© 2005 Pearson Education Inc., publishing as Addison-Wesley Light Spectra of Stars: Temperature determines the spectrum. Temperature Determines: 1. the overall distribution of light energy: Blackbody (Planck) function. Blackbody (Planck) function. 2.spectral lines, by the Excitation and ionization of atoms Excitation and ionization of atoms Astro 7A: Oct 23

© 2005 Pearson Education Inc., publishing as Addison-Wesley Stars Glow by Thermal Emission of Light Cool Warmer Hot Hotter Cool Warmer Hot Hotter Red & Faint White & Bright T = m-K T = m-K Flux at Surface =  T 4 Flux at Surface =  T 4 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 in an object by the emission & absorption lines in the spectrum.You can determine which atoms and their temperature in an object by the emission & absorption lines in the spectrum.

© 2005 Pearson Education Inc., publishing as Addison-Wesley Role of Temperature in Spectral 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.) Hydrogen Atom

© 2005 Pearson Education Inc., publishing as Addison-Wesley A Stars T = ,000 K Strongest H lines, Weak Ca + Abs. lines Wavelength (Angstroms) Flux Balmer   

© 2005 Pearson Education Inc., publishing as Addison-Wesley Stellar Spectra: Hottest to Coolest Flux Wavelength (nm) Balmer  Hot Surf: T=50,000K Cool Surf: T=2,500 K

© 2005 Pearson Education Inc., publishing as Addison-Wesley O Stars Hottest Stars: T>30,000 K Strong H e+ lines no H lines. (H is ionized) No Balmer 

© 2005 Pearson Education Inc., publishing as Addison-Wesley B Stars T = 11, ,000 K Strong neutral He lines (not He+) weak H lines, getting stronger from B0 through B9. B0B9 Balmer lines Balmer 

© 2005 Pearson Education Inc., publishing as Addison-Wesley A Stars T = ,000 K Strongest H lines, Weak Ca + Abs. lines Wavelength Flux Balmer   

© 2005 Pearson Education Inc., publishing as Addison-Wesley F Stars T = K H grows weaker through F9 Balmer 

© 2005 Pearson Education Inc., publishing as Addison-Wesley G Stars T = K Strong Ca +, Fe + and other metals dominate, H grows weaker through the class. Balmer 

© 2005 Pearson Education Inc., publishing as Addison-Wesley Solar Spectrum Balmer 

© 2005 Pearson Education Inc., publishing as Addison-Wesley K Stars T = K Strong metal lines, weak CH & CN molecular bands appear, growing through the class. H lines nearly gone. Balmer  Weak Balmer 

© 2005 Pearson Education Inc., publishing as Addison-Wesley M Stars: T = K strong molecular absorption bands particularly of TiO and VO as do lines of neutral metals. Virtually no H lines anymore. No Balmer 

© 2005 Pearson Education Inc., publishing as Addison-Wesley “L-Type” Stars: the Coolest Stars T = K; strong molecular absorption bands, CaH, LiH Also “metals” Na, K, cesium, and rubidium. No TiO and VO bands.

© 2005 Pearson Education Inc., publishing as Addison-Wesley T dwarfs T < 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 (CH 4 ), water (H 2 O), and neutral potassium.

© 2005 Pearson Education Inc., publishing as Addison-Wesley Stellar Spectra: Hottest to Coolest Flux Wavelength (nm) Balmer  Hot Surf: T=50,000K Cool Surf: T=2,500 K