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Lecture 23 Galaxies and the Universe FINAL EXAM NEXT WEDNESDAY! ► The Final is next Wednesday, May 9th. 5 to 7 pm!!!! ► Required Pencil/pen Equation sheet ► Recommended Calculator Scratch paper TEST DETAILS ► Covers Primarily Lectures 18 - 23 18 multiple-choice questions (2 points each) 12 True/False questions (2 points each) 6 Short answer/problem questions (5 points each) Equation sheet (10 points) ► Mostly conceptual 2/3 new material 1/3 previous material (basic level) Discovery Of The Galaxies ► ► ► ► The discovery of the nature of the galaxies happened only in the last century. During the 1920's, Heber Curtis and Harlow Shapley debated the nature and size of Milky Way Galaxy. Shapley thought that the Milky Way was very large and the "spiral nebulas" were smaller and within it. Curtis argued that the Milky Way was considerably smaller and that the galaxies were island universes comparable in size to the Milky Way and very far away. The Galaxy Takes Shape ► By the 18th century, astronomers think the stars are distant suns. ► But how are they distributed through space? ► Herschel and others attempt to make the first map. Here’s Herschel’s Map: By Early 20th Century: ► ► ► Astronomers have an estimate for the size of the Milky Way. Estimate based on careful study of star brightness and motions. Shaped like a flattened disk Sun is near the center. Empty void outside of the star system. Galaxy is 32,000 LY across But That’s Not Right! ► Modern View: Milky Way is 100,000 LY across! Surrounded by countless other galaxies! ► Why were we wrong? ► How did we find the correct answer? Why Were We Wrong? ► Based on one incorrect assumption: ► Space was totally empty. No dust or gas to obscure our view. We can see to great distances. Harlow Shapley ► Studied two types of common star clusters. Open clusters Globular clusters Open Clusters ► Young stars. ► Hundreds to Thousands of stars. ► Lie all along the Milky Way’s band. Globular Clusters ► Old stars ► 100,000s or millions of stars. ► Very dense. ► About half are clustered around Sagittarius. Shapley’s Assumption ► Star clusters orbit center of the Galaxy. ► Globulars centered around Sagittarius, so they must orbit something in that direction. ► The Galaxy’s center is NOT near the sun, it’s far away, in the direction of Sagittarius. But How Far Away? ► Shapley found the clusters were centered on a point about 25,000 LY away, in the direction of Sagittarius. But How Far Away? ► ► Notice how small Herschel’s Milky Way Map is when put onto Shapley’s Map. In fact, if Hershel’s map represents one end of the Milky Way, Shapley’s implies the Milky Way is over 100,000 LY across! Turning Point ► Major turning point in astronomy. ► It was discovered that the sun was not at the center of a small cloud of stars. ► It was at the edge of a HUGE galaxy of stars. Cepheids In The Andromeda Galaxy Edwin Hubble used the 2.5 meter (100") telescope on Mt. Wilson in 1925 to observe Cepheid variable stars in the Andromeda Galaxy and showed it was over 2 million light years away - well beyond the limit of the Milky Way Galaxy, proving Shapley wrong. ► On the other hand, the Galaxy was much larger than Curtis imagined. ► But What Is Our Galaxy Like? ► The Milky Way is an example of a special type of spiral galaxy called a barred spiral. But What Is Our Galaxy Like? ► 1. Many of the Milky Way’s stars are in the disk of the galaxy. This disk is 100,000 LY across, but very thin (~1,000 LY). The disk contains the spiral arms. But What Is Our Galaxy Like? ► 2. In the center of the disk is the nuclear bulge, which has a foot-ball like shape. This elongated shape is often called a bar, which is why the Milky Way is called a barred spiral. But What Is Our Galaxy Like? ► 3. Surrounding the galaxy is the halo, which contains the globular clusters, a low concentration of wandering stars, and very little dust and gas. The Nuclear Bulge ► The most crowded portion of our galaxy. ► Rather old – little gas and dust for new star formation. The Mass of the Milky Way If all mass were concentrated in the center, the rotation curve would follow a modified version of Kepler’s 3rd law rotation curve = orbital velocity as function of radius Dark Matter? ► By studying how the stars in our galaxy move, we can tell that most of the galaxy’s mass is in the corona, in a form that doesn’t give off light. ► It is called dark