Astronomical Spectroscopy

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Astronomical Spectroscopy
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The Electromagnetic Spectrum
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The Visible Spectrum

• Newton first held a prism up to white light and
  saw the visible spectrum
• Blue light is higher frequency and shorter
  wavelength 400nm
• Red light is lower frequency and longer
  wavelength 700nm
• 1 nanometer (nm) 1x10-9 meters (m)

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• Prisms refract light of different wavelengths
  different amounts resulting in dispersion of the
  colors
• Diffraction gratings diffract light of different
  wavelengths different amounts creating
  interference patterns which disperse the colors
• Grating spectrometers use a diffraction grating
  to disperse the light, then assign a scale to
  measure its wavelength

• The Solar Spectrum
• The Sun contains a continuous source below the
  surface
• Cooler gas in front absorbs at specific
  wavelengths producing a rainbow spectrum with
  dark lines
• Dark lines were first noticed by Wollaston in
  1802
• Fraunhofer identified over 575 of these lines in
  the sun in 1814. They are now named after him

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Kirchof's Laws
1. A heated source emits a continuous spectrum of
radiation Continuous Spectrum 2. A glowing gas
emits light at particular wavelengths
corresponding to its composition Emission Line
Spectrum 3. A cooler gas with a continuous source
behind it emits a continuous spectrum with dark
lines at specific wavelengths corresponding to
its composition Absorption Line Spectrum
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Blackbody Spectrum
Emission from a continuous source reveals the
temperature of the emitting body based on the
shape of the Blackbody Curve Different
temperatures have different intensities at
different wavelengths The Gator Star
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Bohr Atom

• Electrons reside in discrete energy levels
  around the nucleus
• Electrons can be excited by a photon to higher
  energies
• Electrons at higher energies soon drop down to
  lower energies, and emit a photon with an energy
  equal to the difference in levels
• Energy is proportional to wavelength
• This photon always has the same energy i.e. the
  same wavelength!
• Hydrogen Balmer lines come from electrons falling
  from higher energy levels to the level n2, or
  the first excited state of the atom.

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• Every element has a unique set of energies levels
  in its atoms which correspond to a unique set of
  emission or absorption lines in the gas
• Emission and absorption occur at the exact same
  wavelength.
• Emission when the electron loses energy and
  releases a photon
• Absorption when the same electron gains energy by
  absorbing a photon with the same energy and
  wavelength
• Astronomers use this to determine the presence of
  elements in stars, galaxies, planets and nebula

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Doppler Effect
Astronomers use the Doppler effect when looking
at stellar spectra to find shifts in the location
of the expected lines due to a second body such
as a binary star or even extra solar planets!
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This Experiment

• Please DO NOT touch the glass tubes
• Turn off the power supplies when not in use-they
  will eventually burn out!
• DO NOT adjust the slit width as mentioned in the
  lab
• Start the part B. with the scale light OFF the
  scale will produce a secondary spectrum which you
  should ignore, so find the real spectrum first

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Hydrogen Balmer Lines

• H-alpha 656.3 nm
• H-beta 486.1 nm
• H-gamma 434.1 nm
• H-delta 410.1 nm