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

The optical emission line spectrum of a young star

Emission line images Planetary nebula NGC 6543 (blue: Xrays) Green: oxygen; red: hydrogen Orion Nebula

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

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