matter. ► We’re not sure what it is, although there are several very good ideas. Galaxy Diversity ► Even seemingly empty regions of the sky contain thousands of very faint, very distant galaxies of various different types. Galaxies ► Galaxies are classified based on: Size Morphology – What the galaxy looks like Galaxy Size ► There are two basic galaxy sizes. ► Giant galaxies are those like the Milky Way. ► 10,000’s of LY across. ► Have billions of stars. Galaxy Size ► Dwarf galaxies are very common. ► For every giant there are dozens of dwarf galaxies. ► A few thousand light years in diameter. ► Contain millions of stars. Morphology ► There are three basic galaxy morphologies. This is a Spiral galaxy 78% of all galaxies are spiral galaxies What Are The Spiral Arms? Our best theory is the density wave theory. ► Spiral arms are huge shockwaves that travel around the galaxy. ► Trigger new star formation in their path. ► “Lit up” by lots of newly formed O and B stars. Makes them bright and look blue. ► O and B stars quickly die as the density wave moves on to another part of the Galaxy. ► This is an Elliptical galaxy 18% of all galaxies are elliptical galaxies Elliptical Galaxies ► Ellipticals are much like a spiral’s central bulge with no disk. ► Very little dust and gas to form stars. ► Filled with old stars: yellow and red giants. ► HUGE range in sizes! ► Ellipticals range is size from the smallest known galaxies (1,000 LY across and about a million stars) to the largest known galaxies (nearly a million LY across with tens of trillions of stars). This is an Irregular galaxy 4% of all galaxies are irregular galaxies Irregular Galaxies ► Irregulars do not have a well-defined shape. ► Very, very rich in dust and gas. ► Lots of new star formation. Dark Matter In Galaxies ► Based on studies of how galaxies move in clusters: All galaxies contain dark matter. Much of this dark matter is in a halo surrounding the galaxy. Evidence For Dark Matter ► Galaxies rotate as though most of their mass were outside the luminous part of the galaxy. ► In galaxy clusters, galaxies move much faster than expected. Something other than luminous matter must be creating gravity. ► Gravitational Lenses – gravity from dark matter bends light, acting as a lens to magnify and distort the images of galaxies behind the dark matter. Here’s Gravitational Lensing: These arcs are the distorted, magnified images of background galaxies. The distortion is caused by the huge amount of dark matter in the galaxy cluster in the foreground. Where Do Galaxies Hang Out? ► Galaxies gather into galaxy clusters. Loose Groups Most common type of galaxy cluster is a Loose Group. ► Typically contain less than 10 giant galaxies, and a few dozen satellite dwarfs. ► Low space density. ► Infrequent encounters between giant galaxies. ► We are in a Loose Group called the Local Group. ► Our Galaxy Cluster: The Local Group Milky Way Small Magellanic Cloud Large Magellanic Cloud Andromeda galaxy Superclusters: Clusters of Clusters ► ► Individual galaxy clusters group together to form superclusters. Typical supercluster: Rich, massive galaxy cluster at center. Surrounded by dozens of “satellite” loose groups. ► Our supercluster is centered on the Virgo Cluster. Called the Virgo Supercluster. Filaments, Sheets, and Voids ► One final “level” of structure: Superclusters themselves cluster into filaments and sheets separated by vast voids. Appears to be very little of anything inside the voids. The Expanding Universe ► In any direction one observes, the clusters of galaxies appear to be moving away from the Earth. ► Their spectra are all red shifted. ► The farther away the galaxy is, the greater is its red shift (and therefore velocity). ► The Earth only appears to be in the center of the expansion. ► This is just what one would expect if we lived in a uniformly expanding Universe. Static Universe Expanding Universe Cosmology ► In modern science Cosmology is the overall study of the universe. It includes: Overall structure of the universe. Overall properties of the universe. Origin and early history of the universe. Ultimate fate of the universe. Cosmology Tries To Answer Some Very Basic Questions ► How big is our universe? We don’t know for sure. Visible universe is almost 30 billion light years in diameter. Universe is much bigger than that, but how big exactly isn’t known. Olbers’ Paradox Why is the sky dark at night? If the universe is