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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).

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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.

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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.

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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

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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