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Fingerprints in Fingerprints in SunlightSunlight

Understanding SpectroscopyUnderstanding Spectroscopy

Stanford University Stanford University

Solar CenterSolar Center

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How can we How can we study the stars study the stars & Sun?& Sun?

We canWe can’’t get there from heret get there from here Only/primary way of learning about distant Only/primary way of learning about distant

objects is through their light (electromagnetic objects is through their light (electromagnetic spectrum)spectrum)

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

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

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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 Sun & stars emit a continuous spectrum (spectrum (““black bodyblack body””) of EM radiation) of EM radiation

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 distribution of energy The distribution of energy emitted by a radiant source, e.g. the emitted by a radiant source, e.g. the Sun, arranged in order of wavelengthsSun, arranged in order of wavelengths””

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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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Your Simple Your 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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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 SOHO Spacecraft

Hubble’s new Cosmic Origins Spectrograph

Student spectrograph & gas

lamp

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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 – produced when a Absorption lines – produced when a chemical element has absorbed energy chemical element has absorbed energy

Emission lines – produced when a chemical Emission lines – produced when a chemical element has emitted energyelement has emitted energy

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)

Sun

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Build your spectroscopeBuild your spectroscope

Point your Point your spectrograph to an spectrograph to an incandescent light incandescent light or sunlightor sunlight

Next, point your Next, point your spectrograph to a spectrograph to a fluorescent light fluorescent light bulbbulb

What do you see? What do you see? Especially notice the Especially notice the bright green linebright green line

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You should have seen a You should have seen a continuous spectrum with continuous spectrum with some extra bright colored some extra bright colored lineslines

Fluorescent bulb, Fluorescent bulb, old styleold style

Fluorescent bulb, Fluorescent bulb, new stylenew style

MercuryMercury

What do you What do you conclude?conclude?

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Another experimentAnother experiment

Work in teamsWork in teams Take your candleTake your candle Burn a hollow around your wickBurn a hollow around your wick Put salt in the hollow, or pour Put salt in the hollow, or pour

salt onto the flamesalt onto the flame Look for a brief flashLook for a brief flash What do you see?What do you see?

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What did you see?What did you see?

The candleThe candle

Sodium spectrumSodium spectrum

What is salt? What is salt? Sodium chlorideSodium chloride

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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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Reading a spectrumReading a spectrum

A spectrum can be graphed as wavelength vs. intensity

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Measure Here

Location and shape changes of the line give us a lot of additional information

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Spectra tell us Spectra tell us temperaturestemperatures

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

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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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The Sun The Sun ““in H alphain H alpha””

Hydrogen alpha filters allow only light in the 656nm wavelength to pass through. This is the line that appears in the red part of the spectrum when an electron moves from Level 3 to Level 2.

This allows us to see light produced at a particular temperature in the photosphere (surface) of the Sun.

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NASANASA’’s Solar s Solar Dynamics Dynamics Observatory (SDO)Observatory (SDO)

Launched February 2011 to study the Launched February 2011 to study the SunSun

3 instruments, AIA, EVE, HMI3 instruments, AIA, EVE, HMI HMI is from the Solar Observatories HMI is from the Solar Observatories

team at Stanford – my group!team at Stanford – my group! HMI works similarly to a spectroscopeHMI works similarly to a spectroscope

Each SDO image of the Sun is Each SDO image of the Sun is taken in a different spectral taken in a different spectral line (of extreme ultraviolet line (of extreme ultraviolet

light)light)

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IRIS (IIRIS (Interfacenterface RRegionegion I Imagingmaging SSpectrographpectrograph))

Launching sometime Launching sometime after December 2012after December 2012

Goal is to understand Goal is to understand the interface between the interface between the Sunthe Sun’’s photosphere s photosphere (visible surface) and (visible surface) and corona (atmosphere)corona (atmosphere)

IRIS is a spectrograph!IRIS is a spectrograph!

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What are your What are your questions?questions?

Have your students make Have your students make simple spectrographs out simple spectrographs out of CDsof CDs

Use your spectrographs to Use your spectrographs to look at moonlight, look at moonlight, reflected sunlight, reflected sunlight, fluorescent lights, neon fluorescent lights, neon signs, mercury vapor and signs, mercury vapor and sodium streetlights, sodium streetlights, candles, their TV set, etc.candles, their TV set, etc.

Show them how science is Show them how science is done by teaching about done by teaching about the Sun and spectroscopythe Sun and spectroscopy!!

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