Download Taking fingerprints of stars

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
The
Observer
• 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
Emission line spectra
Insert various emission line spectra here
Geometries for producing
emission lines
The
Observer
• 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 optical emission line spectrum of a
young star
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