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Transcript
Stellar and nebular spectra • What we see from different angles. • For a star, gas is its cooler atmosphere layers • What’s going on? …. (Paint/demo) Hydrogen Spectral Line Formation • Transitions in Bohr atom model give rise to different groups • Optical: Balmer (n=2) • UV: Lyman (n=1) • Ionization is transitions to/from infinity. Balmer lines of Hydrogen • Transitions are to/from n=2 level • 3/2 is smallest change, thus lowest energy, thus ?? Color? • What is the Balmer limit shown? Doppler Effect on spectral lines • Stationary source as shown at right emits wave fronts • Moving source stretches wavelengths behind and shortens ahead Doppler Equation Δλ / λ0 = v / c Where C = speed of light V = radial velocity λ0 = rest wavelength Δλ = change in wavelength or “Doppler shift” Doppler-shifted wavelength = λ + Δλ Shown here Δλ is negative (blueshift) Use of Doppler Effect to find Planets • Discover planets around other stars. • More than a 200 now known. Use Doppler to make a Radial Velocity Curve Notes: errors