Blackbody Radiation

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• Blackbody Radiation
• And
• Spectra

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Light is a form of energy.Why is this
important?With very few exceptions, the only
way we have to study objects in Astronomy is via
the light (energy) they emit.
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• The Earths atmosphere is transparent to visible
  light, some infrared, and the radio. It is
  opaque to UV, X-rays, and gamma rays.

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Blackbody Radiation

• Ex. Heat an iron rod with a torch.
• 1st visible color Red Hot
• 2nd visible color Bright Orange
• 3rd visible color Bright Yellow
• 4th visible color Bright White
• 5th visible color Bright Blue
• As you heat an object it gets brighter and emits
  more electromagnetic radiation.

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Blackbody Radiation

• The dominant color or wavelength of the emitted
  radiation changes with temperature.
• Cooler objects emit at longer wavelengths - Red,
  Infrared
• Hotter objects emit at shorter wavelengths -
  Blue, Violet, UV
• Object actually gives off all wavelengths of
  electromagnetic spectrum.

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Blackbody Radiation

• A blackbody absorbs all the electromagnetic
  radiation that strikes it (none is reflected or
  scattered).
• Object is heated and then reemits the energy it
  absorbed at different wavelengths of the EM
  spectrum.

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BlackbodyCurves

• A body with any temperature radiates thermal
  energy, sometimes called blackbody radiation.
• For a given size, hotter objects give off more
  energy than cooler objects, and are bluer.

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Blackbody Curves

• For a given temperature, larger bodies give off
  more energy than smaller bodies, in direct
  proportion to their surface areas.

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Visible light

• Most people can perceive color.
• Different colors correspond to different
  frequencies (or wavelengths).
• The colors of the rainbow are ROY G BIV red
  orange yellow green blue indigo violet.

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Visible light

• In the visible,
• red has the longest wavelength, the smallest
  frequency, and the lowest energy.
• violet has the shortest wavelength, the highest
  frequency, and the highest energy.

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The spectrum

• A graph of the intensity of light vs. the color
  (e.g. the wavelength, frequency, or energy) is
  called a spectrum.
• A spectrum is probably the single most useful
  diagnostic tool available in Astronomy.

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Important points

• The spectrum of a star is approximately a black
  body spectrum.
• Hotter stars are bluer, cooler stars are redder.
• For a given temperature, larger stars give off
  more energy than smaller stars.
• Shorter the wavelength, the greater the
  temperature

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• In the constellation of Orion, the reddish star
  Betelgeuse is a relatively cool star. The blue
  star Rigel is relatively hot.

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The spectrum

• A spectrum can tell us about the temperature and
  composition of an astronomical object.
• There are two major types of spectra of concern
  here
• Continuous spectra - the intensity varies
  smoothly from one wavelength to the next.
• Line spectra - there are discrete jumps in the
  intensity from one wavelength to the next.

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The Line Spectrum

• Upon closer examination, the spectra of real
  stars show fine detail.
• Dark regions where there is relatively little
  light are called lines.

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The Line Spectrum

• Today, we rarely photograph spectra, but rather
  plot the intensity vs the wavelength.

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Emission and Absorption

• If you view a gas against a dark background, you
  see emission lines (wavelengths at which there is
  an abrupt spike in the brightness).

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Emission and Absorption

• If you view a continuous spectrum through cool
  gas, you see absorption lines (wavelengths where
  there is little light).

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Why is this important?

• The line spectrum tells about what elements are
  present in the star.
• Why is this?

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How Light Interacts with Matter.

• Atoms are the basic blocks of matter.
• They consist of protons and neutrons in the
  nucleus, surrounded by lighter particles called
  electrons.

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How Light Interacts with Matter.

• An electron will interact with a photon.
• An electron that absorbs a photon will gain
  energy.
• An electron that loses energy must emit a photon
  giving off light.
• The total energy (electron plus photon) remains
  constant during this process.

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Atomic Fingerprints

• Hydrogen has a specific line spectrum.
• Each atom has its own specific line spectrum.

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Atomic Fingerprints

• These stars have absorption lines with the
  wavelengths corresponding to hydrogen!

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Atomic Fingerprints.

• This cloud of gas looks red since its spectrum is
  a line spectrum from hydrogen gas.

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The Doppler Shift Measuring Motion

• If a source of waves is not moving, then the
  waves are equally spaced in all directions.

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The Doppler Shift Measuring Motion

• If a source of waves is moving, then the spacing
  of the wave crests depends on the direction
  relative to the direction of motion.

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The Doppler Shift Measuring Motion

• Think of sound waves from a fast-moving car,
  train, plane, etc.
• The sound has a higher pitch (higher frequency)
  when the car approaches.
• The pitch is lower (lower frequency) as the car
  passes and moves further away.

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The Doppler Shift Measuring Motion

• If a source of light is moving away, the
  wavelengths are increased, or redshifted.

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The Doppler Shift Measuring Motion

• If a source of light is moving closer, the
  wavelengths are shortened, or blueshifted.

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The Doppler Shift Measuring Motion

• The size of the wavelength shift depends on the
  relative velocity of the source and the observer.

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Using a Spectrum, we can

• Measure a stars temperature by measuring the
  overall shape of the spectrum (essentially its
  color).
• Measure what chemical elements are in a stars
  atmosphere by measuring the lines.
• Measure the relative velocity of a star by
  measuring the Doppler shifts of the lines.