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Chapter 24 - Chapter 24 When galaxies were first discovered...

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Chapter 24 When galaxies were first discovered, it was not clear that they lie far beyond the Milky Way The Hubble Classification: galaxies can be grouped into four major categories – spirals, barred spirals, ellipticals, & irregulars The disks of spiral and barred spiral galaxies are sites of active star formation Elliptical galaxies are nearly devoid of interstellar gas and dust, and so star formation is severely inhibited Lenticular galaxies are intermediate between spiral and elliptical galaxies Irregular galaxies have ill-defined, asymmetrical shapes. They are often found associated with other galaxies. Distance to galaxies: standard candles, such as Cepheid variables and the most luminous supergiants, globular clusters, H II regions, and supernovae in a galaxy, are used in estimating intergalactic distances The Tully-Fisher relation, which correlates the width of the 21-cm line of hydrogen in a spiral galaxy with its luminosity, can also be used for determining distance. A method
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