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Fingerprints in Fingerprints in SunlightSunlight

Understanding SpectroscopyUnderstanding Spectroscopy

Deborah ScherrerDeborah ScherrerStanford University Stanford University

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What is What is light?light?

Light is a form of energy, Light is a form of energy, the electromagnetic the electromagnetic

spectrum.spectrum.

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How can we can study the How can we can study the stars?stars?

Only/primary way of learning about distant objects Only/primary way of learning about distant objects is through their light (electromagnetic spectrum)is through their light (electromagnetic spectrum)

Light has Light has ‘‘fingerprintsfingerprints”” which provide information which provide information about itabout it

How can we How can we ““readread”” these fingerprints and what do these fingerprints and what do they tell us about the star?they tell us about the star?

No matter how good your No matter how good your telescope, a star is only a telescope, a star is only a point of lightpoint of light

We can’t get there from We can’t get there from herehere

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What is the What is the spectrum of spectrum of light?light?

Anything hotter than absolute zero Anything hotter than absolute zero radiates/emits energy, i.e. lightradiates/emits energy, i.e. light Sun & stars emit a continuous spectrum Sun & stars emit a continuous spectrum

(meaning all the colors of visible light). (meaning all the colors of visible light). They also emit radio, microwaves, They also emit radio, microwaves, infrared light, ultraviolet light, X-rays, infrared light, ultraviolet light, X-rays, and/or gamma raysand/or gamma rays

Our eyes see Our eyes see ““whitewhite”” light, which is light, which is made of the spectrum of colors visible made of the spectrum of colors visible in a rainbowin a rainbow

Spectrum = Spectrum = the “colors” of light emitted by the “colors” of light emitted by an objectan object

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What is a What is a spectrograph?spectrograph?

A relatively simple-to-A relatively simple-to-understand scientific understand scientific instrument to look at instrument to look at a spectruma spectrum

Like a prism – breaks Like a prism – breaks light into its colorslight into its colors

Thin, rectangular slit Thin, rectangular slit produces a rectangle produces a rectangle of lightof light

Example output from a

spectrograph

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Most astronomy is Most astronomy is done with done with spectrographs!spectrographs!

Your spectrograph Stanford Solar Center Home-made

spectrograph attached to telescope

NASA‘s Solar Dynamics Observatory (SDO) NASA’s IRIS mission

Student spectrograph & gas

lamp

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Our Simple Our Simple SpectrographSpectrograph

Diffraction gratingDiffraction grating

(similar effect to prism(similar effect to prism

or CD)or CD) Slit & light sourceSlit & light source Scale (optional)Scale (optional) Eye or instrument Eye or instrument

for viewingfor viewing

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What can we learn with What can we learn with a spectrograph?a spectrograph?

Sometimes there are extra bright Sometimes there are extra bright colorscolors

Sometimes there are missing Sometimes there are missing colorscolors

To infraredTo ultraviolet

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Fingerprints in LightFingerprints in Light The extra or missing colors indicate certain The extra or missing colors indicate certain

chemical elements (e.g. hydrogen, helium, chemical elements (e.g. hydrogen, helium, gold, etc.) have affected the lightgold, etc.) have affected the light

Each chemical element changes the spectrum Each chemical element changes the spectrum either by making certain colors brighter or either by making certain colors brighter or removing certain colorsremoving certain colors

Each chemical element has a different and Each chemical element has a different and unique pattern of colors, hence the unique pattern of colors, hence the ““fingerprintfingerprintss””

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Example fingerprintsExample fingerprints

HydrogenHydrogen

HeliumHelium

SodiumSodium

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Absorption vs. Absorption vs. Emission?Emission?

Absorption lines – appear dark and are Absorption lines – appear dark and are produced when a chemical element has produced when a chemical element has absorbed energy absorbed energy

Emission lines – appear extra bright and are Emission lines – appear extra bright and are produced when a chemical element has emitted produced when a chemical element has emitted energyenergy Whether something

produces an absorption or emission spectrum depends upon its temperature, the temperature of any gas between it and the observer, and the observer’s line of site.

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Some Elements on the Some Elements on the SunSun Hydrogen (H)Hydrogen (H)

Helium (He)Helium (He)

Sodium (Na)Sodium (Na)

Oxygen (OOxygen (O2)2)

Iron (Fe)Iron (Fe)

Different types of stars Different types of stars have different spectrahave different spectra

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Want to build your Want to build your own spectrograph?own spectrograph?

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http://solar-center.stanford.edu/activities/cots.html

Your teachers can order class sets for free (just pay shipping)

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What secrets do What secrets do spectra tell us?spectra tell us?

Chemical Chemical CompositionComposition

TemperatureTemperature MovementMovement Magnetic fieldsMagnetic fields

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Spectra tell us about Spectra tell us about compositioncomposition Am emission or absorption line Am emission or absorption line

means a specific chemical means a specific chemical element has been involved with element has been involved with the light you are seeingthe light you are seeing

Careful, though. The element Careful, though. The element could be from the source, or from could be from the source, or from an intervening plasma or gas an intervening plasma or gas cloudcloud

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Spectra tell us Spectra tell us temperaturestemperatures

If you look at the strongest colors or If you look at the strongest colors or wavelength of light emitted by a star, then you wavelength of light emitted by a star, then you can calculate its temperaturecan calculate its temperature

Specific spectral lines are only produced at Specific spectral lines are only produced at certain temperatures, so if you see them you certain temperatures, so if you see them you know what temperature you are looking atknow what temperature you are looking at

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How do spectra tell us How do spectra tell us about movement?about movement? A Doppler shift happens when an object is moving A Doppler shift happens when an object is moving

towards or away from us, as in a siren coming towards towards or away from us, as in a siren coming towards usus

Wavelength is influenced by the movementWavelength is influenced by the movement It works with sound, with light, with any waveIt works with sound, with light, with any wave

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Doppler, continuedDoppler, continued

Motion away Motion away from us from us results in a results in a ““red shiftred shift””

Motion Motion towards us towards us results in a results in a ““blue shiftblue shift

Why don’t they call it a violet shift?

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Spectra tell us about Spectra tell us about magnetismmagnetism

Sunspots are magnetic

storms on the Sun Magnetic fields

cause spectral lines to split into

thirds

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What are your What are your questions?questions?

Thank you!Thank you!Sun Dragon Art image © by Henry Roll. Used with permission.