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Taking the fingerprints of stars, galaxies, and interstellar gas clouds. Absorption and emission from atoms, ions, and molecules. The periodic table of the elements.
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Taking the fingerprints of stars, galaxies, and interstellar gas clouds Absorption and emission from atoms, ions, and molecules
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
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
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
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
Sun’s Fraunhofer absorption lines (wavelengths listed in Angstroms; 1 A = 0.1 nm)
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
Emission line spectra Insert various emission line spectra here
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
Emission line images Green: oxygen; red: hydrogen Planetary nebula NGC 6543 (blue: Xrays) Orion Nebula
Neon Iron 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)
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
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
Molecular emission: vibrational Planetary nebula NGC 2346 left: atomic emission (visible light) right: vibrational molecular hydrogen emission (infrared)
Molecular emission: rotational Rotational CO (carbon monoxide) emission from molecular clouds in the Milky Way