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© 2005 Pearson Education Inc., publishing as Addison-Wesley
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Spectra of Stars:Spectra of Stars:Temperature determines the spectrum.Temperature determines the spectrum.
Temperature Determines:Temperature Determines:
1. the overall distribution of light energy:1. the overall distribution of light energy: Blackbody (Planck) function.Blackbody (Planck) function.2.2. spectral lines, by thespectral lines, by the Excitation and ionization of atomsExcitation and ionization of atoms
Astro 7A: Oct 23Astro 7A: Oct 23
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Stars Glow by
Thermal Emission of Light
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Cool Warmer Hot HotterCool Warmer Hot HotterRed & Faint White & BrightRed & Faint White & Bright
T = 0.00290 m-KT = 0.00290 m-K Flux at Surface = Flux at Surface = T T44
Stars emit light according to the Planck Function (blackbody).Stars emit light according to the Planck Function (blackbody).
© 2005 Pearson Education Inc., publishing as Addison-Wesley
Emission Spectra
• Each type of atom or molecule has a unique set of electron energy levels.
• Each emits its own set of wavelengths of light.
• Unique Emission line spectrum for each atomor molecule.
© 2005 Pearson Education Inc., publishing as Addison-Wesley
Absorption of Light by Atoms & Molecules
• When light shines through a gas, atoms will absorb those photons whose wavelengths match the atom’s electron energy levels.
• The resulting spectrum has all wavelengths (all colors), but is missing those wavelengths that were absorbed.
• You can determine which atoms and their temperature You can determine which atoms and their temperature in an object by the emission & absorption lines in the in an object by the emission & absorption lines in the spectrum.spectrum.
© 2005 Pearson Education Inc., publishing as Addison-Wesley
Role of Temperature inSpectral Lines
Example: Balmer absorption:
Balmer lines only occur if:1. Hydrogen is neutral, not
ionized. (Saha Eqn.)
2. Hydrogen atoms are in n=2 level already, able to absorb a photon. (Boltzmann Eqn.)
HydrogenAtom
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A StarsT = 7500 - 11,000 K Strongest H lines, Weak Ca+ Abs. lines
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Wavelength (Angstroms)Wavelength (Angstroms)
Flu
xF
lux
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Balmer Balmer
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Stellar Spectra: Hottest to CoolestStellar Spectra: Hottest to Coolest
Flu
xF
lux
Wavelength (nm)Wavelength (nm)
Balmer Balmer
Hot Surf:Hot Surf:T=50,000KT=50,000K
Cool Surf:Cool Surf:T=2,500 KT=2,500 K
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O Stars
Hottest Stars: T>30,000 KStrong He+ linesno H lines. (H is ionized)
No Balmer No Balmer
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B Stars
T = 11,000 - 30,000 K Strong neutral He lines (not He+) weak H lines, getting stronger from B0 through B9.
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B0B0
B9B9
Balmer linesBalmer lines
Balmer Balmer
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A StarsT = 7500 - 11,000 K Strongest H lines, Weak Ca+ Abs. lines
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WavelengthWavelength
Flu
xF
lux
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Balmer Balmer
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F StarsT = 5900 - 7500 K H grows weaker through F9 Ca+ grows stronger, weak metals begin to emerge.
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Balmer Balmer
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G StarsT = 5200 - 5900 KStrong Ca+, Fe+ and other metals dominate, H grows weaker through the class.
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Balmer Balmer
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Solar SpectrumSolar SpectrumBalmer Balmer
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K StarsT = 3900 - 5200 KStrong metal lines, weak CH & CN molecular bands appear, growing through the class. H lines nearly gone.
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Balmer Balmer Weak Balmer Weak Balmer
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M Stars: T = 2500 - 3900 K strong molecular absorption bands particularly of TiO and VO as do lines of neutral metals. Virtually no H lines anymore.
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No Balmer No Balmer
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“L-Type” Stars: the Coolest Stars T = 1300 - 2500 K; strong molecular absorption bands, CaH, LiHAlso “metals” Na, K, cesium, and rubidium. No TiO and VO bands.
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T dwarfsT < 1300 K very low-mass objects, not technically stars anymore because they are below the Hydrogen fusion limit (so-called "Brown Dwarfs"). T dwarfs have cool Jupiter-like atmospheres with strong absorption from methane (CH4), water (H2O), and neutral potassium.
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Stellar Spectra: Hottest to CoolestStellar Spectra: Hottest to Coolest
Flu
xF
lux
Wavelength (nm)Wavelength (nm)
Balmer Balmer
Hot Surf:Hot Surf:T=50,000KT=50,000K
Cool Surf:Cool Surf:T=2,500 KT=2,500 K