The Astrophysics Spectator

Issue 1.9, December 8, 2004

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December 8, 2004

This week I add the first pages to the galaxy path. Galaxies are the principal collection of stars in our universe. They range dramatically in size and character. The spiral galaxies are among the most beautiful natural objects we see. Among the spiral galaxies is our own Milky Way galaxy, which counts as a large galaxy.

Galaxies are a challenge for theorist to model, because the orbital motion of the stars is determined by the gravitational field produced by those stars. Galaxies exist in a mathematical regime where approximations are not possible. And the materials that constitute a galaxy—gas, dust, and stars of a range of masses and ages—are in a constant state of change. Gas turn into stars of various masses. Stars explode, creating smaller remnant stars and liberating more gas. These processes play a role in how galaxies form and evolve.

We start the galaxy path with four pages: an introductory page to galaxies, a page describing the basic characteristics of elliptical galaxies, a page describing the basic properties of spiral galaxies, and a page giving an overview of the density wave theory of spiral structure.

In the commentary, I give my justification for studying astrophysics. At the risk of sounding like R. Buckminster Fuller, we profit from astrophysics only as generalists.

Jim Brainerd


The Astrophysics Generalist. The study of astrophysics can be justified if we remain generalists in our study. As generalists we develop an understanding of how our universe became inhabitable, and we learn how our complex universe is composed from a few basic principles. But does being a generalist imply anything about the conduct of research? (continue)

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New Background

Galaxies. Galaxies are gravitationally-bound collections of stars. The galaxies vary in their properties, ranging from the beautiful spiral galaxies to the mundane elliptical galaxies. The physics of galaxies is difficult because the motion of their stars is set by the gravitational field produced by those stars; the study of galaxies is the study of collective phenomena. (continue)

Elliptical Galaxies. Elliptical galaxies are common galaxies in clusters of galaxies. They have little gas and dust, which means that star formation has ceased. As a consequence, these galaxies are red with old star. These galaxies range from the spherical to the highly elliptical, but despite this variation, they do not rotate appreciably. (continue)

Spiral Galaxies. The spiral galaxy is the everyman's galaxy. They have disks of gas and new stars that are embedded in spheroid of older stars. The disks of these galaxies rotate, and within these disks propagate spiral arms of gas and stars. The centers of spiral galaxies appear to have black holes that are tens of millions to billions of times more massive than the Sun. Most of the mass in a spiral galaxy is dark, detectable only through its gravitational influence; its composition is unknown. (continue)

Density Waves and Spiral Structure. The spiral arms are thought to be density waves that propagate through the disk. The wave is associated with the drift in angle of a star's maximum orbital distance from the galactic center. An amplification and feedback mechanism replenishes the wave's energy by extracting energy from the differential rotation of the galactic disk. (continue)


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