Astronomically Far Away: How to Measure the Universe
Credit: European Southern Observatory

Paul Sutter is an astrophysicist at The Ohio State University and the chief scientist at COSI Science Center. Sutter is also host of the podcasts Ask a Spaceman and RealSpace, and the YouTube series Space In Your Face. Sutter contributed this article to Space.com's Expert Voices: Op-Ed & Insights.

Things in space are, appropriately enough, astronomically far away. The distance to the sun and other planets in the solar system is easy enough to calculate, especially because we can toss probes about here and there, and presumably, the probes will know how far they've traveled when they land.

But how can astronomers make the great leaps to measure the distances to the far-flung stars? How can we claim with any certainty the breadth and depth of the Milky Way? And what ruler spans the farthest reaches of the universe, separated from us by seemingly unfathomable oceans of darkness?

I want you to perform a science experiment. Come on, this will be fun. Start by shoving your nose right into the screen. That's it, right here. Close one eye. Now switch to the other. Continue alternating. If anybody gives you distraught glances, smile calmly, and with your sagest voice, tell them, "This is for science."

As you alternate eyes, the words on this screen should leap left and right, covering huge apparent distances. Now, back up to normal reading distance. Continue switching eyes. The words still shift, but not nearly as much as when we were a little more intimate.

Congratulations! You've discovered parallax. It's part of the reason it's useful to have two eyes: With binocular vision, your brain can judge distances without needing to evolve a ruler coming out of your forearm.

It's easy enough to calculate the distance: The span between your eyes forms the base of a long, skinny triangle. The amount of swing a distant object appears to travel as you switch eyes gives you one of the angles in the triangle. With a little bit of high-school geometry, you can know the distance to your desired target.

OK, great, but that's not exactly useful for stars, which are vastly farther away than this screen. But that's fine; we can still play the same game — we just need a different set of cameras. How about, say, Earth in the summer and Earth in the winter?

That's a pretty big triangle. Because we know the distance from the Earth to the sun, we know how wide our binoculars are. And by carefully measuring the teensy-tiny wiggle in a star's position between the seasons, we can compute the distance.

Well, to a point. I mean, most stars are so fantastically far away that we could never hope to measure their parallax, no matter how sophisticated — or big — our triangle gets.

To carry us even farther into the reaches of the cosmos, we need to switch to a different measurement method. This new method is again based on a very simple concept: brightness. If I know exactly how bright something is, then by measuring how bright something looks, I can figure out how far away it is. Farther things look dimmer. Super simple. We just need to know exactly how bright stuff in space is.

Fortunately, nature gives us a few of these "standard candles." One is a kind of star called a Cepheid , which periodically goes from dumb and dim to hot and bright in the matter of weeks or even days. That itself isn't all that special — stars do, after all, change brightness all the time — but what's peculiar about Cepheids is that the time between episodes is proportional to their true brightness. The brighter a Cepheid is in real life (as in, up-close-in-your-stellar-face real life), the longer it takes to cycle back and forth.

Their name doesn't mean anything special. As usual in astronomy, they're named after the constellation where they were first discovered — in this case, Cepheus. And the connection between their true brightness and the time between episodes was discovered about 100 years ago using the ever-reliable parallax method on a few nearby stars. That means we can measure a Cepheid star's cycle (which is supereasy, at least when it comes to astronomy) and immediately know its true brightness (which is superhard, at least until we can build probes that go there and just look). And we can compare the true brightness to how bright it looks, do a little math, take a little nap and tell the world how far away that Cepheid is, reaching even beyond the limited parallax method.

By the way, this is how Edwin Hubble convinced everyone that we should change the name from the Andromeda nebula to the Andromeda galaxy, because the Cepheids there were just a bit too far away to be inside the Milky Way, thereby radically expanding our conception of the true size of the universe.

Also, we're not exactly sure how Cepheids … you know, work. The best we can figure is that it has something to do with the layers of gas surrounding the star. The gas may be (somewhat) cool, hugging close to the star and blanketing the light from our eager telescopes. But the star's intense radiation puffs the gas out farther away from the star, thinning it enough to let the starlight pass through. After a while, the gas layer gets tired of that game, cools off and settles back closer to the star. Unsure of where exactly to live, it cycles back and forth, sometimes days at a time, for centuries.

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That's only our best guess; we actually have only a vague understanding of what powers the variability of Cepheids. But get this: It doesn't matter. I'll say it again, with emphasis: It doesn't matter. When it comes to using Cepheids as a distance-measurement device, what matters most is there's some way to measure the true brightness. The fact that a relationship exists between a Cepheid's period and brightness is all we need. (Well, yeah, if you're an astrophysicist interested in explaining all the goings-on up in space, then it's important. But if you just need a reliable distance estimator, then not so much.)

Cepheids give us rulers to some pretty far-out places — even those beyond our own galaxy. But at even greater distances, their usefulness peters out. If you can't see an individual star anymore, then it's no use to try to measure its periodicity. You need to use something brighter, something more intense, something … super. A supernova, for example.

Bright enough, common enough and reliable enough to use as a standard candle, supernovae (and, specifically, ones known as Type Ia) are so supremely bright that they allow us to measure some truly awe-inspiring distances, over halfway to the edge of the observable universe.

Not bad for a little trigonometry.

Learn more by visiting Paul Sutter's Expert Voices landing page, and by listening to the episode "How do we know far away stuff is…far away?" on the Ask A Spaceman podcast, available on iTunes and on the Web at http://www.askaspaceman.com. Thanks to Harold for the question that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.

Follow all of the Expert Voices issues and debates — and become part of the discussion — on Facebook, Twitter and Google+. The views expressed are those of the author and do not necessarily reflect the views of the publisher. This version of the article was originally published on Space.com .