Do Stars Move?

by Tega Jessa on April 17, 2011

Do Stars Move?


If you look at the night sky carefully you might notice that the stars seem to rise and set just like the sun and moon do. Do stars move in reality? This question doesn’t have a straight forward answer since it can be answered in different ways. The first way it can be answered is do they move in the same way that ancient astronomers thought that the sun and moon in regards to their rising and setting at the horizon. The other way is how they move in the universe.

For the first answer, stars don’t really move when we observe them. The motions are pretty stationary just like the sun. The only reason they seem to move to the casual observer on Earth is that the Earth rotates. So basically, when the part of the Earth facing the sun starts turning away it looks like stars are rising above the horizon. Another piece of evidence is that not all stars set. In ancient times people came up with myths to explain this. The story of Ursa Major and Minor tells us that the constellations never set below the horizon because of the disfavor of the goddess Hera. However the real explanation is that these stars are near the north polar star so rotation does not affect their appearance as much as other constellations.

The second answer is that in a more general sense that stars do move. The simple fact is that due to gravity every object in space moves. The reason that things seem stationary is because of relative motion and frame of reference. Relative motion means that celestial object tend to move at the same speeds. We only notice them moving if their relative velocity is different from ours. Stars move due to being in their own unique orbits around the center of the galaxy. In real time speed the move very quickly but because every other star moves at around the same speed we never notice how quickly we move. The other fact is that as fast as stars move the distance the cover is even bigger. What is fast to us on Earth is a snail’s pace in the larger universe.

In the end we see that the answer to the question “Do stars move?” depends on the frame of reference and our understanding of motion in the wider universe. So when you next time you look up at the night sky know that the stars are moving but not in the way most people think.

We have written many articles about stars for Universe Today. Here are some interesting facts about stars, and here’s an article about the types of stars.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We’ve done many episodes of Astronomy Cast about stars. Listen here, Episode 12: Where Do Baby Stars Come From?


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