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How do we measure the vast distances of the Universe?

Space, as we all know, is big. In fact, it's more than big; it's vastly, hugely, mind-bogglingly big.


But how big is that, exactly, and how do we go about assigning actual, quantifiable distances between planets, nebulae, and other cosmic entities? In this beautifully animated video, produced by the Royal Observatory Greenwich, astrophysicist Olivia Johnson explains how scientists use something called parallax to gauge the distance between the landmarks of our universe.

The film comes via "Measuring the Universe: from the transit of Venus to the edge of the cosmos," a free exhibition currently being held at the Observatory. It's scheduled to run all the way through September 2nd, so be sure to check it out if you find yourself in jolly old England.

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Actually for really huge distances, the distances between galaxies for example, the parallax generated by the orbit of our planet just isn't large enough to measure things accurately. Parallax is really only useful for measuring the distances of nearby stars and is really only good to about 300 light years or so.

But for really huge distances, hundreds of thousands, millions or billions of light years, we need something astronomers call a "standard candle."

To measure distances to nearby galaxies we use stars that vary predictably in brightness, specifically a type of star known as a Cepheid variable. Around the turn of the last century, the astronomer Henrietta Swan Leavitt discovered that the longer the period of variation of a Cepheid variable, the greater its luminosity. Another astronomer, Harlow Shapley, then was able to correlate the brightnesses of Cepheids with those of known types of ordinary stars, tying Leavitt's relative distance scale to an absolute one. This gave us a standard candle.

So, as we observe a Cepheid and see how long it takes for its brightness to vary, we can then plot that information on a now well established graph to find out its intrinsic luminosity, it's absolute magnitude of brightness, with its apparent magnitude of brightness. It then becomes a simple matter to calculate how far away that star is by using the inverse-square law.

But having one standard candle is not enough, for example Cepheids aren't bright enough for us to accurately measure the oldest and most distant galaxies accurately. The distances to those we've had to estimate using red shifted spectra and Hubble's Law. But this has become increasingly inaccurate as other instrumentation improves. So astronomers are always looking out for other ways to measure the distance to distant galaxies with other forms of standard candles.

Recently we now use a specific class of supernova star, in this case Type Ia supernovae. These are far more bright than Cepheid stars and be used accurately to even more huge distances, to the oldest most distant galaxies of all.

It was with this new standard candle, which only became possible for us to use recently*, that we discovered the cosmic acceleration and the need for dark energy to explain that acceleration.

Anyway, the summary is it's very important to know the distances to things in the universe. This is critical in order to make cosmology a science and not just mere speculation.

* This recent technique, using Type Ia supernovae, only became possible with the arrival of pattern recognition software that fully automated the old process of using blink comparators in the old days: