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[The sky in five million years. Credit: ESA/Gaia/DPAC]
If you go outside at night and watch the sky for a moment, it looks fixed, eternal. But if you wait even a few minutes, you see subtle changes. Perhaps a meteor will flash, or a human-made satellite will roll by. A few more minutes and you’ll see the stars have moved to the west, a reflection of the Earth’s rotation.
Go out night after night and you’ll see more changes. The planets slowly slide against the background stars as they (and we!) orbit the Sun. The Moon will appear in a different spot (and in a different phase) every night, and if you’re lucky, a comet might be bright enough to see as well.
But the stars themselves? If you are very diligent, you might see some changing in brightness, what we call variable stars. Some pulsate, some are binaries orbiting each other, some have huge dust clouds around them that intermittently block their light.
But even then, the positions of the stars are fixed. Unchanging, unmoving, unwavering.
But this is a lie, whispered in our ears by our fleeting lifespans. The stars do, in fact, move. The sky is not some velvet canvas upon which the stars are drawn, but a vast volume through which the stars travel.
All the stars you see, and hundreds of billions more, circle the center of the Milky Way galaxy. Their motion is rapid, hundreds of kilometers per second, but they are so remote and the galaxy so huge that it takes hundreds of millions of years to orbit the galactic center even once. But if you observe the sky very carefully, precisely map the positions of the stars over time, you will see them change. And if you are studious, you can then predict where they will be in the future as well.
Behold, the future of the Earth’s sky: The changing positions of the visible stars, projected a staggering five million years into the future (make it full screen and high-res to see it best):
The video is based on the stars’ positions and motions measured by the European Gaia spacecraft, which has been mapping them since 2014. It has found accurate positions for more than a billion stars —a billion!— and has also been able to determine the motions and distances (using parallax) for a subset of more than two million of those. These data were then projected into the future for the five million year timespan of the video, using steps of 750 years between frames (note that early on in the mission some parts of the sky had been incompletely mapped, resulting in dark stripes of no data).
Some things to note: For one, the Milky Way is a flat disk, and we are in that disk, so we see it as a broad swatch of stars across the sky. In the video, this has been remapped as going straight across the middle (in the same way that flat rectangular maps of the Earth show the equator going across the middle). An issue with this sort of map is that the poles get distorted, stretched out (like, again, Greenland and Antarctica looking huge in rectangular full-Earth maps). You’ll see stars move faster at the very top and bottom of the video than closer to the middle due to that, and some take big swings in direction too. That’s not real; the stars aren’t necessarily moving faster or changing their direction. It’s just because the map warps their trajectories.
The map is so big you might have a hard time seeing recognizable constellations. I did, at first! But if you look to the right, just near the edge, you’ll see the stars of Orion, a visual anchor in the sky. At the very beginning of the video, at the upper left, you can find the seven stars of the Big Dipper, too (it’s upside down). But look sharp! Those are relatively nearby stars, and their motions are rapid. Within a few seconds of the video, the familiar shape is gone, blown apart by the stars’ motions. I’ve labeled some of the brighter stars and constellations in the image above, but in even 42 millennia some stars have moved substantially. I had to watch the video carefully to make sure I had the right stars; for example Alpha and Beta Centauri moved a huge amount in that relatively short time.
Remember, this map is just using the star’s motions to predict the future. But there will be other changes, too. For example, Betelgeuse, the reddish star marking Orion’s right shoulder, doesn’t have too long to live. In a few hundred thousand years at most it will explode, going supernova. Rigel, Orion’s left knee, won’t last much longer than that. Many other stars will physically change over five million years, many dying. The video doesn’t show that.
Still, it’s fascinating to watch, and very important scientifically. Mapping the positions, distances, and motions of stars tells us about the galaxy we live in; how massive it is, how structures comes and go, and obviously what things may look like in the coming eons.
For me, personally, though, it is a needed jolt to my philosophy. When I stand under the starry vault and imagine it immutable, I’m basing that on my own ephemerality. Knowing that this view will not endure in perpetuity is a reminder that we don’t own this time, or any time. We’re visitors, moving through, and we should take the opportunity to appreciate it while we can.