infinite and filled with stars, then every line of sight should end on the surface of a star at some point. The night sky should be as bright as the surface of stars! Solution to Olbers’ Paradox: If the universe had a beginning, then we can only see light from galaxies that has had time to travel to us since the beginning of the universe. The visible universe is finite! Cosmology Tries To Answer Some Very Basic Questions ► Is there a center or an edge to the universe? There is no center in space, although in one sense there is a center in time. There is no edge to the universe, although that doesn’t necessarily mean it is infinite. What Is The Evidence? ► It starts with the discovery that the universe is expanding. ► First observed by Edwin Hubble in the 1920s: All but the closest galaxies are moving away from Earth. The more distant the galaxy, the faster it moves away. The Expanding Universe This does not mean that we are at the center of the universe! You have the same impression from any other galaxy as well. Finite, But Without Edge? 2-dimensional analogy Surface of a sphere Surface is finite, but has no edge. For a creature living on the sphere, having no sense of the third dimension, there’s no center (on the sphere!): All points are equal. Alternative: Any point on the surface can be defined as the center of a coordinate system. The Necessity of a Big Bang If galaxies are moving away from each other with a speed proportional to distance, there must have been a beginning, when everything was concentrated in one single point: The Big Bang! ? Looking Back Towards the Early Universe The more distant the objects we observe, the further back into the past of the universe we are looking. Universe cools down as time passes The History of the Universe Universe expands as time passes The Cosmic Background Radiation The radiation from the very early phase of the universe should still be detectable today R. Wilson & A. Penzias Was, in fact, discovered in mid-1960s as the Cosmic Microwave Background: Blackbody radiation with a temperature of T = 2.73 K Cosmology with the Cosmic Microwave Background If the universe were perfectly homogeneous on all scales at the time of recombination (z = 1000), then the CMB should be perfectly isotropic over the sky. Instead, it shows small-scale fluctuations: Measuring the Deceleration of the Universe By observing type Ia supernovae, astronomers can measure the recession speed at large distances It was expected that this would measure the deceleration of the universe, but … Apparent Magnitude of Type Ia Supernovae The Accelerating Universe! Flat decelerating Universe Flat accelerating Universe Red Shift z In fact, SN Ia measurements showed that the universe is accelerating! Confirmation of the Acceleration Observation of the high-red-shift (z = 1.7) SN Ia SN1997ff seems to confirm the acceleration of the universe. What Is Dark Energy? ► Here’s what we know: Somehow it drives a repulsive force that acts on all matter. It gets stronger the more space there is between two globs of matter. It is extremely weak on “local” scales (millions of light years), but gets overwhelmingly strong on “cosmological” scales (tens of billions of light years). What Is Dark Energy? ► Here’s what we don’t know: Dang near everything else about it. ► How did we discover it? The most distant galaxies are not moving away as fast as we would expect. Something is providing a repulsive force that shoves them away from us. But what?! What is our universe made of? ► Atomic Matter: 4% Hydrogen/Helium 98% Other stuff ~ 2% ► Dark Matter: 23% Unknown composition. Not atomic. ► Dark Energy: 73% Very mysterious. Fluctuations in the Cosmic Microwave Background Angular size of the CMB fluctuations allows us to probe the geometry of space-time! CMB fluctuations have a characteristic size of 1 degree. Analysis of the Cosmic Background Fluctuations Analyze frequency of occurrence of fluctuations on a particular angular scale Universe has a flat geometry The Future of Cosmology ► We have learned a great deal about the Universe in just the last few years, and many decades-old problems have been solved by Hubble and WMAP. ► However, many fundamental questions such as the nature of dark mater and dark energy remain unanswered. It is likely that significant progress will be made in our lifetime, so keep an eye out for new breakthroughs in the years ahead. Next Time ► Have a Good Summer