1. 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

2. brightness Bν(T) has units of erg s -1 cm-2 Hz -1 ster -1
Blackbody Equation
Planck radiation law (Planck Function)
2hν
Bν(T) =
c e(hν/kT) - 1
brightness Bν(T) has units of erg s -1 cm-2 Hz -1 ster -1
Rayleigh-Jeans tail hν << kT Bν(T) ~ (2ν2/c2) kT decreasing freq … shallow second power drop
Wien cliff hν >> kT Bν(T) ~ (2hν3/c2)e-(hν/kT) increasing freq … steep exponential drop

3. Blackbody Spectrum
(2ν2/c2) kT
(2hν3/c2)e-(hν/kT)

4. Temperature Scales
°F °C °K

5. Blackbody Spectrum and Temp
position of blackbody depends on temperature …
…so, objects of different temperatures look different colors!

6. λ max (in microns) = 2900 / T (in Kelvin)
1. Wein’s 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
λ max (in microns) = / T (in Kelvin)
Sun has T = 5800 K …
λ max = / 5800 K = 0.5 microns (or 5000 Å)
YOU have T = 310 K …
λ max = / 310 K = microns

7. 2. Stefan-Boltzmann Law L ~ R2 T4
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 = 4πR2 σT4 ( 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 … we’re COOKED!

8. 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

9. Bohr Atom
energy levels

10. electron options once excited by photon, electron has options …
1. re-emits
2. cascades
3. ionized

11. 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!) 

12. H atom vs. H2 molecule

13. 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

14. Spectrum of the Sun

15. Emission and Absorption

16. Creation of Spectra

17. 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 …
Jupiter’s atmosphere absorbs some sunlight, reflects some

18. Earth Radiation
J H K L M
trace gas

19. ………………………………………… 19