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Summary: Nuclear burning in stars Reaction 4 1H Î Min. Temp. 4He 107 o K 3 4He Î 12C 12C + 4He Î 2x108 16O, Ne Î O, Mg Ne, Na, Mg 8x108 1.5x109 O Î Mg, S 2x109 Si ÎFe peak 3x109 Evolution through nuclear burning. Minitial > 3M Nuclear burning all the way to iron. Minitial < 3M Nuclear burning shuts off after He-flash. 1 Galaxies The Hubble Deep Field • Tiny area of sky. • 1/12 angular size of full moon. • Faintest objects ever measured. • 10 days’ exposure with Hubble Space Telescope. • Only 20 stars. • Remaining 5000 objects are galaxies. The Types of Galaxies Spirals Ellipticals Hubble class: Ellipticals • E0 = round Î E7 = elongated Barred Spirals Spirals (SaÎSc, SBaÎSBc): • Central concentration • Tightness of arms • Smoothness of arms Irregulars 2 Spirals Barred Spirals Ellipticals Irregulars What kind of galaxy do we live in? • The Milky Way • Obviously a disk • But where are we located within that disk? 3 Mapping our Galaxy, up until ~1920 What about “spiral nebulae”? False detection of proper motion. Counting stars Î Sun near center of small universe. The Milky Way • Optical view • Infrared view in light of stars • Infrared view in light of glowing dust 4 Need to correct for absorption by dust • F = L/4πr2 • But absorption also affects F 12,000 K, absorbed by dust • Absorption, or larger r ? • Dust absorbs more in blue than in red. • Changes shape of black body curve • Measurements at 3 wavelengths let you detect that change in shape. • Î amount of absorption at each wavelength. Pulsating Variable Stars Time (days) Î Change in size Î • change in temperature • change in luminosity Use F = L/(4πr2) to find r Luminosity Luminosity Î • These stars regularly expand & contract. • Like a big spring. PeriodLuminosity Relation Period (days) 5 Measuring distances inside the Galaxy • Parallax: to 300 pc • Pulsating variables (Cepheids, RR Lyraes) Star clusters in the Milky Way Open Clusters: found in disk of Galaxy. Just a few hundred stars per cluster. Some recently-formed, some middle-aged. Globular Clusters: ~ 150, not confined to disk. Typically 100,00 stars. All are very old. 6 Mapping our Galaxy with globular clusters • Globular Clusters offer key breakthrough (in ~1920). • Distances from pulsating variables. • Spherical distribution in space Î line of sight is not in dusty disk of Galaxy • Result: Sun very far away from center. Our Galaxy (The Milky Way) • 100,000 LY diameter • Sun is 30,000 LY from center • Gas, large fraction of stars in thin disk • ~1000 LY thick • Spiral structure • Spherical halo • ~150 globular clusters • But most halo light is from spherical distribution of stars • Nuclear bulge 100,000 LY Sun Bulge Disk Halo Glob. Cluster 7 Orbits in the Galaxy Stars, gas in disk Stars in halo & nuclear bulge The masses of galaxies • P2 (M1 + M2) = a3 • Velocity = (2πa)/P ∝ √ 1/ a • Measure Doppler shift of absorption lines & emission lines at different points in galaxy. 8 Dark matter • Î large amounts of additional “dark matter” in outer parts of spiral galaxies. Components of our Galaxy (1010 M☼) Nuclear bulge 1 Disk 7 Halo 0.1 Dark matter 55 ORBITAL VELOCITY Î • We expected falling “Keplerian” curve out beyond outermost luminous matter. • But curves do not drop off Our Galaxy observed expected DISTANCE FROM CENTER Î Spiral arms are delineated by young stars & HII regions M74 in the ultraviolet UV to red sequence in M81 (12 Million LY away) The Spiral Arms 9 The spiral structure in the Milky Way From mapping out positions of young objects. At radio wavelengths Formation of Spiral Arms • Differential Rotation • P2 = a3 (Kepler’s 3rd Law) • Gas, stars closer to center orbit in less time than those farther from center. • Î automatic stretching of any feature into a trailing spiral. • But arms should rapidly wind up and disappear 10 Spiral Arms & the Interstate Highway • Density wave • Spiral arms have higher density than space between arms • Excess gravitational attraction slows down gas, stars when they pass through spiral arm in course of their orbits. • Î spiral arms are a traffic jam Formation of Spiral Arms II NGC 3486 • Density waves most likely to occur in presence of external driving force • Example: Satellite galaxy in process of colliding with M51. • “Flocculent” spirals may be due to different process (???) • Waves of star formation form linear structures. • Followed by winding up due to differential rotation. 11 Spirals The Types of Galaxies Also… Irregulars Ellipticals Barred Spirals Where’s the nearest bar? NGC 891 • Milky Way Probably in our Galaxy! M81 Infra-red light shows “Peanut-shaped” nuclear bulge M 31 12 Chemical history of our galaxy • Chemical enrichment • The buildup of the heavy elements through nucleosynthesis. Formation of: Galaxy Globular clusters Sun M67 • Galaxy started with just H, He, Li • H Î He Î C Î O burning has steadily built up carbon, oxygen. Abundance Relative to Solar 1 • Elements like iron built up (somewhat) more recently. Fe/O 0.1 O/H O/H Fe/O Fe/H Fe/H 0.01 0.001 0.01 0.1 1 10 Age (billions of years) Stellar Populations • Population I • Wide range of ages • More enriched • Circular orbits in disk Galaxy • Population II • Nuclear Bulge • Abundances like Sun (enriched) • But elliptical orbits Sun M67 1 Abundance Relative to Solar • Old • Very low heavy-element abundances • Elliptical orbits in halo • huge range of inclinations Globular clusters 0.1 O/H Fe/O Fe/H 0.01 0.001 0.01 0.1 1 10 Age (billions of years) 13 Elliptical Galaxies No gas. No dust. No young stars. NGC 4472 M59, NGC 4647 M87 M59, M60 M32 Spirals Efficiency of star formation Also… Irregulars Ellipticals Barred Spirals 14