The Astrophysics Spectator

Issue 2.34, October 19, 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.

October 19, 2005

The first in a series of pages that describe gravitational lenses is added this week to The Astrophysics Spectator. In principle all objects create gravitational lenses, but in practice only the lenses associated with individual stars, individual galaxies, and clusters of galaxies play a role in astrophysics. The page added this week discusses the gravitational lens created by a star. This page is part of the “General Relativity” survey path.

One of the consequences of the equivalence principle of gravity, the principle that acceleration within a gravitational field is indistinguishable from acceleration by a rocket, is that light travels on a curved path through a gravitational field. This means that starlight passing through the solar system follows a curved path; the stars we see are offset slightly from their true positions. This effect is slight, with the strongest deflection, 1.75 arc seconds, occurring at the Sun's limb.

The deflection of light by a star means that the region of space around a star acts as a lens. Up close, this lens effect is not noticeable; Earth is too close to the Sun to see the efffects of the Sun's gravitational lens. Far away from a star, where the size of the star on the sky is smaller than the angle of deflection of light passing by the star, the effect can be dramatic. The gravitational lens can increase the brightness of objects behind the gravitational lens by several orders of magnitude. A gravitational lens can also create multiple images of an object. For the lens associated with a star, two images of an object are normally created, one on each side of the star. If the object is on a direct line with the star and the observer, a ring circling the star is created. For the lens associated with a galaxy or a cluster of galaxies, more than two images are often seen.

The gravitational lens is a useful tool in astrophysics. Stars within our galaxy can cause stars in more distant galaxies to brighten and dim as the galactic star passes in front of the extragalactic star. By monitoring large numbers of bright stars in distant galaxies, astronomers are able to detect stars in our own galaxy that are too faint to see. This provides us with a method for counting the number of low-mass stars within our own galaxy.

When we look at galaxies and galaxy clusters, we often see the distorted, multiple images of more distant galaxies. By studying how a gravitational lens splits and distorts an image, one can measure how mass of the galaxy or cluster creating the lens is distributed in space.

With this week's issue, the “Gravitational Wave” survey path has been folded into the “General Relativity” survey path.

Publication Notice. The next issue of The Astrophysics Spectator is slated for November 2.


A Star's Gravitational Lens. The gravitational field of every star is a lens. This lens can make a more distant object appear as a ring around the star, or it can make the object appear as two images, one on each side of the star. To see this effect with the Sun, we would have to be several hundred AU away from the Sun. The gravitational lenses of stars within the Galaxy produce rings and double images that are too small to see with any telescope, but we can see the second effect of a gravitational lens: the brightening of objects beyond the lens. (continue)


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