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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