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The Milky Way Galaxy

The Milky Way Galaxy NGC 4103, 55 million LY away edge on spiral galaxy dark dust band 500 LY thick side top center of galaxy obscured by dust The Milky Way Galaxy (1) Our galaxy, the Milky Way, is a rotating disk of stars distributed across a region

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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

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