Spectra

1
Spectra

• Physical Astronomy
• Professor Lee Carkner
• Lecture 4

2
Questions

• 1) Consider violet photons at 400 nm, red
  photons at 700nm and green photons at 550
  photons. If a star peaks at 550 nm will it emit
  more violet or red photons?
• Answer red
• Explain
• 2) Consider two stars at the same distance one
  red and one blue. If the red star looks
  brighter, which star is larger?
• Answer red
• Explain

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Spectroscopy

• Measuring the brightness (magnitude) of stars is
  called photometry
• Measuring the intensity of each wavelength of
  light from the star
• Can use to find
• temperature
• velocity

4
Spectrograph

• Diffraction and interference can separate light
  into wavelengths
• Use more slits, get better separation by
  wavelength
• R lave/Dl nN
• where n is the order and N is the number of
  rulings on the grating and lave is the average of
  the two wavelengths

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Spectral Lines

• If you take a single element, heat it up and pass
  its light through a grating you get a series of
  lines
• pattern of lines
• Determined by Kirchhoff and Bunsen about 1860

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Kirchhoffs Laws

• A hot, dense gas or object produces a continuous
  spectrum
• A hot, thin gas produces emission lines
• A cool, thin gas in front of a blackbody produces
  absorption lines

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Stellar Spectra

• Stars show absorption spectra
• Different stars show different lines at different
  strength

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Bohr Atom

• Angular momentum mvr nh/2p
• Coulomb energy (1/8pe0)(e2/r)
• Kinetic energy ½mv2
• Everything is constant except n
• E -13.6 eV (1/n2)
• Note that hc 1240 eV nm

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Hydrogen Series

• Transitions
• n1
• n2
• n3
• Transition from next level up is called a, from 2
  levels up b, etc.
• The Balmer series is the only one at visible
  wavelengths

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Consequences of Bohr Atom

• those with the energy to move electrons exactly
  between fixed levels
• Ephoton -13.6 eV (1/n2high)-(1/n2low)
• Note that electrons are not really orbiting
• Good for hydrogen only

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Kirchhoffs Laws Explained

• Atoms in a dense state have the quantum states
  blurred into a continuum
• Atoms in a cool, thin gas will absorb only the
  right photons removing them from the spectrum

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Doppler Effect

• For low speeds (vltltc)
• Dl/lrest v/c
• Note that Doppler shifts only give us velocity
  along the line of sight

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Photoelectric Effect

• Will only eject electrons if the incident photons
  have photon energy (hn) greater than the work
  function f
• The maximum kinetic energy is thus
• Demonstrates the quantum and particle nature of
  photons

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Compton Effect

• Photons can be scattered off of free electrons
• If me is the mass of the electron and q is the
  angular change in the photons path
• Dl (h/mec)(1-cos q)

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Next Time

• Read 2.1, 2.3, 2.4, 7.1
• Homework 5.2, 5.10, 5.11, 2.7, 2.8