Astronomical Spectroscopy

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Astronomical Spectroscopy
Spectroscopy is the study of the way matter
absorbs/emits light. a SPECTRUM is like a
fingerprint of an object
collection of photons of different wavelengths,
frequencies, or energies a BLACKBODY
spectrum is the first step to fingerprinting
a blackbody re-emits all energy (i.e.
photons) it absorbs emits a
characteristic distribution of photons
a blackbody curve is the shape of the emitted
light from an object even YOU emit a blackbody
spectrum
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Blackbody Equation
Planck radiation law (Planck Function)
2h?3 1
B?(T) ---------- ---------------
c2 e(h?/kT) - 1
brightness B?(T) has units of erg s -1 cm-2
Hz -1 ster -1 Rayleigh-Jeans tail h? ltlt
kT B?(T) (2?2/c2) kT decreasing freq
shallow second power drop Wien cliff h? gtgt
kT B?(T) (2h?3/c2)e-(h?/kT) increasing freq
steep exponential drop
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Blackbody Spectrum
(2?2/c2) kT
(2h?3/c2)e-(h?/kT)
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Temperature Scales

• F C K

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Blackbody Spectrum and Temp

• position of blackbody depends on temperature

so, objects of different temperatures look
different colors!
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1. Weins Law
location of blackbody curve peak determined by
temperature take derivative of blackbody
equation use blackbody curve as a
THERMOMETER Wobble Law peak moves
left/right ? max (in microns) 2900 /
T (in Kelvin) Sun has T 5800 K
?
max 2900 / 5800 K 0.5 microns (or 5000
Å) YOU have T 310 K
? max 2900 /
310 K 9.4 microns
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2. Stefan-Boltzmann Law
height of blackbody curve determined by
temperature AND SIZE integrate blackbody
equation over all angles and frequencies
hotter object more photons bigger object
more photons Size Law peak moves up/down to
more/less energy
luminous energy surface area X
energy/area E total L 4pR2 sT4
( in erg s -1) L R2 T4 Sun has T
5800 K now, but when it turns into a red giant
T drops to 2900 K L drops by
factor of 16 R increases to 100 X radius
today L rises by factor of
10000 together L increases by factor of 600
were COOKED!
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Spectral Features Light and Matter
atoms are made up of particles protons,
neutrons, electrons H is simplest 1
proton, 1 electron (if neutral) He is
next 2 protons, 2 neutrons, 2 electrons (if
neutral) C is 6 6 protons, 6
neutrons, 6 electrons (if neutral) protons
DEFINES the element Bohr Atom is a model that
explains interaction of light and
atoms invokes particle nature of photons
? discrete amounts of energy are needed
for absorption/emission ? e
orbitals at specific energy levels

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Bohr Atom

• energy levels

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electron options

• once excited by photon, electron has options

1. re-emits 2. cascades 3. ionized
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Atoms are easy, Molecules
molecules are made up of more than one atom
each atom provides options
absorption rotation vibration
He (easy) .. C (not bad)
CO (ugh!) ?
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H atom vs. H2 molecule
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Link to Spectroscopy
each atom/isotope/molecule has a fingerprint a
combination of atomic fingerprints is emitted by
each object a galaxy, a star, a planet, or YOU
emit a complicated spectrum
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Spectrum of the Sun
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Emission and Absorption
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Creation of Spectra
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Four Types of Spectra
1. CONTINUOUS spectrum
BLACKBODY spectrum atoms
are wiggling photons emitted light bulb, Sun
(sort of) 2. ABSORPTION
spectrum atoms catching
photons see dark lines He in Sun, Earth CO2
3. EMISSION spectrum
atoms pitching photons see bright
lines aurorae, meteors 4.
REFLECTION spectrum
combination of catching/pitching bright dark
features Jupiters
atmosphere absorbs some sunlight, reflects some
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Earth Radiation
J H K L M
trace gas trace gas trace gas trace gas
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