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Light and Temperature

Astronomy: The Science of Seeing

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Mastering Astronomy• Go to MasteringAstronomy.com• Register using:• Access Code:• WSSBCW-SNELL-POIND-SPINA-GIGOT-

FLEES• ClassID: NORDGREN2014• We will start using for reading and

homework due next Monday.

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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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Concept Question• Which of the following is true?

1. Red light has more energy than blue light.2. Radio waves travel slower than visible

light waves.3. Gamma waves have less energy than

Radio waves.4. Blue light has a lower frequency than red

light.5. Ultraviolet has smaller wavelengths than

infrared.

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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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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 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

Ener

gy O

utpu

t per

seco

nd

Wavelength

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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

Ener

gy O

utpu

t per

seco

nd

Wavelength

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Concept TestWhich of the following best describes the light from Star A compared to Star B?

1. More ultraviolet light but less visible light.

2. More infrared light but less visible light.

3. More visible light but less infrared light.

4. Less infrared light and less ultraviolet light.

5. More ultraviolet light and more visible light.

V I B G Y O R

visible rangeStar A

Star B

Ener

gy O

utpu

t per

seco

nd

Wavelength

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Concept Test• Imagine the Sun’s surface suddenly

became much cooler (while the Sun’s size remains the same). Compared to the light it now emits, the Sun would emit:1. More ultraviolet light but less visible light.2. More infrared light but less visible light.3. More visible light but less infrared light.4. Less infrared light and less ultraviolet

light.5. More ultraviolet light and more visible

light.

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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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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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Homework #3• For Wednesday 22-Jan:• Read Cosmic Perspectives Chapter 5• Do end of Chapter Questions: 18, 22, 27, 34,

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