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The Nature of Galaxies QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Chapter 17 Elliptical Other Galaxies QuickTi me™ and a TIFF (U ncompressed) decompressor are needed to see this picture. • External to Milky Way – established by Edwin Hubble – used Cepheid variables to measure distance • M31 (Andromeda Galaxy) far outside Milky Way Spiral Qui ckTime™ and a TIFF (Uncompressed) decompr essor ar e needed to see this picture. • Three basic types: – elliptical – spiral – irregular Irregular QuickTi me™ and a TIFF (U ncompressed) decompressor are needed to see this picture. Spiral Galaxies • Similar to Milky Way: – thin disk + nuclear bulge + halo – Disk contains: • dust and gas QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. – H II regions, H I regions, molecular clouds • • • • spiral arms active star formation open clusters mixture of young & old stars – Halo contains: • old stars • Globular Clusters – Bulge contains: • old stars QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Spiral Galaxies QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. me™ and a TIFF (Uncompressed) decompressor are needed to see this picture. QuickTi me™ and a TIFF (U ncompressed) decompressor are needed to see this picture. Barred Spiral Galaxies • Some spirals have bar in center – “barred spirals” QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Quic kTime™ and a YUV420 codec decompress or are needed to s ee this picture. Elliptical Galaxies • Shape ranges from: – spherical to ellipsoidal • Characteristics: Quick Time™a nd a TIFF ( Uncomp res sed) deco mpre ssor are n eede d to s ee this picture . – no disk or spiral arms – old reddish stars • similar to halo or bulge in spirals – little gas or dust – little star formation Quick Time™a nd a TIFF ( Uncomp res sed) deco mpre ssor are n eede d to s ee this picture . Irregular Galaxies • No specific shape – often appear chaotic QuickTi me™ and a TIFF (U ncompressed) decompressor are needed to see this picture. • Often have intense star formation – gravitational interaction with other galaxies? • Mixture of old and new stars QuickTime™ and a TIFF (Uncomp resse d) de co mpres sor a re ne eded to see this picture . QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Interacting Galaxies Cartwheel Galaxy QuickTime™ and a YUV420 codec decompressor are needed to see this picture. NASA/HST QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Simulation by C. Mihos et al., CWRU Galaxy Masses • For spirals: QuickTime™ and a TI FF (Uncompressed) decompressor are needed to see this pict ure. – use Doppler shift; measure galaxy rotation – make rotation curve – calculate mass using Kepler’s Law • For ellipticals: – use Doppler shift; measure stellar orbital velocities – calculate mass using Kepler’s Law • Results – Giant ellipticals and spirals are most massive; – irregulars & dwarf ellipticals least massive QuickTi me™ and a TIFF (U ncompressed) decompressor are needed to see this picture. Mass-to-Light Ratios • ratio of mass to luminosity – for Sun, • M/L = 1 – average star • M/L = 2 to 3 – for entire Galaxy • M/L ~ 100 • 90% of galaxy mass is unseen Extragalactic Distances • Compare apparent and absolute brightness • Variable Stars: – Cepheids, RR Lyrae • Standard Candles: – brightest stars, supernovae, planetary nebulae • Galaxy techniques: – For spirals: • rotation rate gives mass • mass depends on number of stars, hence luminosity – For ellipticals: • range of stellar velocities depends on mass (hence luminosity) Fifteen years ago, a quasar was observed that was found to be located 8 billion light years away. If our universe is approximately 15 billion years old, when did the quasar emit the light that we observe? A. 15 years ago B. 7 billion years ago C. 8 billion years ago D. 15 billion years ago Galaxy Motion • Galaxy spectra: – absorption lines redshifted – more distant galaxies have larger redshift • ALL galaxies moving away Quic kTime™ and a TIFF (Uncompr es sed) dec ompres sor are needed to see this picture. The Hubble Law • Hubble Law: – The more distant the galaxy, the faster it is moving away. v=Hd (H is the Hubble constant, d is distance) Implications of Hubble Law • Every galaxy moving away • Farther away = faster • Conclusion: – Universe is expanding • Predicted by Einstein’s Theory of Relativity • Are we at center? NO – universe same in all directions – there is no center! The Expanding Universe • A uniformly expanding universe – explains Hubble law • example: expanding loaf of raisin bread • Galaxies (like raisins) not moving, not expanding • Space is expanding