The Astrophysics Spectator

Issue 2.37, November 30, 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.

On Wednesday of every fortnight, 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.

November 30, 2005

This issue of The Astrophysics Spectator continues the discussion of gravitational lenses with a page on the lens created by a galaxy. This page is part of the “General Relativity” survey path.

The galaxies close to our own galaxy are too close to act as lens on the images of more distant objects. But a galaxy does not need to be in the most distant regions of our universe for it to magnify more distant images. At a distance of about 100 Mpc, which is over 100 times more distant that our neighboring galaxy, but nearly 100 times closer than the most distant galaxies, the lens of a galaxy that is similar to our own in size and mass magnifies and splits the image of more distance objects. Unlike the lens of a point source, such as a star or a large planet, which only produces two images of a more distant object, the lens of a galaxy can produce one, three, or five images. The multiple images produced by a galaxy's lens are widely separated on the sky, so they are easily resolved with current telescopes. The split images arrive at Earth at different times, with the relative time delay between images ranging from several days to about a month; these time delays are easily observed if the background objects are highly variable.

While each galaxy as a whole has a lens, the stars within a galaxy also have lenses that affect the appearance of more distant sources. These stellar lenses fill a significant fraction of the area on the sky covered by the galaxy when the galaxy is at the edge of the observable universe. When we count up all of the galaxies in our universe, and all of the stars in these galaxies, we find that a large fraction of the sky is covered by gravitational lenses that magnify the images of the most distant objects.

With this abbreviated issue of The Astrophysics Spectator I come to the end of a second year of publication. I wish my readers a merry Christmas, and I hope you check back in with this site next year.

Publication Notice. The next issue of The Astrophysics Spectator will appear on January 3


The Gravitational Lenses of Galaxies. We can see the gravitational lenses of galaxies the size of our own when they are several hundred megaparsecs away, which means they do not need to be at the outer reaches of our universe to be seen. The images produced by a galaxy's gravitational lens is generally more complex than that produced by a star, and because we can resolve a galaxy, we can resolve the multiple images produced by its lens. The time delays associated with a galaxy's gravitational lens range from several days to several months. While all of the gravitational lenses of galaxies at low redshift and the stars they contain cover only a small fraction of the sky, the lenses of galaxies out to high redshift can cover a large enough portion of the sky to make their effect a significant factor in the appearance of more distant objects. (continue)


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