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Light and MatterLight and Matter

Astronomy: The Science of Seeing

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How do you do Astronomy?

• How do Chemists do Chemistry?– Make solutions, mix chemicals …

• How do Biologists do Biology?– Breed fruit flies, (and whatever else

biologists do).

• They devise and conduct experiments in their labs.

• But how do you do that for astronomy?

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Light

• Astronomy is a “passive” science.• We can’t (yet) go to the stars or

other galaxies.

•The Universe must come to us.•We rely on light exclusively!

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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:

•Weigh a planet.

•Take a star’s temperature.

•Tell what’s in the center of a star a thousand light-years away.

•Tell what our Galaxy look like from the outside.

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GoalsGoals

• What is light?• How is it produced?

– Continuum Blackbody radiation (this lecture)

– Line radiation (next lecture)

• How do light and matter affect each other?

• How are we able to learn about the Universe from this light?

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The “Visible” Spectrum

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

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What is Light?

• Light is an electromagnetic wave.• Moves through a vacuum.• Travels at the speed of light (a CONSTANT):

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

related:c =

• The energy is inversely proportional to the wavelength (where h is a constant):

E = hE = hc/

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Radio Optical and infrared

X-rayUV-ray

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What’s the Wavelength Kenneth?

• Arrow 93.1 FM• 93.1 MHz (Mega Hertz) = 93.1 x 106

cycles/secc = 3 x 1010 cm/sec = x 93.1 x 106 cycles/sec

= (3 x 1010 cm/sec)/(93.1 x 106 cycles/sec) = 322 cm =3.22 m

How big is your radio antenna?

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To Sum Up…To Sum Up…

• Radio waves, microwaves, rainbows, UV waves, x-rays, etc are ALL forms of electromagnetic waves.

• They ALL travel through space at the speed of light. c

• The higher the frequency, the shorter the wavelength. c =

• The higher the frequency, the more energetic the wave. E = h

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Three ReasonsAll objects do one or more:1. Reflect light2. Emit light because of their temperature

(blackbody radiation)

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

A person, house, or the Moon: reflect visible light, and because they are warm, emit infrared light.

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The SunTemperature and Light

• Warm objects emit light.– Blackbody radiation

• Emit at a wide range of wavelengths.

• Why?

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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 = h

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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.

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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 = h

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

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Blackbody Laws

• Put another way:

Wien’s Law for peak wavelength (peak):

peak is proportional to 1/T

Stefan-Boltzmann Law for total Flux (F):

F is proportional to T4

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Graphically

peak 1/T

F T4Hot

Hottest

Hotter

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Result

• HOT toasters are BRIGHTER than cool toasters.

• HOT toasters are BLUER than cool toasters.

• What is the peak wavelength for something at room temperature (a cool toaster or a cool person)?

peak = k* 1/T

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

peak = 10-5 m

IR

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BlackbodyBlackbody

• A blackbody is an ideal object that absorbs all radiation falling on it.

• Blackbody radiation is light given off because of an objects 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 blackbody radiation do we see in our everyday life?

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The IR Universe

• In eclipse, there is no reflected light.

• Only blackbody radiation.

• Differences in composition lead to differences in temperature.

Orion – by IRASThe Moon in eclipse.

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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?