The Astrophysics Spectator

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.

The links at the top of each page are Home, which is the current home page of this site, Commentary, which is an index of short essays on topics loosely related to astrophysics, Surveys, which is the index of survey paths, Research, which is the index of research pages and the page leading to recent news items, Background, which is the index page for all background information on astrophysics, including survey pages, simulator pages, tables, bibliographic references, and lists of web resources, Previously, which is an index of previous home pages, and Site Info, which describes the site and its author, and gives contact information.

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.

February 23, 2005

This week is a somewhat light week, with one new page on the “Stars” survey path and this week's commentary.

The new survey page is on polytropic stellar models. These simple models dispense with the details of energy transport through a star. Instead, a simple model relating density to pressure is assumed, and this is used in the equation for hydrostatic equilibrium to calculate a density profile. This profile is close enough to the true structure of a star to be used as the starting point for codes that calculate the precise structure of a star.

Polytropic models are useful tools for understanding some of the features exhibited by stars. First, the position of the surface of the star is shown by these models to be determined by the pressure law. Stars described by hard pressure laws, in which small changes in density produce large changes in pressure, have smaller surface radii than stars described by soft pressure laws, in which large changes in density produce small changes in pressure. If a pressure law becomes too soft, one finds that the polytropic model has no surface. Such stars do not have a static configuration, but must collapse until the pressure law becomes harder.

This page includes a live figure that shows the density of a polytropic star with radius. The plot can be reconfigured by the reader.

Commentary

Academic Freedom, Free Speech, and Harlow Shapley. Harlow Shapley was one of the leading astronomers of the early 20th century. In 1950, he was at the center of astronomy's most infamous episodes, when he and the astronomy community attempted to suppress the publication of Immanuel Velikovsky's Worlds in Collision. This episode is a nice illustration of the restrictiveness of academic freedom as compared to free speech, and it is a warning on how not to engage the public over bad science. (continue)

Background

Polytropic Stars. By assuming a simple law relating density to pressure, the stellar structure of a star is easily calculated. This model ignores much of the interesting physics occurring within a star, but despite this, the results provide an understanding of the factors that determine the stability of a star and the dependence of a star's radius on the gas pressure law. This page contains a live figure that shows the density of a polytropic star as a function of radius. (continue)