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 16, 2005

This week I address three disparate topics: main-sequence stars, the Huygens probe, and academic freedom.

Most stars that we see in the sky belong to a class called the main sequence. A main sequence star is a star that is generating energy at its center by converting hydrogen into helium through nuclear fusion. A star will spend about 90% of its life as a main-sequence star. With the addition of a page describing the structure of these stars, we begin a series of pages on main-sequence stars that will include simulators of stellar structure.

Last month, the Huygens probe landed on Titan, Saturn's largest moon. The data that this probe produced confirms the theory that Titan's water-ice surface is shaped by methane rain. A research page has been added to the “Solar Planetary System” path that describes the probe and its landing onto Titan. The Cassini spacecraft page has been modified to accommodate this new page.

The commentary for this week is on academic freedom. My view of academic freedom, which is shaped by my experiences as a research scientist, is narrower than the concept discussed in the popular press. This article is the first in a series that I will publish over the coming weeks on academic freedom, freedom of speech, and the mission of the university.

Commentary

The Notion of Academic Freedom. Recent controversies over the statements of two university professors and a college president have provoked a discussion of academic freedom. In the first of a series, this week's commentary discusses the concept of academic freedom. I make the point that academic freedom implies a responsibility to question only those topics that are unsettled within a scientific discipline. This makes academic freedom more restrictive than freedom of speech. (continue)

Background

Main-Sequence Stars. Stars spend most of their lives burning the hydrogen at their cores. Stars in this stage of their life are called main sequence stars, because these stars fall along a line, called the main sequence, on a plot of absolute magnitude versus color, or equivalently, of luminosity versus surface temperature. The Sun belongs to this class of star. This new page on the “Stars” path discusses the structure of these stars. (continue)

Research

The Huygens Probe. On January 14, 2005, the Huygens probe landed on Titan, Saturn's largest moon, and the only moon in the Solar System with a significant atmosphere. This new page, which is one of the Research pages and is part of the “Solar Planetary System” path, describes the landing of the probe. Our current knowledge of Titan based on the first analysis of data from Huygens is also given on this page. (continue)

Update

The Cassini Spacecraft. The Cassini page has been updated to accommodate the new page on the Huygens probe and its recent landing on Titan. (continue)