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EmissionEmission

Astronomy: The Science of Seeing

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GoalsGoals

• What is light?• What are the types of light?• Where does the light we see come

from?• Understanding the light of heat.• On a sunny day:

– Why does it seem hotter wearing a black T-shirt versus a white one?

– Why are they different?

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A Spectrum

• A spectrum = the amount of light given off by an object at a range of wavelengths.

Emission lines Absorption linesContinuum

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Temperature and Light

• Warm objects emit light.– Thermal radiation

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Kelvin Temperature

• Kelvin: an absolute scale.• Kelvin is Celsius + 273 degrees.• Water freezes: 0 C 273 K• Water Boils: 100 C 373 K• Room Temp: 80 F 27 C 300 K• Surface Sun: 5800 K

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Atoms in Motion

• Everything is composed of atoms which are constantly in motion.

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Temperature

• The hotter the object, the faster the average motion of the atoms.

HOTTER COOLER

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Atoms and Light

• As atoms move they collide (interact, accelerate).

• Collisions give off energy.• But light IS energy.

E = hc

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Light and Temperature

• The hotter the object the faster the average atom and the more energetic the average collision.

• The faster the atoms the more collisions there are.

COLD

HOT

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Energy and Intensity

• The more energetic the average collision the bluer the average light that is given off.– Since E = hc

• The more collisions that occur the more light that is given off per surface area.

1. Hotter is bluer.(peak at shorter

wavelength)

2. Hotter is brighter.(more intense at all

wavelengths)

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Thermal Radiation Laws

1. Hotter is bluer.– (peak at

shorter wavelength)

2. Hotter is brighter.– (More intense

at all wavelengths)

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Concept Test

Which of the two stars (A or B) is at a higher temperature?a. Star Ab. Star Bc. The two stars have the same temperature.d. It is not possible to infer this relationship.

V I B G Y O R

visible rangeStar A

Star B

Energy Output per second

Wavelength

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Concept TestWhich of the following best describes how Star A would appear compared to Star B?a. Star A would appear more red than Star B. b. Both stars would appear more red than blue.c. Both stars would appear more blue than red.d. Star A would appear more blue than Star B.e. None of the above.

V I B G Y O R

visible rangeStar A

Star B

Energy Output per second

Wavelength

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Thermal versus ReflectionThermal versus Reflection

• Thermal radiation is light given off because of an object’s temperature.

• Don’t confuse with reflected light:– Buses are yellow not because they are hot

enough to emit visible radiation but rather they reflect the yellow light given off by the Sun.

• What kinds of thermal radiation do we see in our everyday life?

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The IR Universe

• Everyday things that are hot radiate in the IR:

• Dust – There are interstellar clouds of dust.

Orion - visible

Orion – by IRAS

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The IR Universe

• Molten Rock – There are lava flows on a moon of Jupiter.

Orion – by IRAS

Io from IRTF.

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The Moon in eclipse.

The IR Universe

• In eclipse, there is no reflected light.

• Only thermal radiation.

• Differences in composition lead to differences in temperature.

Orion – by IRASR. Gendler

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Continuum Concept Test

• The sun shines on a cold airless asteroid made of black coal. What light from the asteroid do we detect?a. No light at all.b. A little reflected visible light.c. A little reflected visible, plus more emitted visible

light.d. A little reflected visible, plus mostly emitted

infrared light.e. A little reflected visible, plus emitted visible and

emitted infrared light.

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A Spectrum

• A spectrum = the amount of light given off by an object at a range of wavelengths.

Emission lines Absorption linesContinuum

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Spectral Line formation?

• Electron has different energy levels: Floors in a building.

• Lowest is called the Ground State.

• Higher states are Excited States.

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Changing Levels• If you add the RIGHT amount of energy to an

atom, the electron will jump up energy floors.• If the electron drops down energy floors, the

atom gives up the same amount energy.• From before, LIGHT IS ENERGY: E = hc/

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Kirchhoff’s Laws

• Light of all wavelengths shines on an atom.• Only light of an energy equal to the difference

between “floors” will be absorbed and cause electrons to jump up in floors.

• The rest of the light passes on by to our detector.

• We see an absorption spectrum: light at all wavelengths minus those specific wavelengths.

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Absorption

• Dark hydrogen absorption lines appear against a continuous visual spectrum, the light in the spectrum absorbed by intervening hydrogen atoms

• Compare with the emission spectrum of hydrogen.

From "Astronomy! A Brief Edition," J. B. Kaler, Addison-Wesley, 1997.

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Emission Lines

• Every element has a DIFFERENT finger print.

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Different stars, different spectra

• Different stars have different types of spectra.

• Different types of spectra mean different stars are made of different elements.

Hot

Cool

Ste

llar

Sp

ectr

a

Annals of the Harvard College Observatory, vol. 23, 1901.

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To Sum Up…To Sum Up…

• EVERY element has a SPECIAL set of lines.– Atom’s fingerprint.

• Observe the lines and you identify the component elements.

• Identify:– Absorption spectrum– Emission emissionLearn about the environment of the element

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Concept Test

• The sunlight we see is thermal radiation caused by the extreme heat of the sun’s surface. However, the very top thin layer of the sun’s surface is relatively cooler than the part below it. What type of spectrum would you expect to see from the sun?a. A continuous spectrum.b. A continuous spectrum plus a second, slightly redder

continuous spectrum.c. A continuous spectrum plus a second slightly bluer

continuous spectrum.d. A continuous spectrum plus an absorption spectrum.e. None of the above.

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The Sun

Courtesy of NOAO/AURA

HOT YouCoolerLow Density

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Helium• The element Helium (He) was first

discovered on the Sun by its spectral lines.

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Doppler Shift

• The greater the velocity the greater the shift.

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So Now…So Now…

• From the presence and position of Spectral Lines we can know:– Composition (H, He, H2O, etc.)

– Movement through space (towards or away)

– How fast?

Vc

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Concept Test

• I spin an object emitting a constant tone over my head. What do you hear?a. A constant tone.b. A tone that goes back and forth between

high and low frequency.c. A constant tone of lower intensity.d. Two constant tones, one of higher

frequency and one of lower frequency.e. One tone going smoothly from low to high

intensity.

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Concept Test

• I spin an object emitting a constant tone over my head. What do I hear?a. A constant tone.b. A tone that goes back and forth between

high and low frequency.c. A constant tone of lower intensity.d. Two constant tones, one of higher

frequency and one of lower frequency.e. One tone going from smoothly from low to

high intensity.

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Homework #8Homework #8

• For Wednesday 24-Sept: Read B5.3-5.5 and B13.1-13.2.

• Do: B5 Problems 38, 57• Do: B13 Problems 19, 27, 32, 53• Don’t forget Topic of Confusion