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The Milky Way is a barred spiral galaxy that looks much like a pinwheel, and just like a pinwheel our galaxy is spinning. Stars in the arms of the galaxy are orbiting about the center, and the entire disk of stars, gas and dust is rotating at approximately 270 kilometers per second (168 miles/second), which translates to 970,000 kilometers/hr ( 600,000 miles per hour).
This rate of rotation means that the Solar System – which is 28,000 light years from the center of the Milky Way – completely orbits the galaxy about every 225 million years. The last time we were in the same place in our orbit, dinosaurs were just starting to appear on the Earth.
Viewed from “above” – what would be North on Earth – the Milky Way spins in the counter-clockwise direction. Of course, if you were to view it from the other side, it would spin clockwise.
To determine the rotation rate of the Milky Way, astronomers mapped star forming regions using the Very Long Baseline Array (VLBA) radio telescope. The molecules in these regions amplify naturally occurring radio emissions. By observing how far these regions shifted over time in three dimensions, they were able to calculate the rotation of the entire galactic disk (and figure the mass of the Milky Way, as well).
Why is the galaxy rotating? In the early Universe, clouds of gas came together to form stars, which then were gravitationally attracted to each other to form larger clusters of stars enveloped in clouds of gas. These “globular clusters” eventually came together through their gravitational attraction and started rotating about the common center of mass. As the rotation picked up, the clusters of stars were squashed flat, eventually forming a disk with a bulge at the center.
To get a good handle on how all this works, think of those old cartoons where a chef is making a pizza – he throws a ball of dough into the air and spins it, and as it spins faster the dough stretches out and becomes flat, making a nice round crust. For a more detailed explanation with great illustrations of the formation of our galaxy, visit How Stuff Works.
If you’re curious about the rotation of other galaxies, you may want to check out Galaxy Zoo, a project in which anyone can help catalog the direction of spin of hundreds of thousands of galaxies in the Universe.
For more on the Milky Way, visit the rest of our section here in the Guide to Space, listen to Astronomy Cast: Episode 99, or visit the Students for the Exploration and Development of Space.