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lect 17 - Galaxies Chapter 24 When galaxies were first...

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Galaxies Chapter 24
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When galaxies were first discovered, it was not clear that they lie far beyond the Milky Way A century ago, astronomers thought that the entire universe was only a few thousand light-years and nothing was beyond the Milky Way Galaxy In 1755, Kant suggested that vast collections of stars lie outside of the Milky Way… What he called “island universes” In 1845, William Parsons built the largest telescope of the 19th century (1.8 m in diameter)
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Sketch of spiral galaxy M51 (telescope had no photographic equipment)
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A modern view of the Spiral Galaxy M51 Note the glowing HII regions (sites of star formation). One arm extends toward the companion galaxy.
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