1. Stellar Spectra Physical Astronomy Professor Lee Carkner Lecture 7

2. Stellar Spectra   If a star shows strong hydrogen lines, does that mean it has a lot of hydrogen?   It means it has a temperature such that the hydrogen atoms are in the right state to emit a lot of the right photons   Too hot, all ionized

3. Distribution   The atoms all have different energies!   The temperature is a measure of the average kinetic energy of the atoms  v rms = (3kT/m) ½

4. Boltzmann Equation  As the atoms collide, the electrons move among energy states  E a =  g a =  g is called the degeneracy   Remember, n is the level (n=1 is ground state)  The ratio of the number of atoms with energy E b to energy E a is: N b /N a = (g b /g a )e -(Eb-Ea)/kT  Can use k = 8.62X10 -5 eV/K 

5. Ionization  We also need to know the ionization state of the atoms   Depends on the partition function, Z   Given by: Z =  g j e –(Ej-E1)/kT  where we sum over all states (starting with j=1)

6. Stellar Atmospheres   Just enough atoms in excited state without too many in ionized state

7. Spectral Type and Lines  O an B stars:  A and F stars:  G and K stars:  M stars: molecules blue white yellow red

8. Next Time  Read: 8.2  Homework: 8.5, 8.6b-c, 8.7, 8.15, 8.16