The Astrophysics Spectator

Issue 2.29, August 3, 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.

August 3, 2005

This week I add three pages to the web site. Despite being on two different survey paths, these pages are intimately related. The first page, which is on the “X-ray Astronomy” path, discusses the observation of clusters of galaxies with x-ray telescopes. The second page, which lies on the “Cosmology” survey path, discusses the microwave background. The third page, which is also on the “Cosmology” survey path, discusses how one can measure the distance to a cluster of galaxies by observing the effects of its hot gas on the microwave background.

In astronomy we have many hierarchies of bound gravitational objects. At the low-end, we have the moons of the planets. As we look out from the Solar System, we find ever-larger systems of gravitationally bound objects, starting with the binary stars, stars bound in clusters, stars and clusters bound in galaxies, and finally, galaxies bound into clusters of galaxies. The galaxies bound within a cluster create a deep gravitational potential that traps gas. This gas is very hot and highly ionized, so it emits x-rays. This means that surveys of the sky in the x-ray provide a means of finding clusters of galaxies. The great interest in the clusters of galaxies is that they provide a measure of the clumpiness of matter at early times in the universe.

On the theme of cosmology, when we look out in any direction, we see the remnant thermal radiation left from earlier times. This radiation is predominately microwaves, and is most often referred to as the cosmic microwave background. Effectively, it is the radiation emitted by a shell of matter that surrounds our visible universe. At its creation the radiation was characterized by a high temperature, but the motion away from us of the matter that released this radiation causes the radiation to be Doppler shifted to a lower energy. The microwave radiation we observe is characterized by a temperature of 2.7Copyright Kelvin. The importance of the microwave background is that its existence is one pieces of evidence for the Big Bang theory of cosmology.

There is a subtle link between galaxy clusters and the microwave background: the hot gas in a galaxy cluster changes the shape of the microwave background spectrum. If we looked at a galaxy cluster with an infrared telescope, we would see the microwave background as skewed to a higher temperature. The microwaves acquire energy as they scatter with the hot electrons in the cluster gas. The strength of this effect, which is called the Sunyaev-Zeldovich effect, depends on two physical properties of the cluster: the density of the gas in the cluster, and the size of the cluster. By measuring the strength of this effect, we have a direct measure of the density times the size of the cluster. When this result is combined with measurements of the x-ray emission from the cluster, we can derive a physical size for the cluster. This permits us to measure the distance to the cluster by measuring the angle on the sky subtended by the cluster.

A news item is included with this issue. This concerns the recent announcement by three astronomers that they have discovered a new planet. This body is a Kuiper Belt object, and appears to be slightly larger than Pluto. This should reopen the debate over whether Pluto should be considered the planet.

Notice. I am taking a vacation from writing, so the next issue of The Astrophysics Spectator will appear on August 24. After that date, new issues will appear fortnightly.

Jim Brainerd


A New Planet? A Kuiper Belt object with a radius comparable to Pluto's has been found. This object is far outside of the orbit of Neptune, with a semimajor axis of 67.5 AU and an orbital period of 560 years. Now the question arises: do we consider this object a planet or a planetoid? (continue)


X-rays from Galaxy Clusters. The largest gravitationally-bound systems in the visible universe are the galaxy clusters. These clusters contain between several hundred and several thousand galaxies in a megaparsec-radius volume. In the gravitational potential well created by these galaxies, hot gas is trapped that emits x-rays. This x-ray emission enables us to find galaxy clusters by surveying the sky with x-ray telescopes. (continue)

Microwave Background Radiation. One of the inevitable consequences of an expanding universe is a bath of background radiation. This radiation is created at an early age of the universe at a very high temperature, but today we see it as a microwave background radiation that is characterized by a temperature of 2.73Copyright Kelvin. (continue)

Measuring Distance with Galaxy Clusters. The hot gas in a galaxy cluster modifies the spectrum of the microwave background when the microwaves scatters with the hot electrons in the gas. This effect, known as the Sunyaev-Zeldovich effect, provides a measure of the density and the physical size of the gas sphere in the cluster. When combined with x-ray observations, one can derive a distance to the cluster. This method provides a Hubble constant that is in agreement with the value derived from supernovae observations. (continue)


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