The Astrophysics Spectator

Issue 2.21, June 1, 2005

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The basic layout of the site is as survey paths, which can be found under the Surveys link at the top of this and most other pages on this site. Each survey begins with a basic overview of the subject. Part of this overview include simulators of astrophysical phenomena that allow the reader to experiment with the phenomena. The later pages in a survey present the subject in greater and more mathematical depth. A path ends with research pages that describe current research projects and results in astrophysics.

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Each Wednesday, a new issue of The Astrophysics Spectator is published that comprises a new home page, a new commentary, whatever news the author notices, and background, research, and simulator pages added to the survey paths. The home page acts as an index to the newly added pages. This site also has an RSS channel, whose link is given at the bottom of the right-hand column of this page.

June 1, 2005

This week I expand the discussion of the diffusion of electromagnetic radiation from the interior of a star on the “Stars” survey path. This discussion appears in two pages: a new page on the principle processes that couple light to matter, and a rewritten page on the diffusion of light through a star.

Radiation interacts with matter through four processes. The first process, Compton scattering, is the collision of a photon with a free electron. This process transfers energy between light and matter; gamma-rays created through nuclear fusion interact with matter through this process. The bremsstrahlung process creates and destroys photons when a free electrons is deflected by an ion; this process replaces the photons that escape from the stellar core. Photo-ionization and recombination create and destroy photons as an atom frees or captures an electron. This process dominates the interaction of radiation with matter in regions way from the core of the star, where the low temperature allows atoms to bind electrons. The final process, the emission and absorption of light during atomic transitions, occurs at the lowest temperatures found in stars; this process, which produces the lines seen in stellar spectra, provides the strongest coupling between radiation and matter.

The light in the interior of a star is always in thermal equilibrium with matter, which gives the light a black-body spectrum. The energy density of black-body light is dependent solely on the temperature of the light. The light escapes from high-temperature regions to low temperature regions through a random walk. This diffusion of light is fastest in regions and at frequencies where coupling of light to matter is weakest.

The rate at which energy diffuses out of a star is proportional to the temperature gradient. More important, the rate of diffusion depends on temperature and density through the four radiative processes discussed above. The rate of diffusion through a star can change dramatically as one radiative process displaces another in dominating the interaction between light and matter. This sets the structure of a star.

Jim Brainerd


Radiative Processes in Stellar Interiors. Four processes are principally responsible for creating, thermalizing, and impeding the flow of radiation in the interior of a star. These processes are Compton scattering, bremsstrahlung emission and absorption, photo-ionization and recombination, and atomic line emission and absorption. The first-two processes are dominant in the cores of stars, where the temperatures are highest, and where few electrons are bound to atoms. Farther out, where the temperature is lower, ionization provides the principle coupling of radiation to matter. Towards the surface of a star, where the temperatures are at their minimum and most electrons are bound to atoms, the atomic line transition provides the dominant means of coupling radiation to matter. (continue)


Radiative Transport in Stellar Interiors. This page has been rewritten to discuss at greater length how radiation diffuses through the interior of a star. Radiation trapped within a star is in complete thermal equilibrium with the star's electrons and ions. It slowly diffuses from a star's core to the photosphere. The power diffusing through any point is proportional to the temperature gradient. But this simplicity masks some very complex physics, because the rate at which photons diffuse depends not only on the temperature gradient, but also upon the details of how light interacts with matter, which is dependent on both the temperature and the density of the matter. (continue)


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