Download Stellar Spectra

10/17/2012
Stellar Spectra
Lecture 5
A STAR IS A HUGE BALL
OF FIRE
YOU CAN PUT A MILLION EARTHS
INSIDE THE SUN
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What is a star?
•
•
•
•
•
Stars “shine” at night (during the day too!).
A star is a self-luminous sphere of gas.
(A HUGE BALL OF FIRE)
(You can put a million earths inside the sun)
Stars are held together by gravity.
n But
what keeps them from collapsing?
Thermal energy from nuclear reactions:
Hydrogen fusion
The Spectra of Stars
• We use a telescope with a spectrograph to
measure and record the spectrum of a star.
• This gives us the brightness of the star at
different wavelengths.
n Like
we discussed with blackbodies.
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Spectra
• Almost all stars show a “continuum”
spectrum with “absorption” lines.
• Some stars show “emission” lines.
– All stars do not have the same
spectrum!
• Interstellar clouds show a continuum with emission
lines
Energy
Examples of Stellar Spectra
Wavelength
OR
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Stellar Spectra
• The spectral (absorption) lines we see in stars
are very important.
• The “missing” photons give us info on:
– Chemistry
– Temperature
– Density
• Kirchhoff’s laws tell us about the region which
gives rise to the spectrum.
Formation of Absorption Lines in a Star
Photons
See this
Star
Photosphere:
“Continuum
Source”
Outer layers are
Cooler -- Absorb
Photons
Like Kirchhoff’s 3rd law
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Example Stellar Spectrum
H
H
H
H
...
4000
6000
5000
7000
 (A) 
The Hydrogen-Balmer spectrum (in the visible) is
present for most stars.
Classification of Stars
• In the late 19th century astronomers categorized
stars according to the strength of the hydrogen
absorption lines in the spectrum.
• They labels these A, B, ... from strongest to weakest.
• Unfortunately, this was the wrong way to do it!
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Classification (cont’d)
• Annie Jump Cannon arranged the spectra of
stars in a sequence which corresponds to the
temperatures.
• Classified over 500,000 stars!
• The spectral sequence is:
O, B, A, F, G, K, M (RNS)
hotter ------------> cooler
• A temperature sequence.
Classifying stars
Annie Jump Cannon (1929)
Prism spectra of a star cluster
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The Harvard GROUP
13 May 1913
Spectral Sequence = Temperature
Sequence
Cooler
Hotter
O
Violet
B
A
Blue
F
G
Yellow
K
M
RNS
Red
Henry Norris Russell (1920)
“Oh, Be A Fine Girl (Guy), Kiss Me”
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ONLY BRIGHT APPLES FALL
GROUNDWARD
KNOCKING ME
REMARKED NEWTON
SOLEMNLY
OUT BEYOND
AMBER FIELDS,
GRACEFULLY
KNOLLS MUST
ROLL,
NEVER
SUBSIDING
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10/17/2012
Spectral Classification (cont’d)
• Each of these class (O, B, etc.) can be
subdivided into tenths, i.e.
G0, G1, ... G9, K0, K1, ... K9
(G0 is hotter than G9)
• The Sun is a G2 star.
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Spectral Sequence
O5V
B5V
A5V
F5V
G4V
K5V
M5V
350 nm
750 nm
Stellar Spectra
Na
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Changes to spectral sequence
• For the first time in over 100 years the
spectral sequence has need for more letters.
• Very low temperature stars discovered with
infrared surveys of the sky.
• Now have L and T stars!
OBAFGKMLT
Spectral Sequence = Temperature
Sequence
Cooler
Hotter
O
Violet
B
A
Blue
F
G
Yellow
K
M
L
T
Red
Contest: Make up your own pneumonic for remembering the spectral sequence –
winner will be recited in class. (Must be publicly presentable).
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10/17/2012
Stellar Photometry & Colors
• It is not necessary to measure the entire
spectrum to determine the spectral peak of a
star.
• We can use color filters to determine the
“colors” of a star.
n By
this we mean how much flux is seen in each
color filter.
n A green filter transmits only green photons.
The UBV System:
A set of color filters which give coarse
spectral information.
1
V
B
Transmission
U
U at 3500 A
B at 4300 A
V at 5500 A
0
3000
5000
7000 A
Wavelength
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Temperature and Colors
•
•
•
•
U at 3500 A => ultraviolet
B at 4300 A => blue.
V at 5500 A => visible (yellow)
A hot star will have more flux in the U filter
than the V filter compared to a cool star.
Colors of a hot star vs. a cool star
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Hot Star
Cool Star
Energy
M
U
V
0
3000
5000
7000 A
O
Wavelength
at U
Flux at U

 Flux


Flux at V Hot Star
Flux at V  Cool Star
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