Stellar Spectra Ay16 Lecture 2 Feb 5, 2008. The Nearest Star SOHO UV Image.
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Transcript of Stellar Spectra Ay16 Lecture 2 Feb 5, 2008. The Nearest Star SOHO UV Image.
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Stellar Spectra
Ay16 Lecture 2
Feb 5, 2008
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The Nearest Star
SOHO UV Image
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1. What is a Spectrum.
2. The Atmospheres of stars.
3. Spectral Lines & Quantum Mechanics (the Bohr atom)
4. Ionization
5. Spectral Types
6. The Hertzsprung-Russell Diagram (intro to the evolution of stars)
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Diffraction/Refraction
Prism refracts
Grating diffracts
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The Solar Spectrum
Fraunhofer Lines in the spectrum of the Sun
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Fraunhofer Lines in the Solar Spectrum (~1817)
Designation Wavelength Origin A 759.4 nm terrestrial oxygen B 686.7 terrestrial oxygen C 656.3 hydrogen (H alpha) D 589.0 neutral sodium (Na I) E 527.0 neutral iron (Fe I) F 486.1 hydrogen (H beta) G 430.3 metal blend H 396.8 ionized Calcium (Ca II) K 393.4 ionized Calcium (Ca II)
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Where do these features come from?
Blackbody
Emission by hot
gas
Absorption by
cold gas in front
of a hot source
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The Bohr
Atom
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•
Hydrogen Atom Energy Levels
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Hydrogen Ionization Fraction
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IONIZATION ENERGIES (in electron volts = eV) Atom 1st 2nd 3rd 4th 5thHydrogen H 13.60Helium He 24.59 54.42Lithium Li 5.39 75.64 122.45Beryllium Be 9.32 18.21 153.90 217.72Boron B 8.30 25.15 37.93 259.38 340.23Carbon C 11.26 24.38 47.89 64.49 392.09Nitrogen N 14.53 29.60 47.45 77.47 97.89Oxygen O 13.62 35.12 54.94 77.41 113.90Fluorine F 17.42 34.97 62.70 87.14 114.24Neon Ne 21.56 40.96 63.45 97.12 126.21Sodium Na 5.14 47.29 71.62 98.91 138.40Magnesium Mg 7.65 15.04 80.14 109.27 141.27Aluminum Al 5.99 18.83 28.45 119.99 153.83
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Ionization Energies
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Stellar Spectral Classification
• Spectroscopy (photographic!) started in the late 19th century.
• Classifiers originally arranged them alphabetically, A-O
• A. J. Cannon figured out the right order from 200,000 objective prism spectra
• O B A F G K M • And astronomers realized it was a well
defined sequence in line strength.
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Digital spectral atlas
(the modern version of the photographic
atlas!)
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Hertzsprung-Russell Diagram
Multicolor photography --- Blue and Visual Plates allowed “colors”
Objective Prism Spectra
Realization that Spectral Type related to
Color related to
Surface Temperature
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Hipparcos
HR Diagram
4907 nearby stars with distances to
< 5%.
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HR Diagram in
Luminosity vs
Temperature
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Solar Spectrum
NOAO Atlas, N. Sharp
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Hydrogen Atom Energy Levels
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