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4/19/17 The Milky Way “Milky Way”: A band of light and a Galaxy Milky Way Composite Photo The band of light we see is really 100 billion stars • Bulge in the center • Dark strip in the middle, from dust Milky Way probably looks like Andromeda. Milky Way Before the 1920’s, astronomers used a “grindstone model” for the galaxy Tried to estimate our location in the galaxy by counting stars in different directions Finding the Center • Harlow Shapely studied globular clusters. • He theorized that they must orbit the true center of the galaxy A Globular Cluster Because some stars are blocked by dust, the true shape of this group of stars was unclear. 1 4/19/17 Finding the Center Parts of Our Galaxy Shapely plotted the locations of the globular star clusters. He found that they are are not centered on the Sun…. …but are centered on a point about 25,000 light years from the Solar System. 100,000 light years 1,000 light years Parts of Our Galaxy Top View Questions: Disk: The Sun Resides in the Disk Nuclear Bulge: The dense central region • How big is the Milky Way? • Where are stars forming (or not forming)? • How much mass is in the Milky Way? Halo: Spherical region surrounding the disk where the globular clusters live. Side View Milky Way Scales Lecture Tutorial: Page 135-137 • Work with a partner or two • Read directions and answer all questions carefully. Take time to understand it now! • Discuss each question and come to a consensus answer you all agree on before moving on to the next question. • If you get stuck, ask another group for help. • If you get really stuck, raise your hand and I will come around. 100,000 light years Mapping the Milky Way The Sun is about halfway out from the center A modern map of the Milky Way (computer-generated diagram) 2 4/19/17 Measuring Distances Standard Candles • To map the Milky Way Galaxy, we need to measure distances to stars. • Parallax only works for nearby stars (within about 1000 light years) • For more distant stars, we use Standard Candles Car Headlights are standard candles: We use them to determine the car’s distance Cepheid Variables ! In 1908, astronomer Henrietta Leavitt discovered a new standard candle using variable stars Period-Luminosity Relation • The connection between a Cepheid’s pulse period and its luminosity. ! These stars are called Cephieds, named after δ Cephei This relation can be used to determine the absolute magnitude of a Cepheid ! These variable stars blink at different rates according to their luminosity (brighter = slower) Henrietta Leavitt Delta Cephei Milky Way: A Spiral Galaxy • Our galaxy seems to be rotating: it has spiral arms Then, compare to the apparent magnitude and find the star’s distance. Pulse Period (in days) Star Formation in Spiral Arms • Spiral arms contain gas and dust. • Stars form out of nebulae in the spiral arms • These are dense concentrations of stars and gas. • Stars orbit the galactic center, pass through the spiral arms as they go. • The spiral arms are denser than the rest of the disk • Stars slow down and pile up in the spiral arms, like cars in a traffic jam. 3 4/19/17 Star Formation in the Milky Way The Disk contains gas, so stars are still forming there. (Population I stars) The Halo has very little gas, and no new stars are forming there. The halo of the galaxy is populated by old stars. (Population II stars) The Center of the Milky Way Stellar Populations • Pop. I: Newer, disk & spiral arm stars, with higher percentage heavy elements • Pop. II: Older, bulge and halo stars, with lower percentage of heavy elements • Heavy elements (metals): anything that isn’t H, He, or Li The Center of Our Galaxy • The density of stars in the Galactic Center is much greater than in the Sun’s neighborhood. • They appear to be orbiting a supermassive black hole at the center of the galaxy Its mass is about 4 Million MSun ! 4