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Taking the fingerprints of stars, galaxies, and interstellar gas
cloudsAbsorption 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
• Electron’s 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 Atom:Extension 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, 6;etc...
• 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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Sun’s Fraunhofer absorption lines
(wavelengths listed in Angstroms; 1 A = 0.1 nm)
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Geometries for producing absorption lines
• 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
The Observer
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Emission line spectra
Insert various emission line spectra here
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Geometries for producing emission lines
• 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
The Observer
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The optical emission line spectrum of a young star
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Emission line images
Planetary nebula NGC 6543(blue: Xrays)
Green: oxygen; red: hydrogen
Orion Nebula
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Spectra of ions• 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)
Neon Iron
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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 molecule’s 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 2346left: 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