For a while, I wondered why pictures of galaxies in space formed rotating disks. Using the Milky Way as an example, we can see everything laid out on a flat, 2D surface, more or less. And for a long time, I wondered why this was the case. Why weren't different solar systems scattered around the galaxy in the shape of a sphere, for example? Well, as it turns out, the answer is simple.
Everything in space is spinning. REALLY fast. The Earth spins at about 1675 kilometers/hour. The Earth rotates around the sun at 30 kilometers/second. The Sun circles the center of the galaxy at 250 kilometers/second, and our galaxy is moving through the universe at about 600 kilometers/second. So, things are always spinning away from each other, but they're anchored by gravity in the middle.
But why does it form a 2D disk? Well, imagine you're holding two buckets of water, one in each arm. If you start spinning (in an open area, safety first!), your arms will raise themselves and start spinning in a circle, and as long as you continue to spin, the buckets will stay elevated, following the path of this flat circle. In this example, your body would be the Sun, the buckets of water would be planets, and your arms would be the gravity keeping them all together.
So, gravity holds everything together, but the spinning is pushing everything away from each other. The natural result, just as with spinning the buckets of water, is that disk shape as shown in the link to the Milky Way Galaxy earlier.
JT Surge
Bibliography:
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/971028e.html
http://www.universetoday.com/26623/how-fast-does-the-earth-rotate/
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