Ask Ethan #19: Why does the Milky Way look curved?

“The fact that we live at the bottom of a deep gravity well, on the surface of a gas covered planet going around a nuclear fireball 90 million miles away and think this to be normal is obviously some indication of how skewed our perspective tends to be.” -Douglas Adams

Even though our main blog has moved, I'm so glad that the questions and suggestions have still been pouring in, because it's time for another entry in our Ask Ethan series! Today's question comes from professional physicist Marty Olsson, who asks:

I have been bothered for a long time why the Milky Way looks curved, sort of like a rainbow... instead of straight like our galaxy is supposed to be.

I guess I don't understand the perspective. Is it because we are not in the galactic plane?

Now, Marty was kind enough to provide an example of exactly what he's talking about, and if you've been using the internet to find outstanding space photos, chances are you've come across something like this yourself.

Now, I'm well aware that not everyone has actually seen the Milky Way themselves in real life; I myself have only seen it on three separate occasions, and none of those happened until I was already well into adulthood.

But when I've seen it, it's always looked like this.

The thing is, the galaxy really is straight! The proposed explanation was actually reasonable, as the spiral arm structure of our galaxy is quite curved; were we significantly out of the galactic plane, we might indeed see something like the curvature of some spiral arms from our perspective.

But we're actually pretty much smack in the middle of our galactic plane, and our galaxy is very much a flat, symmetric disk from our perspective. When we launched the COBE satellite -- rising above the Earth's atmosphere -- we were able to view the entire 360 degree galactic panorama in the infrared, and what we saw definitively showed what lay beyond the light-blocking dust of the Milky Way.

Our galaxy indeed has spiral arms, but we are in the galactic plane, and that plane is flat. So why, then, does it appear so curved in some images, when it just makes a straight (but huge) streak across the sky when we see it both with our own eyes and in some photographs?

The problem comes when we try and look at a larger region of sky than we can capture with a plain photograph and an undistorted lens. Take a look at this beautiful time-lapse video from the 2009 Texas star party, which was taken with a fisheye lens, and notice how some parts of what you see become more-or-less distorted as the time elapses.

Now, that's a nice video and all, but it doesn't do a thing to explain why these photos of the galaxy are as distorted as they are. After all, the horizons aren't distorted, and these photos weren't taken with a lens that distorts space, either.

What might help you get a handle on what's happening is a much simpler task: if you trace the Sun's path out over a day. No matter where on Earth you are, it travels in what appears to be a circle, only part of which is (usually) visible to an observer at a given location.

Yet I want you to think about what this image, above, shows. On the left side, that's the Sun rising in the east. Up atop the image, that's the Sun passing high overhead (this composition was created in Italy) in the southern part of the sky, and then towards the right, it sets in the west. Let me pose the following question to you:

How is it possible to get two points in the sky -- 180 degrees apart -- into the same image?

It's not possible, not unless you distorted something along the way! In this case, we had to distort the composite, stitched-together image in order to display the entire path with a flat horizon! This is a complicated task, similar to the reason why it's impossible to make a flat map of a sphere that simultaneously has all the properties you'd desire!

So what's the story of curved Milky Way panoramas, then?

A beautiful shot like this -- created by Mike Salway, cofounder of the Ice In Space forum -- is not the result of a single shot, long-exposure photo, but rather the result of stitching together and then appropriately distorting a series of long-exposure images of the night sky. Mike was kind enough to detail the process publicly, and I wanted to walk you through some of the concepts involved.

First, here are the thirteen raw images used to stitch together this composite. Note how, in each one, the portion of the Milky Way that appears is completely straight, the way you'd see it with your own eyes.

What you then need to do is stitch these images together, something that necessarily involves the creation of distortion, although not all that much. What you wind up with, though, is something that's not exactly pleasing to the eye to look at.

You see, we know that the horizon has got to be straight, and so when we see something like this, it offends our sensibilities. Distortion is always going to be greatest around the edges; after all, we're projecting a portion of a "sphere" (which is what we can see) onto a flat surface!

So you go to an editor, like photoshop or lightroom, and you apply a severe distortion to the image to flatten that horizon. This works, but it also heavily distorts the sky!

So in the end, you wind up with a very aesthetically pleasing image, and you can see two points 180 degrees apart (or more) at once, but the tradeoff? Your sky is tremendously distorted, although it's beautiful, and you can see far more of it than you otherwise could.

And this isn't even the worst of it: try going for a full 360-degree panorama!

If you want to capture the zenith -- directly overhead -- you have no choice but to deal with the most spectacular of distortions, the same way the poles of a sphere get distorted in a Mercator projection.

But the full 360-degree view is worth it; you just need to remind yourself that what your looking at is distorted in a very particular fashion. It's not the galaxy that's curved, but rather the image, so that we can view it all at once on a flat surface!

And that's why the most beautiful photos of the night sky look like this! Thanks for joining me for another Ask Ethan, and don't forget to check out all the great posts on the new Starts With A Bang; you won't regret it!