The Astrophysics Spectator will not be updated on June 29. The next issue of this site will appear on July 6, assuming I can tear myself way from the Tour de France (Go Lance!).
The Astrophysics Spectator is a web site about modern astrophysics. Here you will find articles on how astrophysical objects work and on current research efforts to more completely understand the universe.
This web site is written for the reader who has some understanding of basic physics, but who is not familiar with the varied phenomena found in our universe. 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.
On the home page is found an addition link. This is the Store link, which leads to reviews of worthwhile books on astronomy and other relates subjects. Links on these pages enable the reader to buy these books from Amazon.com, which helps to financially sustain this web site.
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 22, 2005
This week I create the X-ray Astronomy survey path by adding two new pages to the three pages introduced over the past-two weeks. These pages begin the discussion of the astronomical objects that produce x-rays. The first page gives a simple overview of the various systems outside of our Solar System that light the sky in x-rays. The page that follows discusses compact binary stars, which are among the most brilliant x-ray sources that we see, and which I think are the most interesting objects in the sky (lots of difficult and exotic physics ).
The universe is characterized by many temperatures. Our eye is designed to see light from regions that are around 6,000° K, the temperature of the Sun's photosphere, so when we look into the night sky, we see the photospheres of stars. But there are substantially hotter regions in our universe, and these regions appear in the sky when we observe with x-ray and gamma-ray instruments. Our sky is filled with x-ray sources, and these sources are far more varied than the objects we see with our eye.
Stars with convective outer layers are the most common x-ray sources. Like the Sun, these stars generate magnetic fields that deposit energy into their coronas. While common, they are not particularly bright. For the brightest x-ray objects in the sky, we must turn to the compact binary stars. A compact binary star is a pair of stars, one of which is a compact object—a degenerate dwarf, a neutron star, or a black-hole candidate; the compact object and its companion are close enough together for the compact object to pull the atmosphere from its companion. Another source of x-rays in our Galaxy is the supernova remnant. Farther out, we find the cores of many galaxies emitting x-rays; massive black holes are thought to be the central engine of these active galaxies. Finally, hot gas in clusters of galaxies is seen at x-ray energies.
The compact binary systems fall roughly into two categories: the binary with a degenerate dwarf star as its compact object, and the binary with either a neutron star or a black-hole candidate as its compact object. The principal power source for these systems is the same: gravitational potential energy released as the atmosphere is pulled from the companion star down onto the compact object. This gas usually flows through an accretion disk belting the compact object; much of the luminosity of these systems is from the accretion disk. Radiation is also releases when the gas in the accretion disk enters the atmosphere of a compact star. Of these systems, those containing neutron stars or black-hole candidates are the brighter. If black holes are prove to exists, the proof will come through the study of compact binaries.
In this week's commentary, I use a recent article in Nature on the prevalence of unethical conduct in science as the foil for my own experience with unethical behavior in astronomy and astrophysics.
The Unethical Advantage. The unethical behavior of scientists made another appearance with the publication of a survey of misconduct in Nature that finds unethical conduct to be common. This study brought to mind one of my own brushes with an unethical scientist. Like the authors of the study, I believe unethical actions short of outright plagiarism and the manufacturing of data are corrosive to science, but until the costs greatly outweigh the benefits of unethical behavior, expect it to remain common. (continue)
Astronomical X-ray Sources. A wide variety of astronomical objects produce x-rays that are easily seen by satellites orbiting Earth. These sources range from common and nearby stars to distant compact binary stars scattered throughout our Galaxy to the x-rays produced in very distant active galaxies. This page gives a short overview of most of the x-ray sources in the sky. (continue)
X-rays from Compact Binary Stars. Compact binary stars are stars systems containing a compact object—a degenerate dwarf, neutron star, or black-hole candidate—that is pulling the atmosphere from its companion star. The material pulled from the companion orbits the compact object as an accretion disk. Much of the gravitational potential energy released as the gas flows to the compact objects is radiated away in the accretion disk. If the compact object is a degenerate dwarf or a neutron star, additional energy is released when the gas impacts the atmosphere of the star. These binaries are among the most luminous of x-ray sources in our galaxy. (continue)