For millennia, humanity has been captivated by the night sky. One of the most ancient of all human pastimes, stargazing, has been at the center of our collective consciousness for thousands of years. The way the world looks from atop a hill when all seems quiet and the only thing one can hear is the low breeze blowing through the trees—that is the perfect setup for contemplating the stars. Gaze long and hard enough, and you might find yourself in the throes of an ancient question: Do the stars move? Though our modern understanding of astronomy and physics makes this question seem almost naïve, it must be remembered that for most of our history, humanity has not really known what a star is or where one might find it.

Besides the obvious motion, stars are also moving through space. They orbit the centers of their respective galaxies, and this movement can be quite complex. Our sun, for instance, along with the entire solar system, is not stationary but is moving through space at about 828,000 kilometers per hour (514,000 miles per hour). The solar system is orbiting the center of the Milky Way galaxy at about 230 kilometers per second (143 miles per second), and it takes the solar system about 225 million years to complete one full orbit around the center of the Milky Way. This is known as a "cosmic year." While stars might seem to be stationary when we look at them over shorter periods of time, they are not; they are moving at incredible speeds, and over "astronomical timescales," which are very large periods of time, they cover some substantial distance. But what is even more interesting is that stars are moving not only within their own galaxies but are also part of larger cosmic structures. Galaxies are part of the same thing; they are not stationary but are moving through the universe.

Proper motion is an incredibly interesting aspect of stellar motion. It is the actual movement of a star across our sky when we compare it to more distant stars that form a somewhat fixed backdrop for us. We measure this movement in very small units called arcseconds, where one arcsecond is equivalent to 1/3600 of a degree. Even our most basic instruments allow us to detect proper motion for some of our nearest stellar neighbors. When we discuss proper motion, however, we should keep in mind that the stars we see in our nighttime sky are not actually moving in the same direction or at the same speed. Both local and cosmic factors cause our nighttime stars to have a wide array of proper motions. Meanwhile, a few of our proper motion stars, like Barnard's Star, for example, are actually moving significantly fast when compared to the average speed of a star across our sky.

Along with proper motion, stars are capable of exhibiting radial motion—movement toward or away from the Earth. This motion can be detected through the Doppler effect. If a star is moving away from the Earth, the light from it will appear to be shifted toward the red end; if the star is coming toward the Earth, the light from it will appear to be shifted toward the blue end. This is a rare occasion when color plays a fundamental role in understanding the universe—color perceived with the naked eye or through the lens of a telescope, or in photographs taken with a camera. Once the velocities of stars have been determined, a few members of this exclusive group of celestial objects are used to check and calibrate the speedometers of astronomers. The question "Do stars move?" is not as easy to answer as it seems. The stars that we see at night are constantly moving through space.