stellar spectra ast 112 lecture 7. stellar spectra the interior of a star can be considered a “hot...
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Stellar Spectra
AST 112 Lecture 7
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Stellar Spectra
• The interior of a star can be considered a “hot dense object” that emits a continuous spectrum.
• The interior is surrounded by a much cooler atmosphere.
• What type of spectrum (emission, absorption or continuous) would you expect to see from a star?
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Stars show an absorption spectrum.
* Some stars show emission features as well.
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Stellar Spectra
• Stellar spectra tend to resemble one of these seven spectra
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Stellar Spectra
• Why do stars have different spectra?
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Pioneers of Stellar Spectroscopy
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The Old Classification
• First classified by strength of hydrogen lines
• Old Classification:A, B, C, D, …
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The Old Classification
• Maybe not perfect?
7000 oF
10,000 oF
12,000 oF
15,000 oF
17,000 oF
30,000 oF
60,000 oF
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The Harvard Classification
• More natural order followed (found by Annie Jump Cannon)
• This order follows temperature
• Everything’s great, right??
Hig
her T
empe
ratu
re
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The Balmer Series
• These absorption lines are called the Balmer Series.
• They occur when the electron in hydrogen is in the first excited state and absorbs a photon
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The Balmer Series
• So in order to see Balmer lines, some fraction of the hydrogen atoms must have their electrons in the first excited state.
• The more hydrogen atoms in the first excited state, the darker the absorption lines.
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A Discrepancy
• Quantum Mechanics:
– H absorption lines should get stronger as temperature goes up
• The order of the old scheme looks right
– But temperature is wrong!
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7000 oF
10,000 oF
12,000 oF
15,000 oF
17,000 oF
30,000 oF
60,000 oF
The graph does not agree with the spectra!
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The Harvard Classification
• Saha: Electrons detach at these temperatures!
• If the electron of H is detached, absorption lines don’t happen
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Multiply the top graphs to get the bottom graph.
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This graph peaks at 10,000 K (17,000 oF).
At what temperature would you expect the darkest H absorption lines?
What happens to the H absorption linesabove or below thistemperature?
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Combine Graphs
Graph of line darknesspeaks at 17,000 oF
“A” Star
17,000 oF
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OBAFGKM
• OBAFGKM is correct!
• The physics of the absorption lines agrees completely.
• Spectral type is determined by temperature.
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Structure of Matter vs. Temperature
Cooler:Molecules Form
Warmer:Molecules
Break
HOT:Atoms Ionized
Same atoms. Different interactions. Different spectra.
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Stellar Spectra
We can do this for all of the elements. This is howthe line strength behaves (with temperature) for each element. And it agrees with stellar spectra.
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Stellar Spectra
Decreasing temperature: O B A F G K M
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So what are stars made of?
• Chemical composition does not vary much from star to star
• Cecilia Payne: – All stars made up of:
• About 75% Hydrogen• About 25% Helium• About 1-2% heavier
elements
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Stellar Spectra
If stars have little variation in composition, states of elements determine spectrum.
Temperature determines the state of theelements, and therefore the spectrum.
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Measuring Stellar Temperature (Spectra)
• M type stars: 5000 oF• O type stars: 70,000 oF
• Each spectral type subdivided by number– B0, B1, … , B9– Larger number, cooler star
• Sun is G2• Sirius is A1• Betelgeuse is M2
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Measuring Stellar Temperatures
• Recall Stefan-Boltzmann Law and Wein’s Law
Brightness = sT4
Peak Wavelength depends on T
• When do these laws apply?
• Do they apply to stars?
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Measuring Stellar Temperatures (Color)
• Rank the following stars according to temperature, hottest first:
– Yellow
– Red
– Blue
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Measuring Stellar Temperatures (Color)
• Rank the following stars according to temperature, hottest first:
– Blue (Sirius: 16,500 oF)
– Yellow (Sun: 10,000 oF)
– Red (Betelgeuse: 5600 oF)