Astronomy 162:
Introduction to Stellar, Galactic, & Extragalactic Astronomy
Lecture 7:The Spectra of Stars
Key Ideas:
- Color of a star depends on its Temperature
- Red Stars are Cooler
- Blue Stars are Hotter
- Spectral Classification
- Classify stars by their spectral lines
- Spectral differences mostly due to Temperature
- Spectral Sequence (Temperature Sequence): O B A F G K M
Colors of Stars
Stars are made of hot, dense gas
- Continuous spectrum from the lowest visible layers
("photosphere").
- Approximates a blackbody spectrum.
From Wien's Law, we expect:
- hotter stars appear BLUE (T=10,000-50,000 K)
- middle stars appear YELLOWISH (T~6000K)
- cool stars appear RED (T~3000K)
Spectra of Stars
Hot, dense lower photosphere of a star is surrounded by thinner
(but still fairly hot) atmosphere.
- Produces an Absorption Line spectrum.
- Lines come from the elements in the stellar atmosphere.
Spectral Classification of Stars
Astronomers noticed that stellar spectra showed many similarities.
Can stars be classified by their spectra?
Draper Survey at Harvard (1886-1897):
- Objective Prism Photography
- obtained spectra of >100,000 stars
- hired women as "computers" to analyze spectra
Harvard Classification
Edward Pickering's first attempt at a systematic spectral classification:
- Sort by Hydrogen absorption-line strength
- Spectral Type "A" = strongest Hydrogen lines
- followed by types B, C, D, etc. (weaker)
Problem: Other lines followed no discernible patterns.
Annie Jump Cannon
Leader of Pickering's "computers", she noticed subtle
patterns among metal lines.
Re-arranged Pickering's ABC spectral types, throwing out most
as redundant.
Left 7 primary and 3 secondary classes:
O B A F G K M (R N S)
Unifying factor: Temperature
Spectral Sequence Mnemonics
- Harvard (1920s):
- Oh Be A Fine Girl, Kiss Me
- Berkeley (late `60s):
- Oh Buy A Fine Green Kilo Man
- Caltech (early `80s):
- On Bad Afternoons Fermented Grapes Keep Mrs. Richard
Nixon Smiling
The Spectral Sequence is a Temperature Sequence
Gross differences among the spectral types are due to differences
in Temperature.
Composition differences are minor at best.
Why?: What lines you see depends primarily on the state of
excitation and ionization of the gas.
Example: Hydrogen Lines
Visible Hydrogen absorption lines come from the second excited
state.
- B Stars (15,000-30,000 K):
- Most of H is ionized, so only very weak H lines.
- A Stars (10,000-7500 K):
- Ideal excitation conditions, strongest H lines.
- G Stars (6000-5000 K):
- Too cool, little excited H, so only weak H lines.
Characteristics
O Stars
Hottest Stars: T>30,000 K; Strong He+ lines; no H lines
B Stars
T=15,000 - 30,000 K; Strong neutral He lines; very weak H lines
A Stars
T=10,000 - 7500 K; Strongest H lines, Weak Ca+ lines.
F Stars
T=7500-6000 K; H weaker, Ca+ stronger, weak metals
G Stars
T=6000-5000 K; Strong Ca+, Fe+, other metals, weak H
K Stars
T=5000-3500 K; Strong metal lines, weak CH & CN
M Stars
Coolest Stars: T<3500 K; strong molecule bands (especially TiO),
no H lines (or only very weak).
Modern Synthesis: The M-K System
Understand atomic physics (since 1930s) and better techniques
permit finer distinctions.
Morgan-Keenan (M-K) Classification System:
- Start with Harvard classes: O B A F G K M
- Subdivide each class into numbered subclasses:
e.g., A0 A1 A2 A3 ... A9
- The Sun:
- G2 star
- In Winter Sky:
- Betelgeuse: M2 star (Orion)
- Rigel: B8 star (Orion)
- Sirius: A1 star (Canis Major)
- Aldebaran: K5 star (Taurus)