1. Spectroscopy

2. Atoms and Light • Atomic electron energy levels are a source of discrete photon energies. • Change from a high to low energy state produces a photon. • Atoms can also absorb a photon to excite an electron.

3. Discrete Spectrum • Each atom has its own set of energy levels. • Each atom generates photons at specific frequencies. • The pattern of frequencies identifies the atom. helium neon

4. Absorption Lines • Ionized gases at a star’s surface absorb specific frequencies of light. • These appear as dark lines in a star’s spectrum. • Since gases ionize at different temperatures, the appearance of lines indicate the temperature of the star.

5. Energy states in molecules contribute to stellar spectra. Internuclear distances are quantized in discrete states. Vibrational energy Angular momentum for the molecule is quantized. Rotational energy Molecular Spectra

6. Atoms and molecules can reemit absorbed energy. Fluorescence typically involves three steps. Excitation to higher energy state. Loss of energy through change in vibrational state Emission of fluorescent photon. Fluorescence 10-12 s S1 10-15 s 10-7 s S0 next