Chapter 14 Our Galaxy Copyright 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 View Copyright 2009 Pearson Education Inc We see our galaxy edge on Primary features disk bulge halo globular clusters Copyright 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 1011MSun Copyright 2009 Pearson Education Inc Orbital Velocity Law r v Mr G 2 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 Copyright 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 systems Copyright 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 molecules Copyright 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 starforming clouds Copyright 2009 Pearson Education Inc Gas Cools 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 Copyright 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 stars Copyright 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 stars Copyright 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 Visible Infrared light reveals stars whose visible light is blocked by gas clouds Copyright 2009 Pearson Education Inc X rays X rays are observed from hot gas above and below the Milky Way s disk Copyright 2009 Pearson Education Inc Radio 21cm 21 cm radio waves emitted by atomic hydrogen show where gas has cooled and settled into disk Copyright 2009 Pearson Education Inc Radio CO Radio waves from carbon monoxide CO show locations of molecular clouds Copyright 2009 Pearson Education Inc IR dust Long wavelength infrared emission shows where young stars are heating dust grains 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 are found around short lived high mass stars signifying active star formation Copyright 2009 Pearson Education Inc Reflection nebulae scatter the light from stars Why do reflection nebulae look bluer than the nearby stars Copyright 2009 Pearson Education Inc Reflection nebulae scatter the light from stars Why do reflection nebulae look bluer than the nearby stars For the same reason that our sky is blue Copyright 2009 Pearson Education Inc What kinds of nebulae do you see Copyright 2009 Pearson Education Inc Halo No ionization nebulae no blue stars no star formation Disk Ionization nebulae blue stars star formation Copyright 2009 Pearson Education Inc Much of star formation in disk happens in spiral arms Whirlpool Galaxy Copyright 2009 Pearson Education Inc Much of star
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