizona and the Astronomy Department. And in this module, we're talking about Galaxies and the expanding universe. We start with our view of the Milky Way, here, a beautiful night sky seen from Death Valley in California. One of the few dark places in the continental United States spectacular galaxy, it's hard to see this view. When I poll my class and I teach a large class, only about one in 10 students have ever seen the Milky Way. I recommend it highly, go out to a dark place and see this site once, you'll never forget it. The regions of the Milky Way are well known to us because it's the galaxy we live in, our home galaxy. We live in a disc of stars near one of the spiral arms. The diameter of the disk is about 100,000 light years and it's very thin, a thickness of only 1000 light years. We count about 400 billion stars in our galaxy, most of those are low-mass stars are red dwarfs, much lower in mass than the sun. The sun is in the disk, about 28,000 light years out from the center. So we're in a suburban region of the Milky Way disk, astronomers classify different populations of stars in the Milky Way according to their amount of heavy elements and their ages. In the disk, they characterize it as population one, a younger generation of stars that are often still forming around us. The Orion Nebula is one example of a nearby region where stars are forming right now. These regions contain a lot of gas and dust and also open clusters, the pleiades is a prominent example in the night sky. The central bulge scene in the animation here as the orangish region at the center of our disk contains both populations one and two, a mixture of old and young stars. The older stars are noticeable because they're redder. The halo, which is not really visible in this animation because it consists of a very low density population of stars in an almost spherical cloud around the disk. These stars are older and the halo, as it contains almost no gas or dust. This is where we would find the globular clusters, huge concentrations of up to 100,000 or even a few million stars in swooping elliptical orbits around the galaxy itself. There's also a medium between stars called the interstellar medium. This is the stuff between the stars and stars are made of this. It's mostly a vacuum, about one atom per cubic centimeter, that's incredibly rarified. The air we are breathing is about a billion billion times denser. The interstellar medium is made of about 90% gas, mostly hydrogen and helium and 10% tiny dust particles. The dust particles are made of heavier elements, they include carbonates, silicates, calcium, magnesium, and even some metals. It turns out that we occupy what is called the local bubble, where overlapping supernovae that went off in the distant past have created a region that's 100 of times less dense and 100 of times hotter than the normal material in the interstellar medium. We only recognize this with x-ray observations in the last decade. Here are the orbits of stars in our galaxy. The stars in the disk all orbit the galactic center and in the same direction, like an old style phonograph record, and like a record that was left on a shelf in the sun and got corrugated or warped. The stars in the disk also have upward and downward motions or Z-motions, seen as the wavy yellow lines in this diagram. It's greatly exaggerated. However, the Z-motions are actually quite subtle and small, but they're detectable and we can measure them for our own star and for nearby stars. So stars weave their way in and out of the disk of the galaxy, and that's what gives the disk its thickness of hundreds of light years, otherwise, gravity would make it even thinner. Stars in the bulge in the halo, the bulge being that central concentration of reddish-orange old stars. And the halo stars, we can't really individually see in any image, or all orbiting the galactic center in elliptical orbits, with different orientations and directions of motion essentially random. And so a snapshot of it frozen in time would give the sense of a spherical or nearly spherical cloud. These stars have pretty high velocities because they plunge in and out of the disc of the galaxy and go to quite large distances from the center of our galaxy, several 100,000 light years. We also have spiral arms. These spiral arms are prominent because these are the places where stars are forming. Density waves trigger star formation and the molecular clouds from which stars form are concentrated in the arms. There's plenty of source material for making new stars-short lived, hot O and B stars delineate the arms and make them blue and bright as seen from above. Of course, we live in our galaxy, so we don't really get to see the arms beautifully, we'd have to look at another galaxy to see spiral arms. We live fairly near the Orion arm of our galaxy and that is of course, a distance of about 1000 to 1500 light years away, so we have a close view of those young stars forming in a nearby spiral arm. The spiral arms are caused by a pattern or a density wave, not by the actual motions of those stars. If you followed the stars in their orbits, the Milky Way has rotated dozens of times since it formed, so the spiral arms would get hopelessly wound impossibly small to see. The fact that we only have a handful of spiral arms shows that they represent a pattern, rather than an actual physical rotation of those individual stars. Basically density, waves move in and out of these stellar regions and when the density is higher, star formation is triggered, it then fades as the density wave passes through. Here's what the galaxy looks like in a little snapshot where we zoom in on an area around the sun. You can see that we and the stars near us are doing a quarter of a billion year orbit of the Milky Way center. That's interesting because the Earth is 4.5 billion years old, so that means we've been around the center of our galaxy 18 times since the earth formed. We've also been in and out of the disc of the galaxy because of those Z-motions. But if we measure the speeds of stars in three dimensional space near us, as in the little inset cube, they would be pretty much random. Some stars moving towards us, some away from us, all of the region around us, participating in this large circular motion around the center of our galaxy. Astronomers have opened up their eyes to other forms of electromagnetic radiation. And here we see the disk of the Milky Way, looking towards the Milky Way as we might with our eyes seen in different wavelengths of light. Third from the bottom, you see the visible