Physics 320: Astronomy and Astrophysics

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Physics 320 Astronomy and Astrophysics Lecture
VIII

• Carsten Denker
• Physics Department
• Center for SolarTerrestrial Research

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Problem 6.5
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Problem 6.6
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Problem 6.8
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Problem 6.9
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Stellar Atmospheres

• The Description of the Radiation Field
• Stellar Opacity
• Radiative Transfer
• The Structure of Spectral Lines

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Radiation Field
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Energy Density
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Energy Density (cont.)
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Radiation Pressure
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Solar Spectrum
The spectrum of the Sun The dashed line is the
curve of an ideal blackbody having the Suns
effective temperature.
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Temperature Definitions
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Blackbody
A blackbody radiator has a number of special
characteristics. (1) A blackbody emits some
energy at all wavelength. (2) A hotter blackbody
emits more energy per unit area and time at all
wavelength than does the cooler one. (3) A
blackbody emits a greater portion of radiation at
shorter wavelength than does a cooler on. (4) The
amount of radiation per second per unit surface
area of a blackbody depends on the fourth power
of the temperature.
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Thermodynamic Equilibrium
Excitation, ionization, kinetic, and color
temperature are the same for the simple case of a
gas confined in a box. The confined gas particles
and blackbody radiation will come into
equilibrium, individually and with each other,
and can be described by a single welldefined
temperature. In such a steadystate condition, no
net flow of energy through the box or between the
matter and radiation occurs. Every process (e.g.,
the absorption of a photon) occurs at the same
rate as its inverse process (e.g., the emission
of a photon). This condition is called
thermodynamic equilibrium.
The idealized case of a single temperature can
still be employed if the distance over which the
temperature changes significantly is large
compared with the distance traveled by the
particles and photons between collisions (their
mean free path). In this case, referred to as
local thermodynamic equilibrium (LTE), the
particles and photons cannot escape the local
environment of nearly constant temperature.
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Homework Class Project

• Continue improving the PPT presentation.
• Use the abstract from the previous assignment as
  a starting point for a PowerPoint presentation.
• The PPT presentation should have between 5 and 10
  slides.
• Bring a print-out of the draft version to the
  next class as a discussion template for group
  work
• Homework is due Wednesday October 29th, 2003 at
  the beginning of the lecture!
• Exhibition name competition!

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Homework

• Homework is due Wednesday October 22nd, 2003 at
  the beginning of the lecture!
• Homework assignment Problems 9.1, 9.2, and 9.7!
• Late homework receives only half the credit!
• The homework is group homework!
• Homework should be handed in as a text document!