Chapter 16The Shape of the Milky WayThe Shape of the Milky WaySize of the Milky WayThe (actual) Structure of the M.W.Composition and Mass of the M.W.Age of the Milky Way & Stellar CensusesStellar PopulationsStar ClustersInterstellar MatterInterstellar DustThe “Zone of Avoidance”Reflection NebulasHot Interstellar GasCold Interstellar GasMotion of Stars and Gas in M.W.Measuring the Milky Way’s DiameterMeasuring the Milky Way’s MassThe Galactic CenterEvolution of the Milky WayEvolution by MergersThe Future of the Milky WayChapter 16The Milky Way GalaxyThe Shape of the Milky Way•18th Century: Thomas Wright (English Astronomer) and Immanuel Kant (German Philosopher)•“Flattened swarm of stars”The Shape of the Milky Way•End of 18th Century: Sir William Hershel tries to create the first “map” of the Milky Way•Right idea, but some errors and lots of uncertainty.Size of the Milky Way•Early 1900s: Debate rages. • Joseph Kapteyn (Dutch)•18,000 parsecs across•Sun at the center• Harlow Shapley (American)•100,000 parsecs across•Sun 2/3 of way out•Neither Kapteyn nor Shapley completely right.The (actual) Structure of the M.W.•Models refined by looking at other galaxies•Disk, halo, bulge, globular clusters, spiral arms•Stars very far apart, but closer together in centerComposition and Mass of the M.W.•Gas and dust don’t allow us to see the nucleus of the galaxy at visible wavelengths.•Orbit of stars —> at least 1011 M⊙ inside Sun’s orbit, total mass around 2*1012 M⊙.•Visible stars and gas account for only 10-20% of this value.Age of the Milky Way & Stellar Censuses•Look at how old the oldest stars are.•This terns about to be about 13 billion years.•7-8 stars born each year (on average).•Milky Way will keep shining for billions of years.•Dim cool red dwarfs (0.1 - 0.5 M⊙) are most numerous•Average mass of stars just less than the Sun’s mass•Lower mass —> more common (brown dwarfs)Stellar Populations•Stars generally segregated by type.•Divisions are not set in stone.•Sun is considered a Pop I star.Star Clusters•Groups of stars orbiting the same center of mass.•Open vs globular clusters•Different types of clusters have different types of starsInterstellar Matter•Gas and dust, very low density, limits what we can see.•Most has been pulled into a thin layer in the disk.•Most of the interstellar matter is gas (71% H, 27% He), the rest is dust.Interstellar Dust•Dimming and reddening: comes from scattering of light.•Only 1% of interstellar material is dust, but it has huge implications for what we can see and detect.•“Dark nebula”The “Zone of Avoidance”•The Milky Way itself disrupts our ability to see (visible wavelengths)•Radio and infrared can pass through.Reflection Nebulas•StuffHot Interstellar Gas•Emission nebulas•H II regions (helps us map spiral arms)Cold Interstellar Gas•Very cold, can only be detected with radio telescopes.•H I regions.•Emission from spins of molecules.•Helps us determine density —> patterns of star formationMotion of Stars and Gas in M.W.•Density-Wave Model•Self-propagating star formationMeasuring the Milky Way’s Diameter•First thing to find: how far from the middle are we?•Three main methods.•Look at globular clusters•“Masers”•Detect stars near center•About 8 kpc from center•About 40 kpc acrossMeasuring the Milky Way’s Mass•StuffThe Galactic Center•Super fast orbits•But no object visible at ANY wavelengths•Super-massive black holeEvolution of the Milky Way•Kind of like a scaled up version of solar system formation.•Any theory must explain the high orbital inclinations of Pop II stars and the complete lack of Pop I stars in the halo.•2-stage collapse model.Evolution by Mergers•2-stage collapse can’t explain:•Not all Pop II stars formed during the initial collapse•Pop I stars don’t form at a steady rate•Collisions between galaxies —> bursts of star formation•2-stage collapse predicts “Population III stars”The Future of the Milky Way•Merging with nearby galaxies (Large and Small Magellanic Clouds, Andromeda)•Slowly use up all available gas•Eventually only stellar remnants and dim low-mass red stars.http://i.imgur.com/nEa8qzU.gif
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