Galactic Structure
Our planet Earth is in constant motion. It spins on its axis once a day, and orbits the Sun once a year -- motions that are easy to follow. But one other motion isn't obvious at all: Earth and the rest of the solar system orbit the center of the Milky Way galaxy once every 225 million years.
The solar system is part of what astronomers call the galaxy's "thin" disk. It's like a big pancake -- wide but flat. The disk rotates around the big "bulge" of stars and gas at the galaxy's core.
The disk contains at least a couple of hundred billion stars. We'll talk about the stars in our region of the disk tomorrow.
The hottest, brightest stars form great spiral arms that wrap around the core. Seen from afar, they make the Milky Way look like a pinwheel spinning through space.
The Milky Way's thin disk is only part of the galaxy, though -- like the meat in the middle of a sandwich. The "bread" is a thick disk that extends far above and below the thin disk. It contains far fewer stars, though, and the stars it does have are quite old.
The oldest stars of all inhabit the halo -- a big spherical region that surrounds the disk. Most of the halo consists of "dark matter." It produces no detectable light, but its gravity pulls on the matter we can see. In fact, the dark matter speeds up the solar system as it orbits around the galaxy -- an important contribution to our motion through space.
Script by Damond Benningfield, Copyright 2006
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