f10_nov18 - The Milky Way Galaxy Ast 307 - Nov. 18, 2010...

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Ast 307 - Nov. 18, 2010 ch. 23: The Milky Way Galaxy The Milky Way Galaxy diameter of disk = 100,000 l.y. (30,000 pc) radius of disk = 50,000 l.y. (15,000 pc) number of stars = 200 billion = 2 ! 10 11 disk half-thickness = 1,000 l.y. (300 pc) Sun is in disk, 28,000 l.y. out from center Components of the Milky Way • Disk • younger stars (including newborns) • contains gas and dust • Bulge • mostly fairly old stars, a range in ages • relatively rich in heavy elements (“star ashes”) • Halo • oldest stars, deficient in “star ashes” • contains no gas or dust • location of the globular clusters Stellar Orbits in the Galaxy • Stars in the disk (yellow) orbit the Galactic center: • in same direction • in same plane (like planets) • they bob up and down – due to the gravity of the disk – this gives the disk its thickness • Stars in the bulge and halo orbit the Galactic center: • in different directions • at various tilts from the disk • they have “high” velocities
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Dust mixed with the gas in the interstellar clouds obscures our view because it absorbs visible light. These are the clouds that contract to form stars and planetary systems. Where the Young Stars Are “Side view” above - our view of the Galaxy, in a fish-eye lens; artists’ conception of a “top view” at right. Stars and Dust in a Spiral Galaxy NGC 7331 is a galaxy much like the Milky Way H I Gas (21 cm line) in the Milky Way Dust (top) and Stars in the Milky Way
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The Sun’s galactic orbital motion tells us the mass within Sun’s orbit: 1.0 x 10 11 M Sun Masses from Orbital Velocities The orbital speed ( v ) and radius ( r ) of an object on a circular orbit around the galaxy tells us the mass ( M r ) within that orbit: M r = rv 2 G How orbital speed varies with distance The rotation curve of the Galaxy tells us the distribution of mass The Distributions of Light and Gravity
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Mapping the Gas in the Milky Way Visual Infrared The Central Regions of the Milky Way Swirling gas near center Orbiting star near center The Galactic Center We measure the orbits of stars near the Galactic center. • these measurements must be made in the infrared
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This note was uploaded on 04/30/2011 for the course AST 317 taught by Professor Dinerstein during the Fall '10 term at University of Texas at Austin.

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f10_nov18 - The Milky Way Galaxy Ast 307 - Nov. 18, 2010...

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