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Chapters 23 26 28 (got 93% on the test)

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ASTR 1102
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Chapters 2326 28Chapter 23231 The Sun is located in the disk of our Galaxy about 8000 parsecs from the galactic center Ais an immense collection of stars and interstellar matter far Galaxylarger than a star cluster Locating the Sun Within the Galaxy Early attemptsWe have an edgeon view from inside the pancakelike disk of our own Milky Way Galaxy which is why the Milky Way appears as a band around the skyUntil the twentieth century the prevailing opinion was that the Sun and planets lie at the Galaxys center One of the first to come to this conclusion was the eighteenthcentury English astronomer William Herschel Who discovered the planter Uranus and was a pioneering cataloger of binary star systemsHerschel found approximately the same density of stars all along the Milky Way Therefore he concluded that we are at the center of our GalaxyAccording the Kapteyn the Milky Way is about 17 kpc 17 kiloparsecs17000 parsecs55000 lightyears in diameter with the Sun near its centerThe Problem Interstellar ExtinctionsThe reason for their mistake was discerned in 1930 by Robert J Trumpler of Lick ObservatoryLike the stars themselves dust is concentrated in the plane of the Galaxy As a result it obscures our view within the plane and makes distant objects appear dim an effect called interstellarextinction Thanks to interstellar extinction Herschel and Kapteyn were actually seeing only the nearest stars in the Galaxy Hence they had no idea of either the enormous size of the Galaxy or the vast number of stars concentrated around the galactic centerTo find our location in the Galaxy we need to locate bright objects that are part of the Galaxy but lie outside its plane in unobscured regions of the skyThe Breakthrough Globular ClustersGlobular Cluster a class of star clusters associated with the Galaxy but which lie outside its planeCepheid variables are pulsating stars that vary periodically in brightnessThe longer a Cepheids period the greater its average luminosityA family of pulsating stars closely related to Cepheid variables called RR Lyrae variablesThe light curve of an RR Lyrae variable is similar to that of a Cepheid but RR Lyrae variables have shorter pulsation periods and lower peak luminositiesModernDay MeasurementsGalactic nucleus the center of our Galaxy232Observations at nonvisible wavelengths reveal the shape of the GalaxyIn other words the longer the wavelength the farther radiation can travel through interstellar dust without being scattered or absorbedExploring the Milky Way in the InfraredThe dust emits radiation predominantly at wavelengths from about 30 to 300 nanometers These are called farinfrared wavelengths because they lie in the part of the infrared spectrum most different in wavelengths from visible lightNearinfrared wavelengths that is short wavelengths closer to the visible spectrumThe disk of our Galaxy is about 50 kpc 160000 ly in diameter and about 06 kpc 2000 ly thickThe center of the Galaxy is surrounded by distribution of stars called the central buldge which is about 2 kpc 6500 ly in diameterThe spherical distribution of globular clusters traces the halo of the GalaxyThe Milky Ways Distinct Stellar PopulationIt is estimated that our Galaxy contains 200 billion 2 x 1011 stars
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