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Reminders • Office Hours For Tuesday and Thursday 3:30 – 5pm • Final Exam: Monday May 8th 6:30-9:00pm – Phys1403 – Introductory Astronomy – Instructor: Dr. Goderya • You can come look at your standing in class Bring a Green SCANTRON Sheet (make sure its not crimpled) – No. 2 Pencil and a scientific Calculator – You are allowed to bring 1 page (both sides) reference sheet with equations, constants, conversion factors from textbook or lecture PowerPoint notes. – 50 Multiple Choice Questions – About 12-15 questions from each of Exam 1, 2 and 3. Remaining questions from material not included in exam 3 and material covered till last class. – No Cell Phones or laptops. Last set of homework will be due Wednesday May 3rd. Topics for Today’s Class (Last Class) Foundations of Astronomy | 13e Seeds • Milky Way – General Shape and Structure – Globular Clusters – Stellar Populations – Black Hole in the Center Chapter 15 and 16 • Other Galaxies – Shape of Galaxies and Galaxy Classification Selected Topics – Hubble's Law – Extragalactic Distance Scale • Dark Matter and Dark Energy – What is Dark Matter? – What is Dark Energy? © Cengage Learning 2016 The Milky Way The Structure of the Milky Way (1) Disk Nuclear Bulge Sun Almost everything we see in the night sky belongs to the Milky Way We see most of the Milky Way as a faint band of light across the sky From the outside, our Milky Way might look very much like our cosmic neighbor, the Andromeda galaxy Halo Globular Clusters 1 The Structure of the Milky Way (2) Exploring the Milky Way Using Clusters of Stars Two types of star clusters: 1) Open clusters: young clusters of recently formed stars; within the disk of the Galaxy Galactic Plane Open clusters h and Persei Galactic Center The structure is hard to determine because: 1) We are inside 2) Distance measurements are difficult 3) Our view towards the center is obscured by gas and dust Globular Clusters Globular Cluster M 19 2) Globular clusters: old, centrally concentrated clusters of stars; mostly in a halo around the Galaxy Locating the Center of the Milky Way • Dense clusters of 50,000 – 1 million stars Distribution of globular clusters is not centered on the sun… • Old (~ 11 billion years), lower-mainsequence stars • Approx. 200 globular clusters in our Milky Way Globular Cluster M80 The Mass of the Milky Way (2) Total mass in the disk of the Milky Way: Approx. 200 billion solar masses Additional mass in an extended halo: Total: Approx. 1 trillion solar masses Most of the mass is not emitting any radiation: Dark Matter! …but on a location which is heavily obscured from direct (visual) observation Metals in Stars Absorption lines almost exclusively from hydrogen: Population II Many absorption lines also from heavier elements (metals): Population I At the time of formation, the gases forming the Milky Way consisted exclusively of hydrogen and helium. Heavier elements (“metals”) were later only produced in stars. => Young stars contain more metals than older stars 2 A Black Hole at the Center of Our Galaxy Stellar Populations Population I: Young stars: metal rich; located in spiral arms and disk By following the orbits of individual stars near the center of the Milky Way, the mass of the central black hole could be determined to ~ 2.6 million solar masses Population II: Old stars: metal poor; located in the halo (globular clusters) and nuclear bulge Other Galaxies in the Universe Galaxy Diversity Even seemingly empty regions of the sky contain thousands of very faint, very distant galaxies Large variety of galaxy morphologies: Spirals Ellipticals • Star systems like our Milky Way Irregular • Contain a few thousand to tens of billions of stars. • Large variety of shapes and sizes 10-day exposure on an apparently empty field in the sky Distance Measurements to Other Galaxies: The Hubble Law Galaxy Classification E0, …, E7 The Hubble Deep Field: (some interacting) Sa E0 = Spherical Large nucleus; tightly wound arms E1 Sb E. Hubble (1913): Distant galaxies are moving away from our Milky Way, with a recession velocity, vr, proportional to their distance d: vr = H0*d H0 ≈ 70 km/s/Mpc is the Hubble constant Sc E7 = Highly elliptical E6 Small nucleus; loosely wound arms • Measure vr through the Doppler effect infer the distance 3 The Extragalactic Distance Scale What is Dark Matter? • Many galaxies are typically millions or billions of parsecs from our galaxy. • Typical distance units: Matter that can be seen by its gravitational effects, but does not emit light. Mpc = Megaparsec = 1 million parsec Gpc = Gigaparsec = 1 billion parsec • Distances of Mpc or even Gpc The light we see left the galaxy millions or billions of years ago!! • “Look-back times” of millions or billions of years Dark Matter Not Dark Matter Dark Energy Comprises 73% of Universe What is the Dark Energy? Dark Energy is an unknown form of energy that is present everywhere in the Universe and is hypothesized to accelerate the expansion of the Universe. Dark Energy 73% “Normal Matter” 4% Dark Matter 23% Wikipedia Dark Energy is an Unfinished Story From The Syllabus WE DON’T KNOW WHAT IT IS! But it traces the story of our understanding of the nature of the universe. • An ideal setting for illustrating the process of science: – Science is alive and on-going. – Our ideas change as the data changes. – Scientific debate differs from social/political debate. – Progress in science results from both individual and group efforts. 4 That’s It For This Semester Exam 3 Hi – 86 Low – 0 Average - 65 ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu) ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu) ClassAction: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu) 5