Download Lecture 10-11 - OSU Astronomy

Stellar Spectra
Colors of Stars
Stars are hot, dense balls of gas
• Continuous spectrum from the lowest visible
layers (“photosphere”).
• Approximates a blackbody spectrum with a
single temperature.
From Wien’s Law, we expect:
• hotter stars appear BLUE (T=10,000-50,000K)
• middle stars appear YELLOW (T~6000K)
• cool stars appear RED (T~3000K)
U-B-V Photometry
bU
bB
For a cool star:
bV
For a Sun like star: bU < bB >
bV
bU < bB
< bV
For a hot star: bU > bB > bV
Spectra of Stars
Hot, dense lower photosphere of a star is
surrounded by thinner (but still fairly hot)
atmosphere.
Flux
Hydrogen
• Produces an Absorption Line spectrum.
• Lines come from the elements in the stellar
atmosphere.
Continuum
Absorption Lines
4000
Can we use stellar spectra to distinguish
among different types of stars?
5000
6000
Wavelength
7000
Objective Prism Spectra
Spectral Classification of Stars
1866: Angelo Secchi observed the
spectra of ~4000 stars.
• Divided them into 4 broad classes by
common spectral absorption features.
1886: Draper Memorial Survey at Harvard
• Objective Prism Photography
• Obtained spectra of >220,000 stars
Harvard Classification (1890)
Edward Pickering & Williamina Fleming
made a first attempt to classify ~10,000
stars by their spectra:
• Sorted by Hydrogen absorption-line strength
• Spectral Type “A” = strongest Hydrogen lines
• followed by types B, C, D, etc. (weaker)
Problem:
Edward Pickering
Harvard “Computers” (c. 1900)
The other lines didn’t fit into this sequence.
Annie Jump Cannon
In 1901, Annie Jump Cannon
• Re-ordered the types using strengths of
many absorption lines.
• Many classes thrown out as redundant.
• Left with 7 primary classes:
O B A F G K M
Annie Jump Cannon
Stellar Spectral Sequence
Henry Draper Catalog of Stars
Annie Cannon further refined the spectral
classification system by dividing the
classes into numbered subclasses:
For example, A was divided into
A0 A1 A2 A3 ... A9
Between 1911 and 1924, she classified
about 220,000 stars, published as the
Henry Draper Catalog.
From O0………….to………………..M9
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
OSU (2009):
OSU Buckeys Are Fine Gridiron
Kicking Machines
Cecilia Payne Gaposhkin and
Meghnad Saha
• First comprehensive theoretical
interpretation of stellar spectra.
• Based on the then new atomic physics.
Showed that stars are mostly Hydrogen
and Helium and small traces of all of the
other metals.
The Spectral Sequence
O
B
A
F
G
K
M
Hottest
50,000K
L
Coolest
1300K
Bluest
Reddest
Spectral Sequence is a Temperature Sequence
The Spectral Sequence is a
Temperature Sequence
Gross differences among the spectral types are
due to differences in Temperature.
Differences in chemical composition are minor
at best.
Why?
What lines you see depends primarily on the
state of excitation and ionization of the gas.
Line Strengths
Example: Hydrogen Lines
Visible Hydrogen absorption lines come from
the second excited state (n=2).
B Stars (11-30,000 K):
Most of H is ionized, so only very weak H lines.
A Stars (7500-11,000 K):
Ideal excitation conditions, strongest H lines.
G Stars (5200-5900 K):
Too cool, little excited H, so only weak H lines.
O Stars
• Hottest Stars: T=30,000 K—50,000K
• Strong lines of ionized Helium
• No lines of Hydrogen
B Stars
• T=11,000 - 30,000 K
• Strong lines of Helium
• Very weak lines of Hydrogen
F Stars
• T = 6000 - 7500 K
• weakening lines of Hydrogen
• stronger lines of ionized Calcium
A Stars
• T = 7,500—11,000 K
• Strongest lines of Hydrogen
• Weak lines of ionized Calcium
(Ca+ or Ca II)
G Stars
• T = 5000 - 6000 K
• Strong lines of ionized Calcium, ionized
Iron, & other metals (metals mean
everything except H and He)
• Much weaker Hydrogen lines
The Sun is a G-type Star
(G2)
•
•
•
•
K Stars
M Stars
Cool Stars: T = 3500 - 5000 K
Strongest metal lines
Hydrogen lines practically gone
First weak CH & CN molecular
bands
• Cool stars: T ~3500 – 2000 K
• Strong absorption bands from
metal-oxide molecules (e.g. TiO)
• Weak neutral metal lines
• No lines of Hydrogen
New Spectral Types: L & T
Coolest stars (<2500K) discovered by
new digital infrared surveys.
L stars:
• Temperatures ~1300-2500K
• Strong lines of metal hydrides & neutral
metals.
T dwarfs:
• Strong Methane (CH4) bands, like Jupiter.
• May be failed stars (“Brown Dwarfs”)
Which of the following spectral
classifications represents the hottest
stellar surface temperature?
1)
2)
3)
4)
A
G
K
B
In the spectral classification of stars,
absorption lines/bands by which of the
following would indicate very low surface
temperature?
A)
B)
C)
D)
Mg II
Fe II
TiO
He I
A star was found to be brightest in U, less
bright in B and faintest in V. What
conclusion can be drawn from this
information?
A) The star has high surface T
B) The star has low surface T
C) The star has intermediate surface T, close to
that of the Sun
D) This information is insufficient to draw any
conclusion about the star surface T
The Sun is a G2 star while Enif is K2.
This tells us that Enif is:
A)
B)
C)
D)
Hotter than the Sun
Brighter than the Sun
Fainter than the Sun
Cooler than the Sun