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 9, 2005

This week I add pages to the “Electromagnetic Radiation” path and the “Distance” path. On the first path, I add a page describing the spectra produced by an atmosphere with a temperature that changes with altitude. On the second path, I add a page that describes Cepheid variables in some detail.

The page from the “Electromagnetic Radiation” path describes the propagation through an atmosphere with a variable temperature. Such atmospheres are said to be in local thermal equilibrium (LTE). This type of atmosphere is commonly encountered in stars. When electromagnetic radiation propagates through an atmosphere, it tries to come into equilibrium with the surrounding gas. When the gas has a variable temperature, and the number of interactions of radiation with the gas is small before the radiation escapes into space, then the variable temperature introduces lines into the spectrum of the radiation. These lines provide astronomers with diagnostics of the conditions at the photospheres of stars and other objects.

The page discussing Cepheid variables describes what they are, why they are variable, and how they are used to derive distances to nearby galaxies.

A guide to controlling the applets on this web site is added with this issue. This guide explains how to control a simulator or a live figure with either the mouse or the keyboard.

The two simulators on this web site were updated to simplify their use and to correct a programming error.

This week I comment on how we use our vision in understanding this universe.

Current Commentary

The Worlds We Visualize. Our ability to understand this universe is tied to our vision. We rely on our vision to provide analogies when we think of physical phenomena. We place our data onto plots to use our vision to recognize patterns. We cast our mathematical models of the universe into easily-visualized concepts. Even our mathematics, despite its abstractness, has its basis in our vision. This suggests that we understand the universe by recasting the universe into the concepts of our human vision. (continue)

New Background

LTE Radiation. We can understand astronomical objects because their radiation is out of thermal equilibrium with their surroundings. Even when a gas producing radiation is locally in thermal equilibrium (LTE), the radiation it produces can contain lines if the temperature of the gas varies. The principal diagnostic for conditions at an astronomical object are atomic lines, which contain information about the chemical composition, density, and temperature structure of an emission region. (continue)

Cepheid Variable Stars. One of the prinicpal tools for deriving the distance to nearby galaxies is the Cepheid variable. The bright variable stars of this class have average luminosities that increases with their pulsation periods. This page, which is part of the “Distance” survey path, discusses the characteristics of Cepheid variables, with a short explanation of why they pulsate. The equation that relates their average absolute magnitude to their pulsation period is given. (continue)

Applet Control Guide. Two types of applet are found on the pages of The Astrophysics Spectator: live figures and simulators. Both of these types of applets follow a standard set of guidelines in their design of navigation and control. The Applet Control Guide page describes how to control these applets with either the mouse or the keyboard. (continue)

Updated Pages

The Keplerian Orbit Simulator and The Galactic Disk Orbit Simulator. The Orbit Simulators on these two pages have been updated to bring them in compliance with the general applet guidelines described in the Applet Control Guide. The Plot button has been removed, and its function has been given to the Reset button. All buttons now have mnemonics and tool tips. The Start and Stop buttons are now defined as default buttons, so now they can be selected with the Enter or Return key. I also corrected an error in the Galactic Disk Orbit Simulator that caused the simulator to produced an incorrect orbit when the starting angle was not 0Copyright �. (continue to the Keplerian Orbit Simulator; continue to the Galactic Disk Orbit Simulator)