Milky Way Portrait

The different parts of the Milky Way are not static, but in constant motion. The disk rotates about the Galactic center, and at large radii, the rate of rotation does not trail off, but remains fairly constant. This rotation is in contrast to what we see in a planetary system, where objects (planets) rotate more and more slowly the farther they are from the center. As we'll see in a moment, the constant rotation rate at large distances from the Galactic center betrays the presence of something we cannot see.

The stars in the halo move very differently, plunging through the Galactic disk in elliptical orbits that are randomly oriented. Their orbits are not confined to the Galactic disk, and in some ways seem unaware of it. All of these orbits are centered on what we call the Galactic center region, and give clues as to how the Galaxy formed.

Our part of the Galactic disk orbits the Galactic center at over 136 miles per second (220 km/s). It takes about 225 million years for our region to complete one orbit around the Galactic center. Our solar system has orbited the Galactic center some 15-20 times since it formed. One-quarter Galactic orbit ago, dinosaurs roamed the earth.

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Telescopes Mastery

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