The Astrophysics Spectator

Issue 2.22, June 8, 2005

Home Commentary Surveys Research Background Store Previously Site Info
Logo for 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.

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, 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 8, 2005

This week I begin a discussion of X-ray astronomy. This branch of astronomy has advanced with advances in rocketry. With modern x-ray observatories orbiting Earth, we can study the hot universe: stellar coronas, compact stars such as neutron stars, accretion disks around black-hole candidates, and hot interstellar gases. The page that is added this week described how x-ray telescopes create x-ray images of the sky.

X-ray astronomy presents several hurtles. Cosmic x-rays are absorbed high in the upper atmosphere, so we can only observe x-ray sources efficiently by mounting our instruments onto spacecraft and placing them into orbit around Earth. This limits the size and mass of an x-ray instrument, which severely limits our ability to observe faint x-ray sources. X-rays interact with matter in a complex way; this makes the comparison of x-ray data to the predictions of theory a complex undertaking.

The lowest-energy x-rays can be focused to a point by a mirror, but unlike an optical mirror, an x-ray mirror must be tilted at a very steep angle relative to an x-ray's direction of flight. In practice this is accomplished by casting the mirror as a cylinder, so that the x-rays reflect off of the inside surface of the cylinder. This means that the collecting area of a mirror is much smaller than its surface area. It also means that an x-ray telescope is long.

For higher-energy x-rays, images of a sky of point sources can be created with a shadow box. By placing a mask with holes in front of an x-ray detector, the position of each source can be found from the shadow cast by the mask onto the detector.

The detector at the heart of all modern x-ray telescopes is the CCD. This device is similar to the device found in digital cameras.

The commentary for this week discusses science in fantasy literature and its relationship to the more speculative theories of science.

Jim Brainerd


Fantasy and Aesthetic Science. The popular fantasy literature populates the world with fantastic objects. These objects exists to surprise and delight us with their departure from ordinary life. Science also populates its hidden regions with equally fantastic objects, but for a much different reason: to bring an aesthetic unity to science. Often these objects find their way into the fantasy literature, so that an object born out of a sense of order is used to excite a sense of wonderment in our universe. (continue)


The Physics of Detecting X-rays. X-ray astronomy is a window on the hot universe, the universe of stellar corona, hot interstellar gases, compact stars, and black holes. It is one of several disciplines in astronomy that can only be conducted in space, outside of our protective atmosphere. This page describes the physics of x-ray telescopes. (continue)


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