Motivation:

1
Motivation
0
Detailed spectra of stars differ from pure
blackbodies
2
The Bohr Model of the Hydrogen Atom
0
Postulate L n h
rn a0 n2/Z
Bohr radius a0 h2 / (mee2) 5.2910-9 cm
0.529 Å
En - Z2e2 / (2 a0 n2)
3
Hydrogen Line Series
0
Infrared
Optical
Ultraviolet
4
The Balmer Lines
0
Transitions from 2nd to higher levels of hydrogen
n 1
n 4
n 5
n 3
n 2
Ha
Hb
Hg
The only hydrogen lines in the visible wavelength
range.
2nd to 3rd level Ha (Balmer alpha line)
2nd to 4th level Hb (Balmer beta line)

5
The Balmer Lines
0
6
0
The Cocoon Nebula (dominated by Ha emission)
7
The Fox Fur Nebula (dominated by Ha)
0
8
Quantum-Mechanical Localization Probability
Distributions
0
9
Energy Splitting Beyond Principal Quantum Number
0
10
The Pauli Principle
0
No 2 electrons can occupy identical states
(i.e., have the same n, l, ml, and ms)
11
Gradual Filling of n-Shells
0
12
0
13
Russell-Saunders Coupling
0
Filled shells L S J 0
e2
e1
e3
l1
s1
L
l2
s2
J
l3
s3
S
14
Atomic Energy Levels
0
Hunds Rule 2 For given S, states with larger L
have lower energies
Hunds Rule 1 States with larger S have lower
energies
Landes Interval Rule EJ1 EJ C(J1)
15
Spectral Classification of Stars
0
Different types of stars show different
characteristic sets of absorption lines.
Temperature
16
0
Stellar spectra
O
B
A
F
Surface temperature
G
K
M
17
Spectral Classification of Stars
0
Mnemonics to remember the spectral sequence
18
Hertzsprung-Russell Diagram
0
Absolute mag.
Luminosity
or
Temperature
Spectral type O B A F G K M
19
0
The Balmer Thermometer
Fraction of neutral H atoms in the excited (n
2) state (Boltzmann Equation)
Number of neutral H atoms in the excited (n 2)
state available to produce Balmer lines
Fraction of ionized Hydrogen atoms (Saha Equation)
20
Measuring the Temperatures of Stars
Comparing line strengths, we can measure a stars
surface temperature!
21
Morgan-Keenan Luminosity Classes
Ia Bright Supergiants
Ia
Ib
Ib Supergiants
II
III
II Bright Giants
III Giants
IV
V
IV Subgiants
V Main-Sequence Stars