Different colors of light have different wavelengths

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Different colors of light have different
wavelengths
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What is a Blackbody?

• Absorbs all incident radiation
• Emits radiation that depends only on temperature
• There is no perfect blackbody, but it is a good
  approximation for many things
• heated metal
• the Sun (and all other stars)
• planets asteroids
• the early universe

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Blackbody radiation
0C 32F 273 K
(UV)
(visible)
(Infrared)
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Blackbody Radiation Laws

• The dominant wavelength of radiation emitted by a
  blackbody is inversely proportional to its
  temperature (Wiens Law)
• ?max 2.9 x 10-3 / T
• An object emits energy at a rate proportional to
  the fourth power of its temperature
  (Stefan-Boltzmann Law)
• E sT4

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Cosmic background radiation
2.73 K
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Blackbody radiation is explained by quantum
mechanics

• High temperature objects emit high-energy photons
  (shorter wavelengths)
• Plancks Law
• E h c / ?

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Each chemical element produces its own unique set
of spectral lines when it burns
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The Suns Spectrum
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Spectroscopy(spectral analysis)

• Analyzing components of light received from
  objects
• Determine composition from spectrum
• Can also determine velocity (from redshift)

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Spectrum of hydrogen
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Kirchhoffs Laws

• A hot object or a hot, dense gas produces a
  continuous spectrum
• A hot, rarefied gas produces an emission line
  spectrum
• A cool gas in front of a continuous source of
  light produces an absorption line spectrum

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Emission Nebulae
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Energy levels of a hydrogen atom
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Spectral lines occur when an electron jumps from
one energy level to another
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Absorbtion
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Emission
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What can we learn by analyzing starlight?

• A stars temperature
• by peak wavelength
• A stars chemical composition
• by spectral analysis
• A stars radial velocity
• from Doppler shifts

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Redshift
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Proper motion of Barnards Star
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Summary

• Blackbody radiation
• wavelength decreases (and energy increases) as
  temperature increases
• Spectral Lines
• electron energy levels
• Doppler shift
• redshift/blueshift and radial velocity