Astronomy Jargon 101: Standard Candles

In this series we are exploring the weird and wonderful world of astronomy jargon! If only there was a way to measure the distance to today’s topic: standard candles!

Measuring distances to stuff in space is really, really hard. One technique is to use parallax, the observed wiggle in stellar positions over the course of a year. That technique is fantastic as long as the star isn’t too far away. At a certain distance, your telescope simply won’t be able to accurately measure the wiggle, and you’ll be out of luck.

The key is to find something called a standard candle. If you could look out at a distant object and know for sure exactly how bright it is (in other words, you could know its luminosity), then you could compare that measurement to how bright it appears to be. Using a little bit of trigonometry, you could then calculate a distance.

As an example, if you knew for sure that the brand of flashlight that I had was the exact same as the flashlight that you had, then if I was far away you could measure the brightness of my flashlight versus the brightness of your flashlight and figure out my distance.

Now all we need are some flashlights.

Thankfully, nature has given us a few. The first known were the Cepheids, a kind of star that varies in brightness. Astronomer Henrietta Swan Leavitt discovered that the longer a Cepheid takes to cycle, the brighter it is. By calibrating a few Cepheids using parallax, you can then go out and find any Cepheid you want and figure out how far away it is.

In 1998, two teams of astronomers discovered dark energy – the unexplained accelerated expansion of the universe – by looking at another standard candle: Type-1a supernovae. These kinds of supernovae all go off in roughly the same way, and so it’s possible to compute their true brightness.

Today, astronomers employ a variety of standard candles, from Mira variables to red giant branch stars. But no matter the method, the underlying technique is the same: know the brightness, know the distance.