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What can we learn from light?
• Temperature• Energy• Chemical Composition• Speed towards or away from us
All from the spectrum!
Temperature and Radiation
• Why do different objects give offdifferent forms of light?
• They have different temperatures!– Temperature depends on the motion of
atoms & molecules– Fast motion -> High temp.– Slow motion -> Low temp.
Temperature, Heat, orThermal Energy?
• Temperature: intensity of thermalenergy
• Heat: amount of thermal energy
Two objects can be at the sametemperature, but have different amountsof heat or thermal energy
Kelvin Temperature Scale
• Zero Kelvin (written 0 K) is absolutezero (-459.7ºF)– No heat energy
• Water freezes at 273 K, boils at 373 K.
K = ºC + 273.2
Temperature and Color
• Higher temp = higher E= higher f = shorter λ• What color has shorter wavelength?
– Blue/Violet!– Opposite of faucet handles…
400 nm 700 nm
Spectrum: Wavelength vs.Intensity
• Visible:
• Intensity vs. wavelength plot:
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They show the same information“Dips” in color = black lines
Dips in intensity
Temp. and Color: Blackbodies
• A blackbody absorbs and re-emits alllight that falls on it
• Idealized objects– But still a good model for stars
• They don’t actually look black(misnomer)– Color depends on their temperature
• Intensity vs. wavelengthfor three objects ofdifferent temperatures.
Radiation from aHeated Object
• The curves are high inthe middle and low ateither end.– These objects emit most
intensely at middlewavelengths.
Radiation from aHeated Object
• The total area undereach curve isproportional to the totalenergy emitted.– Hotter object emits more
total energy than coolerobjects.
Radiation from aHeated Object
• Wavelength ofmaximum intensitydepends ontemperature.– The hotter the object, the
shorter the wavelength ofmax. intensity
Radiation from aHeated Object
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• Temperature determinesthe color of a glowingblack body.– The hotter object emits more
blue light than red, and thuslooks blue.
– The cooler object emits morered than blue, andconsequently looks red.
Radiation from aHeated Object
Temp. and Color: BlackbodiesHottest Object
Coolest Object
Blackbody Radiation LectureTutorial: page 57-60
• Work with a partner or two• Read directions and answer all questions carefully.
Take time to understand it now!• Come to a consensus answer you all agree on before
moving on to the next question.• If you get stuck, ask another group for help.• If you get really stuck, raise your hand and I will
come around.
Temperature, Heat, orThermal Energy?
• Temperature: intensity of thermalenergy
• Heat: amount of thermal energy
Two objects can be at the sametemperature, but have different amountsof heat or thermal energy
Comparing Spectra
• Peak at shorter wavelength = highertemperature
• Higher temperature = bluer in color• Larger total area under curve = higher
total energy output
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Example: Solar Spectrum Hydrogen Lines
Lower E,Lower f,Longer λ
Higher E,Higher f,Shorter λ
Visible!
Visible Hydrogen SpectrumLines: Balmer Series
Short λ = High E
Long λ = Low E
Spectral Classification• Get spectral type from line features, predict
temperature• Subdivisions within each letter: 0-9
– 0 is hottest, 9 is coolest– Sun is a G2 star (hotter than a G8 star)
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Spectral Classification
• O, B, A, F, G, K, M– “Oh Be A Fine Girl/Guy Kiss Me”– “Only Boring Astronomers Feel Good
Knowing Mnemonics”• Subdivisions 0-9
– Sun is a G2 star– Predict temperature to 5%
Actual Spectrum from SDSS
Intensity
Wavelength
Balmer
Lines
The Doppler Effect
• How does light tell us the speed of adistant object?
The Doppler Effect
• Definition: “The change in wavelengthof radiation due to relative radial motionbetween the source and the observer.”
Doppler Effect Real Life Example of DopplerEffect
The change in the pitch of a siren on apolice car, fire truck, or ambulance as itzooms past (sound waves)
Astronomers deal with the DopplerEffect of light waves
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Doppler Effect• When something which is giving off light moves
towards or away from you, the wavelength of theemitted light is changed or shifted
V=0Star Light Wave
Doppler Effect
• When the source of light is moving away fromthe observer the wavelength of the emittedlight will increase. We call this a “redshift”.
Doppler Effect
• When the source of light is moving towardsthe observer the wavelength of the emittedlight will appear to decrease. We call this a“blueshift”.
Doppler Effect• “Radial” means “along line of sight”• Doppler Effect happens only if the light
source is moving towards you oraway from you.
Doppler Effect
V=0
Direction of shift tells us directionof light source’s motion
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Amount of shift tells us speedof source’s motion
Stationary
Moving Away
Away Faster
Moving Toward
Toward Faster
Doppler Shifts• Redshift (to longer wavelengths): The source is movingaway from the observer
• Blueshift (to shorter wavelengths): The source is movingtowards the observer
Δλ= shift in wavelengthλ0 = wavelength if source is not moving
V = velocity of sourcec = speed of light
Doppler Effect LectureTutorial: Pg. 73-77
• Work with a partner or two• Read directions and answer all questions carefully.
Take time to understand it now!• Come to a consensus answer you all agree on before
moving on to the next question.• If you get stuck, ask another group for help.• If you get really stuck, raise your hand and I will
come around.
Chapter 7 Recap
• Atoms, electron energy levels,absorbing & emitting light
• Temperature & color• Types of spectra: absorption, emission,
continuous• Spectral Classes: OBAFGKM• Doppler shift & speed