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Announcements

Announcements. Vega. Comet SWAN!. West. Classifying the Stars. 27 October 2006. Today :. Stellar spectra: temperatures and compositions Are hotter stars brighter? (H-R diagrams) Determining sizes of stars Classifying stars, looking for patterns. Stellar Spectra. Pleiades Spectra.

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Announcements

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  1. Announcements Vega Comet SWAN! West

  2. Classifying the Stars 27 October 2006

  3. Today: • Stellar spectra: temperatures and compositions • Are hotter stars brighter? (H-R diagrams) • Determining sizes of stars • Classifying stars, looking for patterns

  4. Stellar Spectra

  5. Pleiades Spectra

  6. Edward Pickering and Harvard “computers,” 1890’s - 1920’s Annie Jump Cannon

  7. Classifying stellar spectra Annie Jump Cannon

  8. Classifying Stellar Spectra “OBAFGKM” Temperature

  9. What are the stars made of? Hydrogen Helium Hydrogen Hydrogen Temperature Calcium Magnesium Sodium

  10. Comte quote “We understand the possibility of determining [celestial bodies’] shapes, their distances, their sizes and motions, whereas never, by any means, will we be able to study their chemical composition. --Auguste Comte (philosopher), 1835

  11. How does temperature affect spectral lines? Cecilia Payne at Harvard, 1924 In the sun, only one H atom in a million is in level 2, ready to absorb visible light!

  12. The Universal Recipe of the Stars • 74% hydrogen (by mass) • 25% helium • 1% other elements (for most stars)

  13. Are hotter stars brighter? Plot known stars on “Hertzsprung-Russell (H-R) diagram”. Luminosity increases vertically; temperature increases to the left. Most stars’ dots lie along a diagonal (“main sequence”), the hotter the brighter.

  14. H-R Diagram Patterns Luminosity = (constant) x (surface area) x (temperature)4 Luminosity For a given size, hotter implies brighter. A bright, cool star must be unusually large (“red giant”). A faint, hot star must be unusually small (“white dwarf”).

  15. H-R Diagram Patterns Most of the stars near us are fainter (and cooler) than the sun; most of the familiar stars in the night sky are brighter than the sun.

  16. Sizes of Main-Sequence Stars Hottest stars are actually somewhat larger Should be white, not green! Reds are greatly exaggerated!

  17. Summary of Stellar Properties

  18. Some familiar examples

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