AST- 105 1st EditionLecture 21Outline of Last LectureI. The Milky Way GalaxyOutline of Current LectureI. Spheroidal ComponentsII. Galactic CenterIII. The Galactic NucleusIV. Mass of GalaxyV. The HaloCurrent Lecture I. Spheroidal Componentsa. Nucleus: spherical 1. Few ly across2. Very dense, active regionsb. Bulge: flattened sphere1. 15,000 ly acrossc. Halo: spherical1. Extends well beyond disk2. Contains individual stars, globular star clusters, and dark matter3. Since there is no gas and dust in the halo, star formation is not presently occurring- old stars only1. Faintd. Motions in spheroid: 1. Circular to elongated orbits2. No common orbital plane or direction (like comets in Solar System)II. Galactic Centera. Old View (before 1915)1. Sun is at center2. Did not account for effect of dust, which obscures distant starsb. Modern View:1. Shapely used globular clusters to find galactic center1. Clusters surround galactic center2. Cluster stars are standard candles for distance determination.c. Standard Candle1. Standard Candle- an object of known luminosity1. Measure apparent brightness2. Apply inverse square law to get distance2. Result:1. Using known positions and determined distances of globular clusters, Shapely found center of distribution.2. Galactic center is located in Sagittarius (teapot shape)3. Sun is about 25,000 ly from galactic center.III. The Galactic Nucleusa. Observations: 1. Best observed using infrared and radio telescopes2. Crowded, active region3. Stellar density: 10^6 stars/ly^34. High5. Speed stars and gas clouds orbit center6. Observe nonstellar and gamma-ray emissionb. Interpretation1. Galactic nucleus contains supermassive black holes2. Gravity of BH accelerates stars and gas clouds to high speed1. High speed stellar orbits indicate MBH = 4 x 10^6 Mʘ3. Density of stars increases approaching center of Galaxy4. Observed nonstellar and gamma-ray emission arise near BH1. Electron-positron annihilation produces gamma-raysIV. Mass of Galaxya. We receive very little light from beyond outer edge of diskb. However, stars in the halo of the galaxy have high speedsc. Thus, there is a large amount of dark mass in the halod. Halo extends to at least 10x the size of the diskV. The Halo a. The Halo is faint, so must contain dark matter – possibilities: 1. “Jupiters” (M = 0.001 Mʘ)2. Brown dwarfs (M < 0.1 Mʘ)b. “WIMPS”- weakly interacting massive particles (new types of subatomic particles)c. “MACHOS”- massive compact halo objects1. Very low mass stars or collapsed stellar remnants (white dwarf, neutron star, or black hole).2. Detecting MACHOs1. Gravitational lensing 1. An object passing between light source and observer bends the light2. Light source temporarily appears brighter3. Millions of stars must be observed, to detect a few lensing events4. Same MACHOs have been detected, but not in large enough number to account for dark matter in
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