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The Milky Way Galaxy

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  1. The Milky Way Galaxy AST 2010: Chapter 24

  2. NGC 4103, 55 million LY away • edge on spiral galaxy • dark dust band 500 LY thick AST 2010: Chapter 24

  3. side top

  4. center of galaxy obscured by dust AST 2010: Chapter 24

  5. The Milky Way Galaxy (1) • Our galaxy, the Milky Way, is a rotating disk of stars distributed across a region • about 100,000 LY in diameter • about 1,000 LY thick • In addition to stars, gas and dust are also found in the thin disk of the Galaxy • The stars around its center form a nuclear bulge AST 2010: Chapter 24

  6. The Milky Way Galaxy (2) • It has 4 major spiral arms plus smaller spurs • The major arms are Cygnus, Perseus, Carina, and a fourth unnamed one, which is hard to detect (on other side of bulge) • The 3 named arms are each about 80,000 LY long • The Sun is near the inner edge of a short arm, or spur, called the Orion arm • which is about 15,000 LY long and also contains the Orion Nebula AST 2010: Chapter 24

  7. Spiral Arms • Hot blue stars delineate spiral structure – like Xmas lights on a tree • The arms are regions where matter is more densely concentrated • Cool orange and red stars are found in and between spiral arms • Interstellar dust limits our view in visible light to dashed circle

  8. Why Spiral Arms? • Stars orbit around center of mass of galaxy, • like planets • Kepler’s laws imply • stars near the center are faster • stars farther out are slower • Differential rotation of stars explains curved shape of spiral arms

  9. Rotation Speeds • Inner Parts: Rise from Zero to few 100 km/sec • Outer Parts: Nearly constant at a few 100 km/sec • Orbital Period: 240 Myr

  10. Rotation  Spiral Arms AST 2010: Chapter 24

  11. Spiral pattern for billions of years? Spiral density waves AST 2010: Chapter 24

  12. Spiral Density Waves: Traffic Jam

  13. Spherical Structure • Nuclear Bulge • Many RR Lyrae stars • A little gas & dust • Galactic Halo: outer sphere with very few stars • Old metal-poor stars • Globular clusters • Dark matter • RR Lyrae Stars

  14. Galactic Center

  15. Galaxy Mapping with Radio Waves • Radio waves are the best for mapping the distribution of hydrogen in the galaxy • Their wavelengths are large compared with the size of interstellar dust grains and thus the waves pass easily through dust Center of the Galaxy AST 2010: Chapter 24

  16. Population I: Disk Stars Ordered, roughly circular orbits in a plane All orbit in the same general direction Orbit speeds similar at a given radius Population II: Spheroid Stars Disordered, elliptical orbits at all inclinations Mix of regular and retrograde orbits Wide ranges of orbital speeds Stellar Populations

  17. Population I • Location: Disk and Open Clusters • Age: Mix of young and old stars • Composition: Metal rich (roughly solar) • 70% Hydrogen • 28% Helium • ~2% "metals" • Environment: Often gas rich, especially for the young stars AST 2010: Chapter 24

  18. Population II • Location: Spheroid and Globular Clusters • Ages: Oldest stars, >10 Gyr • Composition: Metal Poor (0.1-1% solar) • 75% Hydrogen • 24.99% Helium • ~0.01% metals • Environment: gas poor, no star formation AST 2010: Chapter 24

  19. Population I Disk & open clusters Young & old stars Metal-rich Blue M-S stars Ordered, circular orbits in a plane Gas-rich environment with recent star formation Population II Spheroid & globular clusters Oldest stars Metal-poor No blue M-S stars Disordered, elliptical orbits in all directions. Little or no gas & dust, and no star formation Contrast & Compare AST 2010: Chapter 24

  20. Mass of the Milky Way Observe orbital period, P, of stars or interstellar matter vs. distance, D, from center Kepler’s 3rd law  period of orbit determined by mass within orbit D3 = (Mgalaxy + Msun)P2 • earth orbit  sun’s mass • farther from the galactic center, the more mass within orbit FIND: more mass than we see -- dark matter halo AST 2010: Chapter 24

  21. Dark Matter • Two possible explanations for the observed rotation: • Law of gravity is wrong for galaxies • There is additional matter that doesn’t emit detectable radiation (dark matter) • No evidence that gravity behaves differently • The “dark matter” hypothesis is therefore favored • It could be brown dwarfs, black holes, or new exotic particles • Measurements indicate about 90% of the mass in the universe is dark matter! AST 2010: Chapter 24

  22. Major Parts of the Milky Way AST 2010: Chapter 24

  23. Galaxy Formation