Astronomical Spectroscopy

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Astronomical Spectroscopy
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A Review of Wavelength

• When we treat light as a wave, it has wavelength
• We split these wavelengths into regimes

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The Visible Spectrum

• The light we see with our eyes is only a small
  range of wavelengths
• Each color has a different wavelength and
  different properties
• Our suns emission actually peaks in the green
  part of the spectrum

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Spectroscopy in Astronomy

• In astronomy, all we have to study is the light
  coming from distant objects
• What can this tell us?
• Temperature
• Composition
• Information about our line of sight

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A Little History

• Newton used a prism to see the spectrum of the
  sun
• Wollaton (1802) noticed dark lines in the
  spectrum of the sun
• Fraunhofer (1817) also saw these lines
• Some were not in other stars
• Some stars had more/different lines

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How Do You Spread Light?

• A prism works by bending light (also called
  refraction) because it has a different index of
  refraction than air
• n1sin?1 n2sin?2 (n is the index of refraction)
• The colors spread out because different
  wavelengths bend more or less
• A diffraction grating works by creating
  interference, which spreads white light out into
  its different colors

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The 3 Laws of Spectral Analysis

• Kirchoff devised three laws of spectral analysis
  in 1859
• If something is heated to incandescence it will
  create a continuous spectrum
• A gas can be excited by electric currents to
  radiate an emission spectrum that is a pattern of
  lines unique to the composition of the gas
• If a cool gas is placed in front of a source, it
  will absorb energy and show an absorption
  spectrum that is the inverse of its emission
  spectrum

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The Blackbody Spectrum

• The size and shape of the curve tell us how hot
  the emitting object is

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What Causes Emission and Absorption?

• Bohr postulated that the atom was made of a
  nucleus and electrons which were bound at
  discrete energy levels
• Electrons can be excited out of low energy states
  into higher ones
• They can also fall from high states into lower
  states
• These processes give off or absorb energy at
  discrete wavelengths

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Whats Happening Physically?

• As an electron falls from n2 to n1, it gives
  off a photon of a certain wavelength
• This wavelength is always the same, but is a
  different wavelength for each element
• This process happens in reverse absorption

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Emission Spectra
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How Can We Use This?

• Astronomers look at emission or absorption
  spectra and measure the placement of the lines
• Since each element has unique lines, we can tell
  which elements we are looking at
• This can be hard there can be many elements
  mixed together and overlapping each other!
• These lines can also shift due to motion this
  is called the Doppler Effect

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The Doppler Effect

• Wavelengths of objects moving toward us get
  squished and look bluer than they should
• Wavelengths of objects moving away from us get
  stretched and look redder than they should
• If we measure this shift to either shorter or
  longer wavelengths, we can tell if something is
  moving toward or away from us

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Basic Lab Procedure

• There are four stations set up pick one (any
  one) and start there, making sure you get to each
  part
• Answer the questions in the manual as you go
• Please dont tell others the mystery gas let
  them figure it out for themselves!