How We See the Stars in 3D

In Stars by Brian Koberlein2 Comments

Astronomers can measure the distance to about a billion stars using the same effect that lets you enjoy a 3D summer blockbuster.

There are lots of clues our minds use to determine whether something is close to us or far away, such as its apparent size, its position relative to other objects and its motion. But since humans typically have two eyes, we can also gauge an object’s distance using an effect known as parallax. When you walk along a sidewalk, you’ll notice that closer objects appear to move past you more quickly than more distant objects. This means that if you look at an array of objects from two slightly different positions, the closer objects will appear to have shifted more than distant objects. Since your eyes have different positions, each eye gets a slightly different view of the world, and closer objects will appear more different than farther objects. Our brains combine these two views to give us a 3D view of the world.

Some directors showing off their 3D cameras.

Some directors showing off their 3D cameras.

3D movies take advantage of this effect to give an “in your face” movie experience. Rather than filming one movie, directors of 3D movies film two, where each movie is filmed from a slightly different perspective (typically the same separation as human eyes). Once in the theater, the two perspectives are shown together. By itself that would just make for a blurry movie, so they are shown on a special screen so that the light reflected from one projector is polarized differently than the other. Polarization is a property of light orientation that our eyes don’t typically notice, but polarized light can be blocked by things like polarized sunglasses.

How 3D glasses work.

How 3D glasses work.

This is why you have to wear special glasses in a 3D movie. These glasses are similar to sunglasses, but each lens blocks a different orientation of light. This ensures that each of your eyes only sees one of the two movies. Since each movie has a different perspective, your eyes have different views similar to the real world, and your brain integrates them as a perception of depth.

A simple example of parallax. Credit: NASA, ESA, and A. Feild (STScI)

A simple example of parallax. Credit: NASA, ESA, and A. Feild (STScI)

So how does this effect work for stars? You don’t notice any depth to the night sky because your depth perception only works for a distance of about 6 meters (20 feet), and the stars are light years away. But because the Earth revolves around the Sun, our view of the stars changes slightly over the course of a year. So over the course of a year the closer stars will appear to shift relative to more distant stars. This shift is incredibly small. If you held a single sheet of paper edge-on at arm’s length, the thickness of the paper would be about three times wider than the largest shift of the closest star.

Astronomers have been able to measure the parallax of the closest stars for 80 years. We’re now able to measure stellar distances to more than 1,600 light years. The Hipparcos spacecraft, for example, measured the distances of about 2.5 million stars to varying degrees of accuracy. The Gaia spacecraft is expected to measure the distances of a billion stars.

As impressive as that is, you might be wondering how we measure the distances of farther stars? After all, our galaxy alone is 100,000 light years across. That’s an interesting story for another time.


  1. If you want to get a sense of what the stars look like in 3d, I wrote a web app for that. It’s got about 15k stars from Hipparcos. But rather than a 2-AU baseline like Hipparcos and Gaia, the default is set to 1 light year.
    To view in 3d, select “Stereo”, You can change the baseline if you like. You can also swap the images by choosing “crosseye” or “parallel”. With crosseye, you cross your eyes until the 2 images merge. With parallel, you gaze with an infinite stare, like those books of patterns where a 3d dolphin pops out if you do it right. You can also print it and view it in Brian May’s “Owl” 3d viewer.

    Another way of getting a 3d feel is to take a journey “through” your favorite constellation. Set the ship speed to a non-zero number and press “Play”.

    It’ll be fun to improve this with Gaia data one day!

  2. This is amazing! I just hope that one day, we can all see these up close and personal. For now, this is good enough.

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