Download Light - Dan Caton

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