You've Never Seen a Picture of the Milky Way's Spiral

By Rebecca J. Rosen

A little primer on the images we have of space, and how we get them.


I think we all have some sort of impression of what our galaxy, the Milky Way, looks like from space. Perhaps you had one of these posters hanging on the wall of your seventh-grade science classroom:

Or perhaps you've seen one of these pictures of a spiral galaxy from NASA:

And from some combination of all these images, and, also, from some vague knowledge that NASA has images of all sorts of faraway galaxies, it's easy to believe that a picture of the Milky Way like the ones above exists. But one doesn't. The above images are all either artists' rendering of the Milky Way or of other spiral galaxies in our universe.

The reason is simple: In order to get that long view of the Milky Way's arms, we'd need a spacecraft to be outside the galaxy. We've never even sent one outside our solar system (though we are very close).

We do, however, have incredible pictures of the Milky Way's center, which we can capture from very near Earth with our biggest telescopes.

These telescopes -- the Hubble is the best known but there are others -- work by taking grayscale images of space through red, green, and blue light filters, and then combining those images into one picture, much the same way that a digital camera works. For the Hubble telescope, which captures visible light, the images are pretty much what you would see if you could travel way into space to see things with your own eyes. For another NASA telescope, Chandra, which captures X-ray light (which is not visible to the human eye), scientists shift the wavelengths down to visible light, much like, as Jonathan McDowell, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics, explained it to me, "playing the same tune in a different key." The result is that images from Chandra, and another NASA telescope, Spitzer, which captures infrared light, are not shown in true color.

I also asked McDowell about images like the one at the top of this post, in which the stars appear to have an extra, almost unnatural twinkle. Do scientists add those diamond-like spikes to make the stars appear more starry? McDowell explained that no, those spikes wouldn't appear if you saw the stars with your naked eyes. Rather, that extra twinkle is the incidental effect of the rods that support one of the telescope's mirrors. As the light from the stars enters the telescope, it refracts around the support rods, and the effect is the extra twinkle. 

Images: NASA.

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