Stellar Spectra

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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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• What is a Spectrum.
• The Atmospheres of stars.
• Spectral Lines Quantum Mechanics (the Bohr
  atom)
• Ionization
• Spectral Types
• The Hertzsprung-Russell Diagram (intro to the
  evolution of stars)

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Diffraction/Refraction

Grating diffracts
Prism refracts
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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 5th
• Hydrogen H 13.60
• Helium He 24.59 54.42
• Lithium Li 5.39 75.64 122.45
• Beryllium Be 9.32 18.21 153.90
  217.72
• Boron B 8.30 25.15 37.93
  259.38 340.23
• Carbon C 11.26 24.38 47.89
  64.49 392.09
• Nitrogen N 14.53 29.60 47.45
  77.47 97.89
• Oxygen O 13.62 35.12 54.94
  77.41 113.90
• Fluorine F 17.42 34.97 62.70
  87.14 114.24
• Neon Ne 21.56 40.96 63.45
  97.12 126.21
• Sodium Na 5.14 47.29 71.62
  98.91 138.40
• Magnesium Mg 7.65 15.04 80.14 109.27
  141.27
• Aluminum 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
• lt 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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