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Issue 3.03

The Astrophysics Spectator

February 1, 2006

This week on The Astrophysics Spectator, two more pages are added to “The Structure of Our Universe” topic path. The new pages are introductions to our Galaxy, the Milky Way, and to the structure of our expanding universe. Under the news section I discuss the implications of the recent launch by NASA of the New Horizons spacecraft to Pluto.

The Milky Way Galaxy we live in is a spiral galaxy. Our universe is filled with such galaxies, although our own galaxy is an unusually large example. As with other spiral galaxies, our own Galaxy has three components: a central bulge that is thought to contain a massive black hole, a disk of bright stars and gas, and an invisible halo of dark matter. The gas and largest stars in the Galactic disk form sets of spirals that twist around the center of the Galaxy.

When we look beyond our own galaxy, we see billions of more galaxies. Most of these galaxies are moving away from us, with the more distant galaxies moving away faster than the nearby galaxies. This motion away from us is the motivation for the big bang theory of cosmology, the theory that the universe expands with time.

The most striking aspect of modern cosmology is how it combines both the idea that Earth occupies a favored position in the universe with its opposite. In the ancient cosmologies, Earth sat at the center of the sphere of stars. When we look out from us, we see ourselves at the center of an expanding sphere, a sphere that has a surface that emits microwave radiation. But while we see ourselves in the center of the universe, the theory that fits the observations says that we would see this very thing at every point in the universe. The theory is that no point within the universe is the center of the universe, but every point in the universe appears to by the center to anyone sitting there.

Next Issue: The next issue is scheduled for February 15.

Jim Brainerd

The Structure of the Universe

The Milky Way Galaxy. Our solar system sits in the disk of a massive spiral galaxy called The Milky Way Galaxy, named after the hazy band on the sky. Our Galaxy is an unusually large spiral galaxy. This new page gives a short description of our Galaxy's structure. (continue)

The Universe. When we look out beyond the Milky Way, we see billions of more galaxies stretching to the outer limits of our observable universe. These galaxies have two striking properties: they are not uniformly spread across space, and they are on average moving away from us. The most distant galaxies are much younger than our own Galaxy. At the outer limit of our vision, we cease seeing galaxies; instead we see a photosphere that completely encloses our universe. (continue)

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