3 Light and Temperature Astronomy: The Science of Seeing.
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Transcript of 3 Light and Temperature Astronomy: The Science of Seeing.
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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?