Taking

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Taking Fingerprints of Stars, Galaxies, and
Other Stuff

• Absorption and Emission from Atoms, Ions, and
  Molecules

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Periodic Table of Elements

• Universe is mostly (97) Hydrogen and Helium (H
  and He)
• The ONLY elements created in the Big Bang were H,
  He, and a little lithium, Li
• All heavier elements have been (and are still
  being) manufactured by stars via nuclear fusion
• Each element has characteristic set of energies
  where absorbs or radiates

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Plancks Theory, 1901

• Light with wavelength ? has frequency
• ? c/?
• can exchange energy with matter (atoms) in
  units of
• E h?
• h is Plancks constant
• h 6.625 ? 10-34 Joule-seconds

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The Bohr Atom

• Model of Hydrogen atom
• Introduced by Niels Bohr early in 1913
• to explain emission and absorption of light by H
• 1 proton ( nucleus) orbited by 1 electron

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The Bohr Atom

• Electron orbits have fixed sizes - orbitals
• Not Like Planets in a Solar System
• atomic orbitals are QUANTIZED
• only some orbital radii are allowed
• was very confusing to physicists
• first deduced by physicist Neils Bohr
• Movement of electron e- between orbitals requires
  absorption or radiation of energy
• jump from lower to higher orbital ? atom absorbs
  energy
• jump from higher to lower orbital ? atom emits
  energy

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Bohr Atom
Absorption of Photon kicks electron to
higher orbital

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Bohr Atom
Emission of Photon makes Electron drop to
lower orbital

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Absorption vs. Emission

• Atom absorbs photon if electron kicked up to
  a higher energy state
• Atom emits photon if electron drops down to a
  lower state
• Again, only a certain set of energy states is
  allowed
• set of states depends on the atom or molecule

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Ensembles (Groups) of Atoms

• States of individual H atoms in a group are not
  identical
• Some electrons are in low states and are more
  likely to absorb photons
• Some electrons are in high states and more
  likely to emit photons
• What determines the distribution of states of a
  group of atoms?

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Ensemble of Atoms in Low States
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Ensemble of Atoms in Low States

• Photons from Star at correct ? are absorbed,
  and thus
• removed from the observed light
• Absorption Line

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Absorption lines

• Discovered in Solar spectrum by Fraunhofer
• called Fraunhofer Lines
• Lines because they appear as dark bands
  superimposed on rainbow of visible spectrum

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Ensemble of Atoms in High States
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Ensemble of Atoms in High States

• Photons at correct ? are emitted, and thus
  added to any observed light
• Emission Line

Dark Background
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Emission line spectrum
Appear as Bright Bands on Faint Background
Spectrum Why the Background??
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All real gas clouds have atoms in both states,
with one state usually dominant
Absorption Emission
More absorption if more atoms in low
state More emission if more atoms in high state
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Why would an ensemble of atoms (i.e. a gas cloud)
be in High or Low state?

• Some other mechanism (besides light) must be at
  work! But what?

TEMPERATURE T
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Effect of Thermal Energy

• If T ? 0 K (ensemble of atoms is very cold), most
  atoms are in low state
• can easily absorb light
• If T gtgt 0 K (ensemble of atoms is hot), the
  thermal energy kicks most atoms into high
  state
• can easily emit light

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Sidebar LASER

• Electrons in the medium (gas, solid, or diode) of
  a LASER are driven to high state by external
  energy
• Emit simultaneously and with same phase
• External Energy
• electrical
• optical (external light source, flash lamp)

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Sidebar LASER

• External source maintains energy inversion
• more electrons in high state, even during and
  after emission

high
Emission
low
After Emission
After Driving
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Geometries for producing absorption lines
1 2
The Observer

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

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Suns Fraunhofer absorption lines
(wavelengths listed in Angstroms 1 Å 0.1 nm)
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Geometries for producing emission lines
1 2
The Observer

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

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Emission line spectra
Insert various emission line spectra here
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What Wavelengths are Involved?

• Depends on the Size of the Gaps between Energy
  States in the atoms

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Energies of H Orbitals
Limiting Energy
Energies of Orbitals of H Transitions
between Orbitals
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Ionization of Hydrogen
Limiting Energy

• If electron absorbs sufficient
• energy E to rise above the upper
• limit of energy for a bound
• electron, then the electron becomes
• ionized
• electron escapes the proton

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Relate Size of Gap to Wavelength of Light

• Larger gaps or jumps in energy (both absorbed
  and emitted) ? photon carries more energy
• Recall
• Larger ?E ? Shorter ? ? bluer light
• Smaller ?E ? Longer ? ? redder light

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Relate Size of Jump to the ? Absorbed or
Emitted

• Very Small ?E ? Very Long ? ? radio waves
• Very Large ?E ? Very Short ? ? X rays

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Sidebar A Transition withVery Small ?E ? Very
Long ?

