Copyright © 2009 Pearson Education, Inc. Me and HawkingCopyright © 2009 Pearson Education, Inc. Black Holes • T=hc3/8πGkM = hc/4πRS • S = A/4 • tevap = 2x1067 (M/Mo)3 • Anything smaller than 1.7x1011 kg will evaporate by nowCopyright © 2009 Pearson Education, Inc. Chapter 14 Our GalaxyCopyright © 2009 Pearson Education, Inc. 14.1 The Milky Way Revealed Our goals for learning: • What does our galaxy look like? • How do stars orbit in our galaxy?Copyright © 2009 Pearson Education, Inc. What does our galaxy look like?Copyright © 2009 Pearson Education, Inc. The Milky Way galaxy appears in our sky as a faint band of light.Copyright © 2009 Pearson Education, Inc. Dusty gas clouds obscure our view because they absorb visible light. This is the interstellar medium that makes new star systems.Copyright © 2009 Pearson Education, Inc. All-Sky ViewCopyright © 2009 Pearson Education, Inc. We see our galaxy edge-on. Primary features: disk, bulge, halo, globular clustersCopyright © 2009 Pearson Education, Inc. If we could view the Milky Way from above the disk, we would see its spiral arms.Copyright © 2009 Pearson Education, Inc. How do stars orbit in our galaxy?Copyright © 2009 Pearson Education, Inc. Stars in the disk all orbit in the same direction with a little up-and-down motion.Copyright © 2009 Pearson Education, Inc. Orbits of stars in the bulge and halo have random orientations.Copyright © 2009 Pearson Education, Inc.Copyright © 2009 Pearson Education, Inc. Thought Question Why do orbits of bulge stars bob up and down? A. They’re stuck to the interstellar medium. B. The gravity of disk stars pulls them toward the disk. C. Halo stars knock them back into the disk.Copyright © 2009 Pearson Education, Inc. Thought Question Why do orbits of bulge stars bob up and down? A. They’re stuck to the interstellar medium. B. The gravity of disk stars pulls them toward the disk. C. Halo stars knock them back into the disk.Copyright © 2009 Pearson Education, Inc. Sun’s orbital motion (radius and velocity) tells us mass within Sun’s orbit: 1.0 × 1011MSunCopyright © 2009 Pearson Education, Inc. Orbital Velocity Law • The orbital speed (v) and radius (r) of an object on a circular orbit around the galaxy tell us the mass (Mr) within that orbit. € Mr=r × v2GCopyright © 2009 Pearson Education, Inc. What have we learned? • What does our galaxy look like? — Our galaxy consists of a disk of stars and gas, with a bulge of stars at the center of the disk, surrounded by a large spherical halo. • How do stars orbit in our galaxy? — Stars in the disk orbit in circles going in the same direction with a little up-and-down motion. — Orbits of halo and bulge stars have random orientations.Copyright © 2009 Pearson Education, Inc. 14.2 Galactic Recycling Our goals for learning: • How is gas recycled in our galaxy? • Where do stars tend to form in our galaxy?Copyright © 2009 Pearson Education, Inc. How is gas recycled in our galaxy?Copyright © 2009 Pearson Education, Inc. Star–gas–star cycle Recycles gas from old stars into new star systemsCopyright © 2009 Pearson Education, Inc. High-mass stars have strong stellar winds that blow bubbles of hot gas.Copyright © 2009 Pearson Education, Inc. Lower-mass stars return gas to interstellar space through stellar winds and planetary nebulae.Copyright © 2009 Pearson Education, Inc. X rays from hot gas in supernova remnants reveal newly made heavy elements.Copyright © 2009 Pearson Education, Inc. A supernova remnant cools and begins to emit visible light as it expands. New elements made by supernova mix into interstellar medium.Copyright © 2009 Pearson Education, Inc. Multiple supernovae create huge hot bubbles that can blow out of disk. Gas clouds cooling in the halo can rain back down on disk.Copyright © 2009 Pearson Education, Inc. Atomic hydrogen (H2) gas forms as hot gas cools, allowing electrons to join with protons. Molecular clouds form next, after gas cools enough to allow atoms to combine into molecules.Copyright © 2009 Pearson Education, Inc. Molecular clouds in Orion Composition: • Mostly H2 • About 28% He • About 1% CO • Many other moleculesCopyright © 2009 Pearson Education, Inc. Gravity forms stars out of the gas in molecular clouds, completing the star–gas– star cycle.Copyright © 2009 Pearson Education, Inc. Radiation from newly formed stars is eroding these star-forming clouds.Copyright © 2009 Pearson Education, Inc. Summary of Galactic Recycling • Stars make new elements by fusion. • Dying stars expel gas and new elements, producing hot bubbles (~106 K). • Hot gas cools, allowing atomic hydrogen clouds to form (~100–10,000 K). • Further cooling permits molecules to form, making molecular clouds (~30 K). • Gravity forms new stars (and planets) in molecular clouds. Gas CoolsCopyright © 2009 Pearson Education, Inc. Thought Question Where will the gas be in 1 trillion years? A. Blown out of galaxy B. Still recycling just like now C. Locked into white dwarfs and low-mass starsCopyright © 2009 Pearson Education, Inc. Thought Question Where will the gas be in 1 trillion years? A. Blown out of galaxy B. Still recycling just like now C. Locked into white dwarfs and low-mass starsCopyright © 2009 Pearson Education, Inc. We observe the star–gas–star cycle operating in Milky Way’s disk using many different wavelengths of light.Copyright © 2009 Pearson Education, Inc. Infrared light reveals stars whose visible light is blocked by gas clouds. Infrared VisibleCopyright © 2009 Pearson Education, Inc. X rays are observed from hot gas above and below the Milky Way’s disk. X raysCopyright © 2009 Pearson Education, Inc. 21-cm radio waves emitted by atomic hydrogen show where gas has cooled and settled into disk. Radio (21cm)Copyright © 2009 Pearson Education, Inc. Radio waves from carbon monoxide (CO) show locations of molecular clouds. Radio (CO)Copyright © 2009 Pearson Education, Inc. Long-wavelength infrared emission shows where young stars are heating dust grains. IR (dust)Copyright © 2009 Pearson Education, Inc. Gamma rays show where cosmic rays from supernovae collide with atomic nuclei in gas clouds.Copyright © 2009 Pearson Education, Inc. Where do stars tend to form in our galaxy?Copyright © 2009 Pearson Education, Inc. Ionization nebulae
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