Taking “Fingerprints” of Stars, Galaxies, and Other Stuff Absorption and Emission from Atoms, Ions, and Molecules

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

Planck’s Theory, 1901 Light with wavelength  has frequency  = c/ can “exchange” energy with matter (atoms) in units of: E = h h is “Planck’s constant” h = 6.625  10-34 Joule-seconds

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 +

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

Bohr Atom Absorption of Photon “kicks” electron to “higher” orbital + - - +

Bohr Atom Emission of Photon makes Electron “drop” to “lower” orbital - - +

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

“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?

Ensemble of Atoms in “Low” States + -

Ensemble of Atoms in “Low” States Photons from Star at “correct”  are absorbed, and thus removed from the observed light Absorption Line + -

Absorption “lines” Discovered in Solar spectrum by Fraunhofer called “Fraunhofer Lines” “Lines” because they appear as dark bands superimposed on “rainbow” of visible spectrum

Ensemble of Atoms in “High” States + -

Ensemble of Atoms in “High” States Photons at “correct”  are emitted, and thus added to any observed light Emission Line + - Dark Background

Emission line spectrum Appear as Bright Bands on “Faint Background Spectrum” Why the Background??

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 + - + - + - + - + - + -

Why would an ensemble of atoms (i. e 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

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 >> 0 K (ensemble of atoms is hot), the thermal energy “kicks” most atoms into “high” state can easily emit light

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)

Sidebar: LASER External source maintains “energy inversion” more electrons in “high” state, even during and after emission high Emission low After “Driving” After Emission

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

Sun’s Fraunhofer absorption lines (wavelengths listed in Angstroms; 1 Å = 0.1 nm)

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

Emission line spectra Insert various emission line spectra here

What Wavelengths are Involved? Depends on the Size of the “Gaps” between Energy States in the atoms

Energies of H Orbitals Limiting Energy Energies of Orbitals of H “Transitions” between Orbitals

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

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

Relate Size of “Jump” to the  Absorbed or Emitted Very Small E  Very Long   radio waves Very Large E  Very Short   X rays

Sidebar: A Transition with Very 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 High-E State Low-E State

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

Bohr Atom: Extension 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

Optical Emission-Line Spectrum of “Young Star” Intensity  (in Angstroms Å, or units of 10 nm)

Emission line images green  oxygen red  hydrogen Planetary nebula NGC 6543 (blue: X Rays) Orion Nebula green  oxygen red  hydrogen

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)

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

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

Rank Molecular Transitions by Energy UV, Visible, IR  Electronic NIR  Vibrational Radio  Rotational Radio  H “spin flip” @  = 1420 MHz

Molecular Emission: Vibrational Transition Planetary nebula NGC 2346 Electronic Transition (visible light) Vibrational Molecular Hydrogen Transition (IR)

Molecular Emission: Rotational Transition Rotational CO (carbon monoxide) Emission from Molecular Clouds in “Milky Way”

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

1. Filter Spectrometer Filters in Rotating “Filter Wheel” Sequence of “Monochrome” Images thru Different Colors (How the images in the laboratory were created)

2. Prism Spectrometer Recall: Optical Dispersion n 

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

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

Long  “dispersed” by smallest angle  2. Prism Spectrometer Red Blue White Light In Long  “dispersed” by smallest angle 

2. Prism Spectrometer Problems: Glass absorbs some light Ultraviolet light Why you can’t 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”

3. Grating Spectrometer “Interference” of Light Different  Interfere Red  Different  Interfere at Different   Blue 

3. Grating Spectrometer White Light In Red Blue Long  “diverges: at largest angle  Long  “dispersed” by largest angle  Can be constructed for all wavelengths

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