1. February 28, 2006 Astronomy 2010 1 Chapter 24: The Milky Way Galaxy M51 Whirlpool Galaxy 31 million LY distant 30,000 LY across

2. February 28, 2006Astronomy 20102 How Do We Determine the Shape of the Galaxy?  We are in the galaxy and can’t easily step back to see what it looks like.  William Herschel: measure the distance and direction to stars and make a map. Dust obscures view of much of the galaxy.Dust obscures view of much of the galaxy.  Harlow Shapley: measure the distance and direction to globular clusters scattered above and below the galaxy away from the dustaway from the dust  Modern: use radio and infrared light

3. Mosaic of 51 wide-angle photographs. Made over a three year period from locations in California (USA), South Africa, and Germany, the individual pictures were digitized and stitched together to create an apparently seamless 360 by 180 degree view.

5. February 28, 2006Astronomy 20105  NGC 4103, 55 million LY away  edge on spiral galaxy  dark dust band 500 LY thick

6. side top

7. February 28, 2006Astronomy 20107 center of galaxy center of galaxy obscured by dust obscured by dust

8. Our Milky Way Galaxy

9. February 28, 2006Astronomy 20109 The Milky Way Galaxy  stars grouped in galaxies  our galaxy: the Milky Way roughly disk shaped, 100,000 LY diameterroughly disk shaped, 100,000 LY diameter 1000 LY thick1000 LY thick  central spherical nuclear bulge,  4 major spiral arms plus smaller “spurs” arms: Cygnus, Perseus, Sagittarius-Carinaarms: Cygnus, Perseus, Sagittarius-Carina fourth unnamed arm – hard to see (on other side of bulge)fourth unnamed arm – hard to see (on other side of bulge) 80,000 LY long80,000 LY long  Sun in short spur – the Orion arm – between Perseus and Carina 15,000 LY long15,000 LY long also contains Orion Nebulaalso contains Orion Nebula Sun in middle of disk, 70 LY from central planeSun in middle of disk, 70 LY from central plane

10. Spiral Arms Spiral Arms   hot blue stars delineate spiral structure – like Xmas lights on a tree   arms: regions where gas and dust more densely concentrated   need dust to replenish short lived blue stars   cool orange and red stars found in and between spiral arms   interstellar dust limits our view in visible light to dashed circle

11. Why Spiral Arms?   stars orbit around center of mass of galaxy – like planets   Kepler’s Laws: closer in – faster farther out – slower   differential rotation of stars explains curved shape of spiral arms

12.  Rotation Speeds Inner Parts: Rise from Zero to few 100 km/secInner Parts: Rise from Zero to few 100 km/sec Outer Parts: Nearly constant at a few 100 km/secOuter Parts: Nearly constant at a few 100 km/sec  Sun has V rot =220 km/sec at R=8.5 kpc  Orbital Period: 240 Myr

13. Rotation  Spiral Arms

14. February 28, 2006Astronomy 201014 Spiral pattern for billions of years? Spiral density waves

16. Spiral Density Waves: Traffic Jam

17. Spherical Structure Nuclear Bulge Many RR Lyrae starsMany RR Lyrae stars A little gas & dustA little gas & dust Galactic Halo: outer sphere with very few stars Galactic Halo: outer sphere with very few stars Old metal-poor starsOld metal-poor stars Globular clustersGlobular clusters dark matterdark matter RR Lyrae StarsRR Lyrae Stars RR Lyrae stars RR Lyrae stars pulsate like Cepheid Variablespulsate like Cepheid Variables distance scaledistance scale

18. Galactic Center

19. February 28, 2006Astronomy 201019 Galaxy Mapping with Radio Band  radio is best for mapping the distribution of hydrogen in the galaxy  most of the hydrogen gas is not ionized because O and B stars are rare  radio waves pass easily through dust

20. 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

21. February 28, 2006Astronomy 201021 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.

22. February 28, 2006Astronomy 201022 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

23. February 28, 2006Astronomy 201023 Contrast & Compare 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.

24. February 28, 2006Astronomy 201024 Mass of the Milky Way Observe orbital period, P, of stars or interstellar matter vs. distance, D, from center Kepler’s Third Law  period of orbit determined by mass within orbit: D 3 = (M galaxy + M sun )P 2 D 3 = (M galaxy + M sun )P 2  earth orbit  sun’s mass  farther from the galactic center, the more mass center, the more mass within orbit within orbit FIND: more mass than we see -- dark matter halo

25. February 28, 2006Astronomy 201025 Dark Matter  Two possibilities to explain the observed rotation: Law of gravity is wrong for galaxies.Law of gravity is wrong for galaxies. There is additional matter that doesn’t emit detectable radiation (dark matter).There is additional matter that doesn’t emit detectable radiation (dark matter).  No evidence that gravity behaves differently.  The “dark matter” hypothesis is favored. Could be brown dwarfs, black holes, or new exotic particles.Could be brown dwarfs, black holes, or new exotic particles.  Measurements indicate about 90% of the mass in the universe is dark matter!

26. Galaxy Formation