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Light and Temperature

Astronomy: The Science of Seeing

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Goals

• 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 is it hot in my car?

3Open Cluster NGC 290: A Stellar Jewel Box Credit: ESA & NASA; Acknowledgement: E. Olszewski (U. Arizona)

"[Of celestial bodies] We may determine their forms, their distances, their sizes, and their motions - but we can never know anything of their chemical composition; and much less, that of organized beings living on their surface.“

Philosopher Auguste Comte, 1835

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What you see is all you get!

• So you need to squeeze EVERY last drop of information out of the light we get.

• This semester we’ll see how we can use light to:

1. Take a star’s temperature.

2. Weigh a black hole.

3. Tell what our Galaxy looks like from the outside.

4. See the beginning of the Universe.

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The “Visible” Spectrum

• When you think of “light”, what do you think of?

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Light

• Travels at the speed of light (a CONSTANT):

c = 3 x 1010 cm/s• The wavelength (l) and frequency (n)

are related:

c = ln• The energy of light is:

E = hn = hc/l

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E = hc/l

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Three ReasonsAll objects do one or more:1. Reflect light because of color or smoothness2. Emit light because of their temperature

(thermal radiation)

3. Emit or absorb light because of their composition(spectral lines)

A person, house, or the Moon: reflects visible light, and because each is warm, emits infrared light.

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Reflecting Light

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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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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 TestWhich of the following best describes how Star A would appear compared to Star B?

1. Star A would appear more red than Star B.

2. Both stars would appear more red than blue.

3. Both stars would appear more blue than red.

4. Star A would appear more blue than Star B.

5. None of the above.

V I B G Y O R

visible rangeStar A

Star B

Ene

rgy

Out

put p

er s

econ

d

Wavelength

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Concept Test

Which of the two stars (A or B) is at a higher temperature?

1. Star A2. Star B3. The two stars have the

same temperature.4. It is not possible to

infer this relationship.V I B G Y O R

visible rangeStar A

Star B

Ene

rgy

Out

put p

er s

econ

d

Wavelength

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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/l

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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/l

• 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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Graphically

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Graphically

lpeak = const/T

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Result• If everything with a temperature gives off

light, why doesn’t we see everything glowing? lpeak = k/T

lpeak = (3 x 10-3 m/K) * 1/ 300 K

lpeak = 10-5 m IR

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Thermal 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 World• Everyday objects (at everyday

temperatures) emit thermal radiation in the IR, this is why we equate IR with HEAT.

http://www.x20.org/library/thermal/blackbody.htm

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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

Betelgeuse

Orion Nebula

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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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The Greenhouse Effect

• Why is my car hot on a summer day?

• At T = 6000 K, the Sun radiates mostly visible light.Windshield is transparent to visible light.

• Car seat absorbs this visible light and warms up to 400 K.

• At T = 400 K, my seat radiates mostly at longer wavelengths in the IR. Windshield is opaque in the IR.

• Result: Energy is TRAPPED inside the car!

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Venus and Earth

• Certain gases act the same way as your windshield: Carbon Dioxide (CO2).

• Venus – Runaway greenhouse effect. • Earth – Could that happen here?