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Galaxies look stationary, so why do scientists say that they rotate?
Category: Space
Published: November 17, 2016
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
Galaxies do indeed rotate. This rotation is what gives typical galaxies a flattened round shape, a bit like how throwing and spinning pizza dough makes it round and flat. In terms of the tangential speed of its parts, galaxies rotate at an incredibly high speed. For instance, our entire solar system speeds along at about 500,000 miles per hour as it takes part in the galaxy's rotation. So why do galaxies look so frozen in place if their parts are moving so quickly? It's because galaxies are unimaginably huge. An object traveling at high speed across a very long distance appears to be moving slowly when viewed from far away. This is not a psychological effect. The object is actually traveling very slowly when its speed is expressed in terms of the percentage of the total distance it has to travel.
For example, pretend that you are in a sports car traveling at 200 miles per hour relative to the ground. This is a high speed compared to what humans usually experience. At this speed, you can get to the next city block in a few seconds. To an observer that is situated so as to be able to see your entire journey to the next city block, you are seen as traveling blazingly fast. However, now consider that you are traveling at 200 miles all the way across the United States from New York City to San Francisco. Even at this high speed, it would still take you 15 hours to complete this journey (assuming you don't get in an accident or get thrown in jail along the way). To an observer that is situated so as to be able to see your entire journey across the country (such as an astronaut on the International Space Station; pretending the car is visible from this distance), you are seen as traveling incredibly slow. In fact, the observer might even think you are not moving at all. This makes more sense if we express your speed not in miles per hour, but in percent of the overall trip per hour. For the trip across the country, you are traveling at about 7 percent of the journey per hour. In contrast, to drive to the next city block, you are traveling at about 500 percent of the journey per hour.
The same concept applies to the galaxy. The distance that our solar system has to travel in order to make one full trip around the galaxy is 9 × 1017 miles. Even though our solar system is speeding along at about 500,000 miles per hour as part of the galaxy's rotation, it will still take us about 200 million earth years to complete one trip around the galaxy's center. In terms of completing a single trip around the galaxy, our solar system is traveling at 0.0000000000005 percent of the journey per hour. To a distant alien astronomer in another galaxy who can see our entire galaxy, our galaxy is rotating so slowly that it looks like it is not rotating at all. The same things happens when we look at other galaxies.