Now we begin…..

http://sdo.gsfc.nasa.gov/gallery/potw.php?v=item&id=87

Stellar Atmospheres I

• The Description of the Radiation Field

• Stellar Opacity

• Radiative Transfer

Stellar Atmosphere

• Light comes from the Star’s Atmosphere

• Atmosphere is the layers of gas overlaying opaque interior of star

• Flood of photons pour from these layers

• releasing energy produced by the thermonuclear reactions, gravitational contraction and cooling in the star’s center

• 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 Hwavelength. 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 SunResults from Skylabhttp://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 -500km-

0km

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 FieldSpecific and Mean Intensity

• Specific Intensity

• Mean Intensity

• Energy Density

Blackbody radiation is isotropic and has

http://spiff.rit.edu/classes/phys440/lectures/radfield/radfield.html

Description of the Radiation FieldThe Specific Energy Density

of a Black Body Radiation Field

Specific Radiative Flux

• Net Energy having a wavelength between andd that passes each second through unit area in the direction of the +z axis

Resolved SourceFlux independent of r

Unresolved SourceFlux decreases as 1/r2

Radiation Pressure

• Photon momentum exerts pressure

Blackbody Radiation

Radiation Pressure is 1/3 of the Energy Density for

Blackbody Radiation

Stellar Opacity

• Solar Spectrum What determines features of Spectrum?

Stellar OpacityTemperature and Local Thermodynamic Equilibrium

http://spiff.rit.edu/classes/phys440/lectures/lte/lte.html

What is temperature anyhow????

What is Thermodynamic Equilibrium???

Stellar Opacity

Definition of Opacity

• 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 traveleddensity of gasabsorption coefficient

Absorption/Attenuation Coeffiecient

• Absorption coefficient

Attenuation coefficient=(opacity) x (density)

[density]=kg/m^3

[opacity]=m^2/kg

[Attenuation coefficient]=1/m

Optical Depth

• 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