Taking the fingerprints of stars, galaxies, and interstellar gas

cloudsAbsorption 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 2346left: 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