AST- 105 1st EditionLecture 23Outline of Last LectureI. Dark MatterII. Globular Cluster DynamicsIII. Cluster EvolutionIV. Close BinariesV. Blue Stragglers Outline of Current LectureI. Historical Understanding of GalaxiesII. Types of Galaxies Current Lecture I. Historical Understanding of Galaxiesa. Nebulae (clouds): faint, fuzzy objects in skyb. Messier’s List (1781):1. 103 brightest nebula2. Do not move relative to fixed stars thus are not coments1. Examples:1. M1 = Crab Nebula2. M31 = Andromeda Nebula3. M51 = Whirlpool Nebulac. Island Universe Model1. Island Universe = self-contained stellar system outside of Milky Way = galaxy2. By 1910, 15,000 nebulae were known3. Hypothesis: most nebula are galaxies4. Counter view:1. Milky Way Galaxy is entire universe2. Contains all nebulaed. Island Universe Model Confirmed1. 1984: Hubble found Cepheid variable stars in Andromeda nebula = M312. Cepheid Variable1. Average luminosity in a standard candle2. Brightness regularly oscillates3. Period (time between peaks) is relative to luminosity4. Observe period (P); use period-luminosity relation to find L5. Observe apparent brightness (B); use inverse-square law (B = L/d^2) to find distance d from B and L.3. Brightest Cepheids have L >10^4 Lʘ1. Can be detected to large distance4. Result from P-L relation:1. M31 lies well outside of Milky Way2. Distance = 2 x 10^6 ly3. Most “nebulae” are “island universes” = galaxiesII. Types of Galaxiesa. Spiral1. Disk with spiral pattern and spheroid (= bulge and halo)1. Examples: Milky Way and Andromeda Galaxy (M31)2. Contain gas and dust in disk3. Contain young star (in disk) and old stars (in disk, bulge and halo)4. Disk rotates rapidly5. Many spiral galaxies (including Milky Way) have ventral bars.b. Elliptical 1. Smooth, symmetric structure2. Vary in apparent shape from round to flattened1. Examples:1. Dwarf companions of M312. M87: giant elliptical in Virgo galaxy cluster3. Spheroid only4. Little gas and dust5. Old stars only6. Stars orbit have no common plane, shape, nor directionc. S0 (“S-zero”)1. Disk but NO gas, dust, or spiral structure2. Old star only3. May be formed by removing gas and dust from spirals4. Irregluar:1. Asymmetric structure2. High gas and dust content3. Vigorous star formation1. Examples: Magellanic Clouds (companions of the MW)d. Galaxy Clusters1. Most galaxies are found in groups and clusters2. Contain 10’s to 1000’s of galaxies3. Sizes: 10^6 to 10^7 ly across4. Clusters are prominent in large-scale distribution of galaxies in
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