Milky Way AST2002 Prof. Voss Page 1 Milky Way faint band of light visible by eye circling sky galaxies kuklos milky circle to ancient Greeks image: via lactea southerncross_milkyway image: milky road 15-06 to Romans only in 20th century known as star system image: ~ 75,000 ly diameter 15-01b image: ~200 billion stars 15-03b Harlow Shapley image: image: 47 Tucanae 1 million stars shapley 15-03a studied clusters Globular 105-106 stars near center Open in disk the Jewel Box a few hundred stars based on work of Henrietta Leavitt image: leavitt image: Cepheid variable stars 15-02b image: throbber period-luminosity relation Shapley calibrated variable stars absolute distance mapped location of image: Globular Clusters 15-04 in center of galaxy Mxx - Messier Object cataloged in 1800's 11/28/2001 Milky Way AST2002 Prof. Voss Page 2 NGCxxxx - New General Catalogue of astronomical objects Milky Way Galaxy: Disk component stars metal-rich Population I circular orbits in plane faster rotation near center image: image: 15-06 15-05 image: 15-10 /predust and gas obscure center concentration at visible wavelengths Spherical component stars image: nuclear bulge at center 15- image: halo - thinly scattered 09a 15-09b metal-poor Population II random elliptical orbits globular clusters age > 11 billion years metal - elements heavier than Helium Element building cycle - Nucleosynthesis original galaxy material 80% Hydrogen 20% Helium (by weight) higher Z elements require processing through image: generations of stars 15-12 metal-rich - younger 2nd (and higher) generations metal-poor - older 1st generation stars galactic corona extends 7× further postulated from motions of stars and star clusters rotating more rapidly than expected suggests Dark Matter (nonluminous) Galactic formation suggested from star populations similar to solar system condensed from image: spherical cloud of gas 15-14a a b c d e image: younger stars 15-15a metal-rich 11/28/2001 Milky Way AST2002 Prof. Voss Page 3 circular flat orbits youngest in spiral arms image: mapped in our galaxy 15-20 image: 15-15b Density Wave theory spiral arms are compression waves trigger star formation not completely understood: star formation effects spiral arms ages of clusters ~7 billion, composition, and distribution Nucleus of Milky Way blocked at visible λ ⇒ seen in radio, IR image: γ UV, X-ray, -ray 15-23a penetrate clouds radio, IR, X, γ source image: Sagittarius A* galacticcenterIR at very center image: image: 15-25a gc_1meter core less than 2 AU 2-3 million solar masses massive black hole? ⇒ Chandra X-ray image 11/28/2001.