Taking the fingerprints of stars, galaxies, and interstellar gas clouds

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Taking the fingerprints of stars, galaxies, and
interstellar gas clouds

• Absorption and emission from atoms, ions, and
  molecules

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The periodic table of the elements

• The universe is mostly (97) hydrogen and helium
  H and He (and a little lithium, Li) were the only
  elements created in the Big Bang
• heavier elements have all been (and are still
  being) manufactured in stars, via nuclear fusion
• Each element has its own characteristic set of
  energies at which it absorbs or radiates

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The Bohr Atom

• Hydrogen atom consists of a proton (nucleus)
  orbited by an electron
• Unlike a satellite, electron cannot orbit at
  arbitrary distances from nucleus
• electron has specific, fixed set of orbitals
• atomic structure is quantized
• quantized structure first deduced by physicist
  Neils Bohr
• Electrons movement between orbitals requires
  absorption or radiation of energy
• jump from lower to higher orbital energy
  absorbed
• jump from higher to lower orbital energy emitted

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Bohr AtomExtension to other elements

• Although H is the simplest atom, the concept of
  electron orbitals applies to all elements
• Neutral atoms have equal numbers of protons (in
  nucleus) and electrons (orbiting nucleus)
• He has charge of 2 Li, 3 C, 6etc...
• The more electrons (protons) characterizing an
  element, the more complex its absorption/emission
  spectrum

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Absorption lines

• First discovered in spectrum of Sun (by an
  imaging scientist named Fraunhofer)
• Called lines because they appear as dark lines
  superimposed on the rainbow of the visible
  spectrum

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Suns Fraunhofer absorption lines
(wavelengths listed in Angstroms 1 A 0.1 nm)
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Geometries for producing absorption lines
The Observer

• Absorption lines require a cool gas between the
  observer and a hot source
• scenario 1 the atmosphere of a star
• scenario 2 gas cloud between a star and the
  observer

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Emission line spectra
Insert various emission line spectra here
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Geometries for producing emission lines
The Observer

• Emission lines just require a gas viewed against
  a colder background
• scenario 1 the hot corona of a star
• scenario 2 cold gas cloud seen against empty
  (colder) space

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The optical emission line spectrum of a young star
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Emission line images
Green oxygen red hydrogen
Planetary nebula NGC 6543 (blue Xrays)
Orion Nebula
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Spectra of ions
Neon
Iron

• Emission lines from heavy ions -- atoms stripped
  of one or more electrons -- dominate the
  high-energy (X-ray) spectra of stars
• Ions of certain heavier elements (for example,
  highly ionized neon and iron) behave just like
  supercharged H and He

Wavelength (in Angstroms)
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Molecular spectra

• Molecules also produce characteristic spectra of
  emission and absorption lines
• Each molecule has its particular set of allowed
  energies at which it absorbs or radiates
• Molecules -- being more complex than atoms -- can
  exhibit very complex spectra
• Electrons shared by one or more atoms in
  molecule will absorb or emit specific energies
• Change in molecules state of vibration and/or
  rotation is also quantized
• Vibration, rotation spectra unique to each
  molecule

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Molecular spectra (cont.)

• Electronic transitions mostly show up in the UV,
  optical, and IR
• Vibrational transitions mostly show up in the
  near-infrared
• Rotational transitions mostly show up in the
  radio

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Molecular emission vibrational
Planetary nebula NGC 2346 left atomic emission
(visible light) right vibrational molecular
hydrogen emission (infrared)
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Molecular emission rotational
Rotational CO (carbon monoxide) emission from
molecular clouds in the Milky Way