1ASTR 1020: Stars & GalaxiesMarch 10, 2008• Reading: Chapter 19, section 19.1-19.2.• MasteringAstronomy Homework on The Milky Way is due March 19th(Wednesday).•Extra credit (1 pt) observing opportunities at SBO. See CULearn.• Exam 2 on Friday(covers Chapters 15.3 to 19.2). Astronomy Picture of the DayThe James WebbSpace TelescopeLaunch = 2013Mirror diameter = 6.5-meters (21.3 ft)Last Week: Star Wars• Battle for Gravitational Equilibrium:– White dwarfs: electron degeneracy pressure– Neutron stars: neutron degeneracy pressure– Black holes: gravity wins!Today: The Milky WayThe Galactic CenterMilky Way Topics• Basic anatomy- structure, contents• Looking at the Milky Way at different wavelengths2Milky Way Factoids• 100-200 billion stars• 100,000 light years acrossSun is located ~28,000 light years from core, in the Orion ArmMilky Way AnatomyDisk: includes spiral armsYoung, new star formation---------------------------Bulge & Halo: older stars, globular clustersDisk is very thin!Why is this an artist’s conception?Galaxy “NGC 891” Æ nearly but not quite edge-onClicker Question: The ages of stars suggest that the bulge and halo of the Milky Way formed before many of the stars in the disk. Which would you expect to have more heavy metals (higher metallicity)?a) Halo and bulge starsb) Disk starsc) No difference• (B) Disk stars are continually forming out of gas that is more and more “polluted” by heavy metals.• The OLD globular clusters found in the halo were formed a long time ago before the galaxy was so polluted– they have very low “metallicities”3Contents: Really Hot stuff• Bubbles of hot gas blown out by supernovae• T= million degrees K• Mixing with rest of galactic gas Æenrichment with heavy elementsSuperbubbles & Fountains• Supernovae can burst hot gas out of the galaxy• “Enriches” gas between galaxies• May rain back down and mix into galaxy?Artists’ conceptions!Fast electrons & magnetic fields• Æ synchrotron emission(prominent in X-rayand radio)• Traces hot gas bubblesX-ray image of a supernova remnantWarm stuff• Gas & dust heated by stars • Gas- emission lines from hydrogen (H-alpha) and other elements (ionization nebulae)• T~ 10,000 near hot young starsDust• Absorbs visible and UV light• Transparent to longer wavelengths (red, IR, radio)• Emits IR light (thermal spectrum)Cold stuff• Molecular clouds• Dark, dusty, cold• (10-30K)• Emit molecular emission lines in far IR, radio4Cold hydrogen• Even the coldest hydrogen emits a faint emission line in the radio• Wavelength 21-cm (radio)• Change in energy levels of nuclear configurationThe Star-Gas-Star
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