Despite my Ph.D. bona fides, at heart I still consider myself to be an amateur astronomer—someone who gazes at the night sky for the sheer joy of it, whether through a telescope’s eyepiece or a computer screen or with just their eyeballs.
I’ve been at it for more than four decades, and of all the things I’ve seen—planets, galaxies, exploding stars, stellar nurseries, and more—the one thing I’ve never seen for myself is a star that has moved.
Oh, certainly, the stars rise and set every night, but that’s just a literal reflection of Earth’s rotation, what astronomers call diurnal motion. No, what I mean here is relative motion: a star that has moved noticeably out of position compared with those around it. In all those years, Orion still looks like Orion and the Big Dipper like the Big Dipper. After all this time, to my eyes, the stars still seem fixed and unchanging.
On supporting science journalism
If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
But that’s an illusion borne of our limited lifespan and our not-keen-enough vision. The stars do move, just so slowly as to be imperceptible. It can take centuries for their subtle motion to become sufficiently substantial to detect.
It was no less a luminary than the astronomer Edmond Halley (you may have heard of the comet named after him) who was first to provide evidence for this. In a brief letter to the Royal Society in 1718 he noted that three of the night sky’s brightest stars—Sirius, Arcturus and Aldebaran—had moved substantially since they were mapped by the Greek Hipparchus circa 130 B.C.E. although it was later found he was in error about Aldebaran, the other two stars had indeed moved, and the fundamentals of his arguments remained sound: The stars could no longer be considered “fixed” in the sky.
What was once almost beyond belief is now relatively common knowledge: Today we know that—of course!—all the stars move and at great speeds; the sun orbits the center of the Milky Way at roughly 800,000 kilometers per hour. A jet moving that rapidly would cross the continental U.S. in less than 20 seconds. Despite that zippy velocity, the stars are so far away—tens of trillions of kilometers, at least—that this motion is reduced to an apparent crawl: one so glacial, as Halley realized, that it takes centuries to notice by eye.
Astronomers call this proper motion; “proper” is used in the old-style sense meaning “belonging to.” It plays a big role in modern astronomy.
All the stars you can see are actually moving through space as they each orbit the center of the galaxy. They move at different speeds in different directions and are at different distances, however, which means the motion we see for each star is different, too. The largest factor is distance; stars that are closer to the sun appear to move more rapidly because of perspective, just as nearby trees seem to whiz by as you drive on a highway while distant buildings or mountains move much more slowly.
The closest stars have the highest proper motion. The speed record is currently held by Barnard’s Star, a dim bulb of a red dwarf just under six light-years away. Even then it’s not exactly flying across the sky; this star would take nearly two centuries to move half a degree on the sky, the equivalent distance as the apparent size of the full moon. Still, that’s enough to easily record its motion with even a small telescope over the course of just a year or so.
Sirius, the brightest star in the night sky, has a proper motion that’s much slower than Barnard’s Star but still fast enough that its celestial position shifted more than half a degree between the times of Hipparchus and Halley; no wonder the eminent astronomer noticed.
The effects of stellar proper motion can be seen easily nowadays. One way is to look at, for example, Hubble Space Telescope images of the same patch of sky taken years apart. After that amount of time, together with Hubble’s amazingly sharp vision, some motions are obvious.
An animation of Hubble Space Telescope images captured in 2001 and 2020 shows the proper motion of a portion of the Cygnus Loop Nebula, an expanding bubble of gas produced by a dying star some 20,000 years ago. Despite its seemingly subtle positional shift, the material seen here is moving through interstellar space at more than half a million miles per hour—fast enough to travel from Earth to the moon in less than 30 minutes.
Credit: NASA/ESA/STScI/Ravi Sankrit (STScI)
In fact, astronomers sometimes must account for proper motion when using telescopes such as Hubble. If they target a star using vintage coordinates from, say, the 1970s, they could wind up pointing the telescope to the wrong part of the sky and completely missing the star! (I’ve actually seen this happen; it’s a real and important issue.)
A star’s proper motion can be used to determine if an object seen near it is actually nearby at all—like a candidate exoplanet would be—or is instead a background galaxy or some other far-distant imposter. As the star moves, a planet would be seen to move along with it, while a galaxy would remain fixed. Images taken a few years apart can determine the difference. Another use is to “weigh” a black hole; astronomers have used the motions of the stars closely orbiting Sagittarius A*, the black hole in the center of the Milky Way, to calculate its mass and have found it to be a whopping four million times that of the sun.
A star’s proper motion is measured in two components: its north-south movement and its east-west movement. That gives us its two-dimensional motion in the sky, what we call its transverse motion. There are also ways for astronomers to gauge a star’s motion toward or away from us (its so-called radial velocity), and by combining that with its proper motion, we can find a star’s total three-dimensional motion through space.
Over the years astronomers have used this technique to build a 3-D map of the stars in the sky, which can play a critical role in understanding our local Milky Way neighborhood. Stars moving in the same direction at the same speed may be associated with one another even if they’re far apart in the sky; this is how the nearest star in the universe to us, Proxima Centauri, was found to be a third member of the Alpha Centauri star system. This can also be used to find stars that are members of stellar clusters, which can be difficult to determine from just distance alone.
Building this sort of 3-D map is the goal of the European Space Agency’s Gaia mission, which has measured the positions and proper motions of more than one billion—yes, billion with a B—stars in our galaxy and even in other galaxies.
Using Gaia data, astronomers have been trying to determine the velocity through space of the Andromeda Galaxy with respect to our own. Andromeda is the Milky Way’s nearest large neighbor and is located about 2.5 million light-years away—but it’s getting closer all the time. Radial velocity measurements show that it’s approaching us at a little more than 100 kilometers per second, which makes it seem like a collision is inevitable—although not for another four billion years or so.
Andromeda’s transverse motion remains uncertain, however. Its sideways motion may be small, allowing for a collision, or it may be on par with the galaxy’s radial velocity, in which case Andromeda will swing wide and miss us. So will it collide or not? That’s unclear. But the good news is that certainty will come easily—provided we have patience. As time passes, the motions of the stars are easier to measure because they move farther and farther from their initial position, making the motion more obvious.
Sooner or later, the stars will tell us whether Andromeda and the Milky Way will be getting much, much better acquainted. If so, then in a few eons we may have to make our proper introduction.