Emission

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Emission

• Astronomy The Science of Seeing

2
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 does it seem hotter wearing a black T-shirt
  versus a white one?
• Why are they different?

3
A Spectrum

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

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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.

COOLER
HOTTER
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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.

HOT
COLD
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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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Thermal Radiation Laws

• Hotter is bluer.
• (peak at shorter wavelength)
• 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 A
• b. Star B
• c. The two stars have the same temperature.
• d. It is not possible to infer this relationship.

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Concept Test

• Which 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.

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Thermal versus Reflection

• Thermal radiation is light given off because of
  an objects temperature.
• Dont 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.

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The IR Universe

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

Io from IRTF.
Orion by IRAS
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The IR Universe

• In eclipse, there is no reflected light.
• Only thermal radiation.
• Differences in composition lead to differences in
  temperature.

Orion by IRAS
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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?
• No light at all.
• A little reflected visible light.
• A little reflected visible, plus more emitted
  visible light.
• A little reflected visible, plus mostly emitted
  infrared light.
• 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.

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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/l

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Kirchhoffs 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
Hot

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

Stellar Spectra
Cool
Annals of the Harvard College Observatory, vol.
23, 1901.
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To Sum Up

• EVERY element has a SPECIAL set of lines.
• Atoms fingerprint.
• Observe the lines and you identify the component
  elements.
• Identify
• Absorption spectrum
• Emission emission
• Learn 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 suns surface.
  However, the very top thin layer of the suns
  surface is relatively cooler than the part below
  it. What type of spectrum would you expect to see
  from the sun?
• A continuous spectrum.
• A continuous spectrum plus a second, slightly
  redder continuous spectrum.
• A continuous spectrum plus a second slightly
  bluer continuous spectrum.
• A continuous spectrum plus an absorption
  spectrum.
• None of the above.

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The Sun
Courtesy of NOAO/AURA
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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

• From the presence and position of Spectral Lines
  we can know
• Composition (H, He, H2O, etc.)
• Movement through space (towards or away)
• How fast?

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Concept Test

• I spin an object emitting a constant tone over my
  head. What do you hear?
• A constant tone.
• A tone that goes back and forth between high and
  low frequency.
• A constant tone of lower intensity.
• Two constant tones, one of higher frequency and
  one of lower frequency.
• 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 constant tone.
• A tone that goes back and forth between high and
  low frequency.
• A constant tone of lower intensity.
• Two constant tones, one of higher frequency and
  one of lower frequency.
• One tone going from smoothly from low to high
  intensity.

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Homework 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
• Dont forget Topic of Confusion