1. What Are the Primary Properties of Stars
Mass
Size
Temperature
Wien’s Law
Stefan Boltzman Law
Luminosity
L = 4πR2σT4
Chemical Composition

2. Not All Are Measureable For All Stars
Stellar Observables
Not All Are Measureable For All Stars
Parallax
Distance and in some cases absolute magnitude
Proper Motion
Apparent Brightness
Colors
Spectrum
Morphological Type
Radial Velocity
Parameters: Temperature and Pressure
Chemical Composition

3. Stellar Types
These Are All Dwarfs O5 B1 A1 F3 G2 K0 M0

4. Metallicity and Temperature Effects in Stellar Spectra

5. Type
Color
Temperature
Principal Features
Examples
Relatively few absorption lines in visual. Lines
of ionized helium, doubly ionized nitrogen, O
Blue
>25000
10 Lacertae
triply ionized silicon, and other lines of highly
ionized atoms. Hydrogen appears only weakly.
Lines of neutral helium, singly and doubly
11000
ionized silicon, singly ionized oxygen and
Rigel B
Blue to
magnesium. Hydrogen lines are more
Spica
25000
pronounced than in O stars.
Strong lines of hydrogen. Also lines of singly
7500
ionized magnesium, silicon, iron, titanium, A
Blue to
Sirius
calcium, and others. Lines of neutral metals
11000
Vega
show weakly.
Hydrogen lines weaker than in A stars but are
Blue
6000
still conspicuous. Lines of singly ionized F to to
calcium, iron, and chromium, and also lines of
Canopus
White
7500
neutral iron and chromium are present, as are
Procyon
lines of other neutral metals.
Lines of ionized calcium are the most
conspicuous spectral features. Many lines of
White
5000
ionized and neutral metals are present.
Sun G to to
Hydrogen lines are weaker than in F stars.
Capella
Yellow
6000
Bands of CH, the hydrocarbon radical, are
strong.
Orange
3500
Lines of neutral metals predominate. The CH
Arcturus K to to
bands are still present.
Aldebaran
Red
5000
Strong lines of neutral metals and molecular
Betelgeuse M
Red
< 3500
bands of titanium oxide predominate.
Antares

6. Ions and Stars Ion = a positively or negatively charged atom
Ca+ is a positively charged calcium atom
Ca- is a negatively charged calcium atom
Astronomical (and Atomic Physics) Notation
Ca I = Neutral Calcium
Ca II = Singly Ionized Calcium = Ca+
Ca III = Doubly Ionized Calcium = Ca++

7. Spectral Types, Temperatures, and Color
Why Is The Spectral Type Sequence a Temperature Sequence?
Originally it was not!
The types started at A
When it realized that the types are dominated by temperature the sequence was temperature sorted and redundant types deleted.
How was the connection made between the types and temperatures?

8. Spectra and the Planck Function
An Application of Wien’s Law: λ = 0.29/T
Note in the theoretical Planck functions as T decreases that the Flux maximum moves to the red and the absolute flux level decreases
This is also what happens in the observed stellar spectra.. The spectral type sequence is a temperature sequence.