Transcript Stellar Atmospheres

Now we begin…..
http://sdo.gsfc.nasa.gov/gallery/potw.php?v=item&id=87
Stellar Atmospheres I
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The Description of the Radiation
Field
Stellar Opacity
Radiative Transfer
Stellar Atmosphere
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Light comes from the Star’s
Atmosphere
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Atmosphere is the layers of gas
overlaying opaque interior of star
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Flood of photons pour from these
layers
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releasing energy produced by the
thermonuclear reactions, gravitational
contraction and cooling in the star’s
center
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The temperature, density and
composition of the atmospheric layers
determine the features of the star’s
spectrum
No solid surface…
Photosphere: Segment of star that emits
light. Typically defined to be the region
down to an optical depth of 2/3.
Chromosphere: In the Sun, a thin layer just
above the photosphere that is visually more
transparent than the photosphere. The
spectrum of the light generated here is
dominated by Hwavelength. Temperature
of Chromosphere is up to 20,000K.
Transition Region: In the Sun, a region
between the Chromosphere and Corona.
Corona: In the Sun, a type of plasma
atmosphere that extends millions of
kilometers into space. High temperature.
Temperatures/Densities in the Sun
Results from Skylab
http://history.nasa.gov/SP-402/p2.htm
Region
Height
Density
Temperature
Interior
<-2000km
-500km
10-6 g/cm3
10-6 g/cm3
15,000K+
8500K
Photosphere
-500km0km
10-6 g/cm3
0.5*10-6 g/cm3
8500K
5770K
Chromosphere
0 km
2100km
0.5*10-6 g/cm3
10-13 g/cm3
5000K
5770K
Transition
Region
2100km
2200km
10-13 g/cm3
10-14 g/cm3
5770K
50,000K
Corona
2200km
18000km
10-14g/cm3
5*10-18g/cm3
50,000K
1,800,000K
Description of the Radiation Field
Specific and Mean Intensity
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Specific Intensity
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Energy Density
Mean Intensity
Blackbody radiation is isotropic and
has
Il = Bl
http://spiff.rit.edu/classes/phys440/lectures/radfield/radfield.html
Description of the Radiation Field
The Specific Energy Density
of a Black Body Radiation Field
Specific Radiative Flux
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Net Energy having a wavelength
between andd that passes
each second through unit area in
the direction of the +z axis
Resolved Source
Flux independent of r
Unresolved Source
Flux decreases as 1/r2
Radiation Pressure
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Photon momentum exerts
pressure
Blackbody Radiation
Radiation Pressure is 1/3 of
the Energy Density for
Blackbody Radiation
Stellar Opacity
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Solar Spectrum
What determines features
of Spectrum?
Stellar Opacity
Temperature and Local Thermodynamic Equilibrium
What is temperature anyhow????
What is Thermodynamic Equilibrium???
http://spiff.rit.edu/classes/phys440/lectures/lte/lte.html
Stellar Opacity
Definition of Opacity
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Consider a beam of parallel light
rays traveling through a gas.
Any process that removes
photons from this beam of light is
called absorption
Absorption includes Scattering!!
True absorption is by electronic
transitions in atoms (and
sometimes molecules)
Change in Intensity is
Proportional to:
distance traveled
density of gas
absorption coefficient
Absorption/Attenuation Coeffiecient
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Absorption coefficient
-dI = aIdx
I(x) = I0 exp(-ax)
a = k lr
Attenuation coefficient=(opacity) x (density)
[density]=kg/m^3
[opacity]=m^2/kg
[Attenuation coefficient]=1/m
Optical Depth
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Unit-less Measure of amount of
attenuation.
Accounts for varying density
[density]=kg/m^3
[opacity]=m^2/kg
[Optical Depth]=[density][opacity][distance]=1
Optical Depth
http://spiff.rit.edu/classes/phys440/lectures/optd/optd.html