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1
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- Today: Finish
- Chapter 6 –Starlight and Atoms
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2
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- Energy absorbed/emitted depends on upper and lower levels
- Higher energy levels are close together
- Above a certain energy, electron can escape (ionization)
- Series of lines named for bottom level
- To get absorption, lower level must be occupied
- Depends upon temperature of atoms
- To get emission, upper level must be occupied
- Can get down-ward cascade through many levels
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3
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- The higher the temperature, the higher the typical level
- Collisions can knock electrons to higher levels,
if moving atoms have enough kinetic energy
- At T ~
300 K (room T)
almost all H in ground state (n=1)
- At T ~ 10,000 K many H are in first excited state (n=2)
- At T ~ 15,000 K many H are ionized
- Because you have highest n=2 population at ~10,000K
you also have highest Balmer line strength there.
- This gives us another way to estimate temperatures of stars
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4
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- Different atoms hold on to electrons with different force
- Use weakly held electrons to sense low temperatures (Fe, Ca, TiO)
- TiO molecule is destroyed above 4000K
- Ca has lost 1 electron by ~5000K, but still has others to give lines
- Use moderately held electrons to sense middle temperatures (H)
- Below 6000 K most H electrons in lowest state – can’t
cause Balmer lines
- Above 15,000K most H electrons completely lost (ionized)
- Use tightly held electrons to sense high temperatures (He, ionized He)
- Below 10,000K most He electrons in ground state – just like H,
no visible absorption lines
- Above 15,000K most H has lost one electron, but still has a second one
to cause absorptions
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5
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- O B A F G K M scheme
- Originally in order of H strength – A,B,etc Above order is for
decreasing temperature
- Standard mnemonic: Oh, Be A
Fine Girl (Guy), Kiss Me
- Use numbers for finer divisions:
A0, A1, ... A9, F0, F1, ... F9, G0, G1, ...
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6
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- Somewhat complicated – we must correct for temperature effects
- Regular pattern:
- More of the simplest atoms:
H, then He, ...
- Subtle patterns later – related to nuclear fusion in stars
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7
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- Effect similar in light and sound
- Waves compressed with source moving toward you
- Sound pitch is higher, light wavelength is smaller (bluer)
- Waves stretched with source moving away from you
- Sound pitch is lower, light wavelength is longer (redder)
- v = velocity of source
- c = velocity of light (or sound)
- l = apparent wavelength of light
- lo
= original wavelength of
light
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8
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- Star moving toward you at 200 km/sec = 2.0´105 m/s
- Speed of light c = 3.00 ´ 108 m/s
- Original Ha lo= 0.65647 mm
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- Star moving toward you at 200 km/sec = 2.0´105 m/s
- Speed of light c = 3.00 ´ 108 m/s
- Original Ha lo= 0.65647 mm
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10
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- Car with horn blowing, moving away from you at 70 MPH.
- Speed of sound is ~700 MPH = 1000 ft/sec
- Original horn pitch is 200 cycles/sec Þ lo ~ 5 ft
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Notes
