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Issue 3.04

The Astrophysics Spectator

February 15, 2006

This issue of The Astrophysics Spectator contains some of the new, some of the old, and some of the philosophical. The new is a page introducing galaxies, which have only been recognized as galaxies distinct from our own over the past-two centuries. The old is a review of the book The History and Practice of Ancient Astronomy, which describes not just the theories of the ancient astronomers, but also their instruments and their observations. The philosophical is in a new commentary on the habit of theorists to spin new astrophysical theories even when data is to sparse to support any theory.

From a historical standpoint, out understanding of galaxies is recent. The galaxies were at first called nebulae, grouping them with the gas nebulae of our own Galaxy. The idea that galaxies were outside of our own Galaxy was first pursued in the 19th century, but broad agreement that they were large collections of stars similar to our own Galaxy was not reached until the 20th century.

Galaxies run a range of sizes, with the largest galaxies containing about a trillion stars. Some galaxies are free of dust and gas, being composed of only old stars. Other galaxies contain gas, from which new stars are continually formed. They are classified by their appearance into three broad categories: ellipticals, spirals, and irregulars. Massive black holes are though to lurk at their centers; when the galaxies are young, these black holes generate massive amounts of energy. Many of the most distant galaxies are visible in this state.

These characteristics of galaxies are discussed in a new introductory page on “The Structure of Our Universe” survey path.

Next Issue: The next issue is scheduled for March 1.

Jim Brainerd


The Urge to Explain (February 15, 2006). Phenomena without explanation is as intolerable as a grain of sand in our shoes. When a phenomena is first seen, theorists flood the journals with theories to explain the phenomena. But astrophysics is almost always too complex to be solved on the basis of a handful of observations. Serious theories for a phenomena come after observers have answered some key questions, first of which is where in the universe are the objects creating the phenomena. (continue)

The Structure of the Universe

Galaxies. The visible universe contains tens of billions of galaxies, many containing tens or hundreds of billions of stars, and some, such as our own galaxy, containing trillions of stars. These systems are varied in their shape and size, ranging from the elliptical galaxies that contain only old stars to the spectacular spiral galaxies that contain many young, large stars. This page provides a short introduction to the galaxies. (continue)

Book Review

The History and Practice of Ancient Astronomy by James Evans. The patterns of discovery seen in ancient astronomy are still repeated in modern astronomy. This book by James Evans very effectively describes the astronomical achievements of the Babylonians and the Greeks by explaining their astronomical instruments and their observational methods. If astrolabes and sun dials appeal to you, then so will this book. (continue)

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