astro110-01 lecture 15 light and matter (cont’d)...23/02/09 habbal_astro110-01 lecture 15 10...
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123/02/09 Habbal_Astro110-01 Lecture 15
Astro110-01Lecture 15
Light and Matter (Cont’d)
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What have we learned?• Three basic types of spectra
— continuous spectrum— emission line spectrum— absorption line spectrum
• Light tells us what things are made of—Each atom has a unique fingerprint—We can determine which atoms
something is made of by looking for theirfingerprints in the spectrum
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What have we learned?
• How light tells us the temperature ofplanets and stars—Nearly all large or dense objects emit a
continuous spectrum that depends ontheir temperature
—The spectrum of that thermal radiationtells us the object’s temperature
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How light tells us the speed ofa distant object:
The Doppler Effect
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Mathematically speaking
vrad/c = (λshift – λrest)/ λrest
Example: If observe Hα spectral line(normally at 656.285 nm) from the starVega at 656.255 nm,
then Vega is moving at:vrad/c = - 4.57 10-5 or 13.7 km/s away from
us
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The Doppler Effect
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Doppler effect: Same for light
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Measuring the Shift
We generally measure the Doppler effect fromshifts in the wavelengths of spectral lines.
Stationary: Reference
Moving Away
Away Faster
Moving Toward
Toward Faster
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The amount of blue or red shifttells us an object’s speed toward
or away from us:
The Doppler Shift of an Emission-Line Spectrum
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Doppler shift tells us ONLY about the part of anobject’s motion toward or away from us.
Star 1 is moving directly away fromus. Doppler shift tells us its fullspeed
Star 2 is moving across our line ofsight, but not toward or away fromus. Doppler shift = 0
Star 3 is moving diagonally awayfrom us. Doppler shift tells us thepart of the star’s speed away fromus. But not the part of the speedacross our line of sight
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Thought Question
• It is moving away from me.• It is moving toward me.• It has unusually long spectral lines.
I measure a line in the lab at 500.7 nm. Thesame line in a star has wavelength 502.8 nm.What can I say about this star?
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Thought Question
• It is moving away from me.• It is moving toward me.• It has unusually long spectral lines.
I measure a line in the lab at 500.7 nm.The same line in a star has wavelength502.8 nm. What can I say about this star?
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The Doppler Shift of an Emission-Line Spectrum
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MeasuringRedshift
Doppler Shift of Absorption Lines
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MeasuringVelocity
Determining the Velocity of a Gas Cloud
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MeasuringVelocity
Determining the Velocity of a Cold Cloud of Hydrogen Gas
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What have we learned?
• Light tells us the speed of a distant object:—The Doppler effect tells us how fast an
object is moving toward or away from us
• Blueshift (shortened wavelength):– objects moving toward us
• Redshift (lengthened wavelength):– objects moving away from us
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Reviewing the properties ofspectra
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Continuous Spectrum
Emission Line Spectrum Absorption Line Spectrum
Spectra of astrophysical objectsare usually combinations of these
three basic types
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Continuous Spectrum
• The spectrum of a common (incandescent)light bulb spans all visible wavelengths,without interruption.
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Emission Line Spectrum
• A thin or low-density cloud of gas emits light only atspecific wavelengths that depend on its compositionand temperature, producing a spectrum with brightemission lines.
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Absorption Line Spectrum
• A cloud of gas between us and a light bulb canabsorb light of specific wavelengths, leaving darkabsorption lines in the spectrum.
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Chemical Fingerprints
Each type of atom has a unique spectralfingerprint
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Transitions yielding emissionlines
• Downwardtransitionsproduce a uniquepattern ofemission lines.
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Transitions yielding absorptionlines
• Because thoseatoms canabsorb photonswith those sameenergies, upwardtransitionsproduce a patternof absorptionlines at the samewavelengths.
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Production of Absorption Lines
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Wien’s Law λ max = 2.9 106 / T (K)
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Some Exercises
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Thought Question Which letter(s) labels absorption lines?
A B C D E
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Thought Question Which letter(s) labels absorption lines?
A B C D E
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Thought Question
A B C D E
Which letter(s) labels the peak (greatestintensity) of infrared light?
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A B C D E
Thought Question
Which letter(s) labels the peak (greatestintensity) of infrared light?
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Thought Question Which letter(s) labels emission lines?
A B C D E
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Thought Question Which letter(s) labels emission lines?
A B C D E
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Interpreting an ActualSpectrum
• By carefully studying the features in aspectrum, we can learn a great dealabout the object that created it.
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What is this object?
Reflected Sunlight: Continuous spectrum ofvisible light is like the Sun’s except that some ofthe blue light has been absorbed—object mustlook red
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What is this object?
Thermal Radiation: Infrared spectrumpeaks at a wavelength correspondingto a temperature of 225 K
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What is this object?
Carbon Dioxide:Absorption lines arethe fingerprint of CO2in the atmosphere
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What is this object?
Ultraviolet Emission Lines:Indicate a hot upper atmosphere
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What is this object?
Mars!