light, the familiar view to the eye. And you can see the raggedy nature of the Milky Way where the thin disk of stars is not obvious, and that's because those dark regions are dust, the dust obscures the stars, it doesn't mean there are no stars there. To see the dust through the dust and to see the disk more clearly, go up to the infrared view of our galaxy, and there you can see, seeing through the dust with infrared eyes, the thin disk of a galaxy and then the bulge at the center. At other wavelengths, both long and short, we see different forms of radiation. The 21 centimeter radiation at the top is from very cold gas, atomic hydrogen, this is the gas that will make stars. And at the bottom, we see x-ray and gamma ray emission, which tend to come from individual high energy events, like supernovae and neutron stars, and high energy regions of gas caused by excited phenomena of dying stars. Here, in summary form, are the main views of our galaxy, the visible light that we're familiar with at the top. The infrared light coming from stars, the radio emission cold coming from molecules with temperatures of tens of kelvin above absolute zero, and the x-ray emission from hot gas with temperatures of hundreds of thousands of kelvin. The galaxy is rotating, and herein came the first surprise about the Milky Way. We just thought the Milky Way was made of stars and gas and dust through most of the 20th century. But later in the century, observations emerged which would create a conflict and a problem in astronomy, a problem is not still fully solved. Think of the rotation speed of stars moving out from the center of our galaxy. If we imagine that most of the gas mass of the galaxy is concentrated in the galactic center, which is true because the bulge is there, the nucleus of the galaxy is there, and the densest regions of the disc are there, then the rotation speed of stars moving out towards the edge of the disk should decline. An interesting and extreme version of this is the solar system, essentially all the mass of the solar system is in the sun and the orbits of the planets and their speed and distance from the sun is seen on the left. It's given by kepler's laws, going at the inverse square root of the velocity. So that's what we would expect if the mass of the galaxy was concentrated in the center. But on the right, you can see what's called a rotation curve, that is the velocity of stars orbiting the center of our galaxy as we move out from the center of the galaxy in tens of thousands of light years. And you can see the sun, but you can also see that the velocities do not decline, they actually increased slightly, going outwards. That's completely counterintuitive, and it's completely against the expectation if most of the mass of the galaxy was in the visible stars that we see, because most of the galaxy is interior to the sun's orbit. So what is possibly making the motions of stars so far out go so fast. We can even use these motions to calculate the mass of the galaxy using Kepler's Third Law. And here we get from the simple calculation, two times 10 to the 12 solar masses, that's two trillion solar masses for the total mass of the Milky Way galaxy. But we can count stars in the galaxy, and the total number of stars down to the dimmest red dwarfs, which are not very massive stars at all is 10 times less. In other words, there's far too much mass in the galaxy to be accounted for by the stars that we see. The galaxy you see in the lower left is Andromeda, so that's a nearby spiral, which we know to be doing the same thing, its rotation speeds are also high, moving out through the disk. And in the lower right, you see an animation. On the left side of the animation is what you might expect if most of the mass of the galaxy was in the center or near the center. You can see the speeds near the center of orbiting stars are quite fast, but at the periphery they're slower as kepler's Law would predict. On the right is the situation we actually see, where the speeds near the center are fast, but the speeds going further out barely declined at all. That is the counterintuitive result that astronomers found about 40 years ago. This is evidence for what we call dark matter in the Milky Way. Again, the motions in the Milky Way are faster than would be predicted by kepler's Laws, if the mass was distributed the way the light is, and where the stars are. There's really only two logical explanations for this, one of them is pretty ominous. It's that we don't understand gravity on the scale of galaxies, and that means that Newton's law of gravity is wrong. Now, remember, this would have nothing to do with Einstein's theory being a superior theory of gravity, because the regime we're in is of weak gravity, general relativity doesn't apply. Newton's law should work perfectly well, and it does everywhere we've tested it, but it's important to remember that Newton's law has only been tested as an inverse square law with the law that Newton published all those years ago in the solar system itself. That's the only lab we have to test Newton's law. We don't have a direct test of the force of Newton's law moving with distance on a scale of the galaxy, unless we measure the galaxy itself. And so the attraction of an unseen form of matter, causing high speeds in the Milky Way is attributed to something we call dark matter. And you see a little diagram on the right, indicating that the luminous material of the galaxy disk and halo is embedded in a much larger halo of something we don't really understand that we just call dark matter. So alternative is if we trust our theory of gravity, there's 5 to 6 times more dark matter than luminous matter in our galaxy. The luminous stuff is confined to the disk and the halo. The dark matter is found in the halo and far beyond the luminous disk to much larger distances, and that was an extraordinary surprise. It took a while to realize that this had to be the case. So what is the dark matter? The short answer, we don't know, but there are now several lines of evidence indicating that six or seven times more invisible than visible matter is in our galaxy. Again, the diagram recapitulates the issue that if you thought all there was in the galaxy was the stars that it's made of. You'd expect them to move slower as you move out from the center of our galaxy, but that's not what we observed. The rotation speed does not decline with radius, either violating Kepler's law or Newton's law of gravity, or having an unseen halo of dark mass driving the faster motions, and that's the end of this topic.
