7PTYS_206_light_heat_from_planets_stars

# Long wavelengths usually described as waves

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Long-wavelengths usually described as waves Radiowaves, microwaves Shorter-wavelengths usually described as particles Photon energy depends on frequency – ‘blue’ photons are more energetic than ‘red’ photons …but total energy also depends on the number of photons Not a hard rule x-rays still show diffraction Radiowaves can still be described by photons Waves or particles?

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PYTS/ASTR 206 – Light and Heat from Planets and Stars 11 Anything with heat (and that’s everything) glows Temperature in Celsius or Fahrenheit are conveniences Temperature in Kelvin directly measures how much heat a material has Temperature in Kelvin is always positive Nothing actually has a temperature of absolute zero The amount of radiation released depends on this temperature Blackbody radiation Hot things radiate a lot of energy Power radiated per square meter = σ T 4 σ is the Stefan-Boltzmann constant 5.67*10 -8 W m -2 K -4
PYTS/ASTR 206 – Light and Heat from Planets and Stars 12 Some things emit radiation more efficiently than others Measured by emissivity ε (from 0 – terrible to 1 – perfect) A perfectly efficient (ε=1) material is called a black body Emissivity and absorptivity are the same A blackbody (perfect emitter) is also a perfect absorber Blackbodies radiate at all wavelengths – with perfect efficiency They radiate at some wavelengths more than others Planck curve Blackbody curve The sun radiates most at visible wavelengths That’s not a coincidence! Total Power radiated per square meter = ε σ T 4

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PYTS/ASTR 206 – Light and Heat from Planets and Stars 13 Blackbody radiation depends only on temperature Emissivity=1 Higher temperature means: More radiation… Power radiated per square meter = σ T 4 Peak radiation at shorter wavelengths A star with a surface at 3000 K radiates in the infrared A star with a surface at 6000K (like the Sun) radiates in the visible part of the spectrum The Blackbody Curve
PYTS/ASTR 206 – Light and Heat from Planets and Stars 14 Which of these stars is hotter:

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PYTS/ASTR 206 – Light and Heat from Planets and Stars 15 Which of these stars is hotter: Are the inside of sunspots hotter or cooler than the rest of the Sun’s surface ?
PYTS/ASTR 206 – Light and Heat from Planets and Stars 16 Which of these stars is hotter: Are the inside of sunspots hotter or cooler than the rest of the Sun’s surface ? Darker means less radiation means cooler! (But not by much still ~4500 K)

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PYTS/ASTR 206 – Light and Heat from Planets and Stars 17 Wavelength of peak emission Wien’s law Davedarling.com
PYTS/ASTR 206 – Light and Heat from Planets and Stars 18 Add up all the energy emitted by the Sun Over all wavelengths In all directions Over its entire surface Solar power = 4 * 10 26 Watts The US generates about 1.5 * 10 19 Joules/year (in 2007) The sun does this 27 million times every second

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PYTS/ASTR 206 – Light and Heat from Planets and Stars 19 That energy gets spread over a large area… More spread out further from the Sun At the Earth @ 1AU distance becomes What about at 2 AU?
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• Spring '07
• ShaneByrne

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