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Download Views of Spiral Galaxies Views of our Milky Way
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Views of Spiral Galaxies Astro 250: Galaxies page 1 Astro 250: Galaxies page 3 Astro 250: Galaxies page 2 Views of our Milky Way the Milky Way as a typical galaxy Optical sun nucleus, bulge spiral arm Sun nucleus 500 pc Disk Halo 30,000 parsecs 30,000 parsecs Infrared (COBE) Astro 250: Galaxies page 4 Astro 250: Galaxies page 5 Tracing the structure of the Milky Way • Globular clusters: spherical halo of old stars Probing the structure of the Galaxy: ! Astro 250: Galaxies page 7 Galactic Rotation • differential rotation • inner parts rotate faster than outer parts orbital period of Sun: 230,000,000 years • • the Sun is 20 galactic years old • distance to MW center: 8,500 pc • Mass of Milky Way (via Kepler’s Laws): 1011 Msun if mass follows light BUT what we see V orb distance • DARK MATTER = 90% of galaxy mass distance Astro 250: Galaxies page 6 Tracing the structure of the Milky Way • Young O,B Stars: delineate spiral arms • most luminous stars around • dust blocks more distant ones • we live in the “Orion Arm” Astro 250: Galaxies page 8 Tracing the structure of the Milky Way • HI 21cm and Molecular Cloud maps • Each line-of-sight has many clouds at different distances • Use differential rotation of galaxy to estimate: • cloud distance using cloud radial velocity Astro 250: Galaxies page 9 Astro 250: Galaxies page 10 Tracing the structure of the Milky Way • HI 21cm and Molecular Cloud maps • IR, Radio mapping • differential rotation to get distances What keeps spiral galaxies spiral? Astro 250: Galaxies page 11 • Age of Galaxy: • ~ 12 billion years • ~ 50 galaxy rotations • differential rotation: arms would have wound up long ago • youngest stars found in arms... • Arms must be fixed patterns through which stars move. • Spiral Density Wave Theory: Spiral Density Waves Astro 250: Galaxies page 12 • Arms must be fixed patterns through which stars and gas move. • Localized density increase —> enhanced star formation Spiral Arm close-up Astro 250: Galaxies page 13 Astro 250: Galaxies page 14 Astro 250: Galaxies page 15 Astro 250: Galaxies page 16 Classifying Galaxy Types Nucleus Arms Sa Sb large tight medium Barred Spirals same as spirals, but with central bar medium the “Hubble Tuning Fork” Sc small loose Ellipticals • no spiral arms • little or no dust • dominated by Pop II stars • size between 106 and 1013 Msun Astro 250: Galaxies page 17 Astro 250: Galaxies page 18 Classifying Spiral Galaxies M101 M81 Astro 250: Galaxies page 19 General galaxy characteristics Astro 250: Galaxies page 20 Spirals Ellipticals Irregulars Mass (Msun) 109-1012 106-1013 108-1011 L (Lsun) M/L 108-1011 2-10 106-1011 10-20 108-1011 1-10 D (kpc) 5-50 2-200 1-10 Stars Pop I and II Pop II Pop I and II Spectrum A to K G to K A to F ISM gas, dust little gas gas, dust Environs small groups rich clusters low density Numbers 20 % 55 % 25 % + Dark Matter
