Astronomers have transformed star data from the Gaia and Hipparcos missions into a video that predicts stellar motions millions of years from now.
It’s tempting to think of stars as our still companions in the night. But they’re not still at all — stars are moving all the time. It’s just that most of them zoom about the galaxy on timescales too long for humans to appreciate. (Barnard’s Star and Cygnus 61 make for two notable exceptions.)
Since July 2014 Gaia has repeatedly photographed the entire sky with the aim of eventually determining precise distances to more than 1 billion stars across the galaxy. Gaia does this by looking for parallax, the slight apparent motion of stars that’s actually due to the spacecraft’s motion in its orbit. (Hold a finger in front of your eyes, then view it with one eye at a time — your finger will appear to move due to the same perspective effect.)
By measuring precise positions, Gaia also sees stars’ proper motions, their movement across the sky. Doppler shifts in spectra measured by other surveys provide the stars’ movement toward and away from Earth, so ultimately we’ll have not just the distances but also the full 3D motion of each of these stars.
That said, Gaia’s still very much in the middle of its mission: The full dataset isn’t expected until 2022. So astronomers don’t have distances and proper motions from Gaia alone just yet. Instead, they’re calculating this information based on early Gaia images, along with previous data from the Hipparcos mission.
The video above shows this selection of 2,057,050 stars seen by both spacecraft and predicts their future motion based on their current positions and velocities. The video is plotted in galactic coordinates, so the plane of the Milky Way spans the center of the image. Orient yourself by Orion (to the right) and the Pleiades (to the left) of the frame.
A Matter of Perspective
Predicting stellar motions 5 million years into the future isn’t easy, and it comes with its share of caveats. The most obvious one is that of perspective — close stars appear to move quickly while faraway stars, such as those in the galactic plane, appears to move very slowly.
Another effect you’ll notice is that stars near the top and bottom centers of the image appear to accelerate and decelerate as they swoop across the galaxy. This is a spurious effect thanks to the rectangular projection that transforms the celestial sphere into a flat image — the stars’ current velocity, which the astronomers used to predict their future positions, is constant.
Another, less obvious effect comes from the fact that no velocity is an island — when you measure a star’s velocity, you have to measure it with respect to something else. In this case, all the motions you see are measured relative to the Sun in its galactic orbit. So although stars in the galaxy’s halo move quite slowly relative to the galaxy’s center, they appear to move quickly in the video because their speeds are calculated relative to the Sun.
That’s not all — there’s one more word of caution. Gaia’s tracking stars, but not interstellar dust clouds. So the clouds in the video don’t move. Likewise, stars currently hidden inside or behind those clouds (which Gaia doesn’t see) aren’t accounted for. So as the video goes on, you’ll notice some areas that appear depleted of stars, when in reality, other stars currently out of view would move in to fill these areas.
Into the Future
The tie-in to Hipparcos data is useful to astronomers, but we’re all really waiting for the full Gaia dataset. The next expected data release comes in April 2018, which will include not only 1 billion stars’ positions, but also their distances and proper motions. It won’t be to the high standards of the full 2022 catalog, but it’ll come one step closer!
Read more about Gaia, the making of this video, and future data releases in the ESA press release.
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PUBLISHED; April 17, 2017 at 03:00AM