• Due to spin flip of e- in Hydrogen Atom
• ?E hc/? ? 9.4 10-25 Joules
• ? ? ? 0.21 m 21 cm
• ? ? ? 1420.4 MHz - RADIO Wave

Low-E State
High-E State
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Sidebar 21-cm Radio Wave of H

• First observed in 1951
• Simultaneously Discovered at 3 observatories!!
  (Harvard, Leiden, Sydney)
• Measures the H in interstellar matter
• Map of Spiral Arms in Milky Way Galaxy

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Bohr AtomExtension to other elements

• H is simplest atom, BUT concept of electron
  orbitals applies to all atoms
• Neutral atoms have equal numbers of protons (in
  nucleus) and electrons (orbiting nucleus)
• He has 2 protons 2 electrons Lithium (Li), 3
  each Carbon (C) , 6 each, etc. ...
• In atoms with more electrons (and protons), the
  absorption/emission spectrum is more complicated

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Optical Emission-Line Spectrum of Young Star
Intensity
? (in Angstroms Å, or units of 10 nm)
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Emission line images
Planetary nebula NGC 6543 (blue X Rays)
Orion Nebula
green ? oxygen red ? hydrogen
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Spectra of ions
Neon
Iron

• Emission lines from heavy ions dominate
  high-energy (X-ray) spectra of stars
• atoms stripped of one or more electrons
• Ions of certain heavier elements (e.g., neon and
  iron with only one electron) behave much like
  supercharged H and He

Wavelength (in Angstroms)
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Spectra of Molecules

• Also have characteristic spectra of emission and
  absorption lines
• Each molecule has particular set of allowed
  energies at which it absorbs or radiates
• Molecules are more complicated than atoms
• Resulting spectra are also more complicated
• Electrons shared by one (or more) atoms in
  molecule absorb or emit specific energies
• Changes in state of vibration and/or rotation are
  also quantized
• Vibration, rotation spectra unique to each
  molecule

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More on Molecular Spectra

• Transitions between different orbitals of
  molecules (electronic states)
• mostly in ultraviolet (UV), optical, and infrared
  (IR) regions of spectrum
• Transitions between different Vibrational
  states
• mostly in the near-infrared (NIR)
• Transitions between different Rotational states
• mostly in the radio region

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Rank Molecular Transitions by Energy

1. UV, Visible, IR ? Electronic
2. NIR ? Vibrational
3. Radio ? Rotational
4. Radio ? H spin flip _at_ ? 1420 MHz

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Molecular Emission Vibrational Transition
Planetary nebula NGC 2346
Electronic Transition (visible light)
Vibrational Molecular Hydrogen Transition (IR)
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Molecular Emission Rotational Transition
Rotational CO (carbon monoxide) Emission from
Molecular Clouds in Milky Way
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Q How Can We Measure Spectra?

• A With a Spectrum Measurer
• A SPECTROMETER
• Splits light into its constituent wavelengths
• Common Mechanisms for Splitting Light
• Optical Filters
• Blocks light except in desired band
• Dispersion of Glass Differential Refraction
• Prism
• Diffraction Grating

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1. Filter Spectrometer
Filters in Rotating Filter Wheel Sequence of
Monochrome Images thru Different Colors (How
the images in the laboratory were created)
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2. Prism Spectrometer
Recall Optical Dispersion
n
?
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2. Prism Spectrometer

• Refractive Index n measures the velocity of
  light in matter
• c velocity in vacuum ? 3 ?108 meters/second
• v velocity in medium measured in same units
• n ? 1.0

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2. Prism Spectrometer

• Refractive index n of glass DECREASES with
  increasing wavelength ?
• Make a glass device that uses optical dispersion
  to separate the wavelengths
• a PRISM

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2. Prism Spectrometer
?Blue
?Red
White Light In
Long ? dispersed by smallest angle ?
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2. Prism SpectrometerProblems

• Glass absorbs some light
• Ultraviolet light
• Why you cant get a suntan under glass
• Infrared light
• Images taken in different ? will overlap
• Dispersion Angle ? is a complicated function of
  wavelength ?
• Spectrometer is difficult to calibrate

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3. Grating Spectrometer
Interference of Light
?Red
?
Different ? Interfere at Different ?
?
?Blue
?
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3. Grating Spectrometer
White Light In
?Red
?Blue
Long ? diverges at largest angle ?
Long ? dispersed by largest angle ? Can be
constructed for all wavelengths
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3. Grating Spectrometer

• Uses Diffraction Grating
• works by interference of light
• Regularly spaced transparent opaque regions
  
• Can be made without absorbing glass
• Used at all wavelengths (visible, UV, IR, X-Rays,
  )
• Dispersion angle ? is proportional to ?
• Easy to calibrate!
• Images at different ? can still overlap

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