In this series, we explore the weird and wonderful world of astronomy jargon! If only there was one way to measure the distance to today’s topic: standard candles!
Measuring distances to objects in space is really, really difficult. One technique is the use of parallax, the observed wobble in star positions over the course of a year. This technique is fantastic as long as the star isn’t too far away. At a certain distance, your telescope just cannot measure the shake accurately and you are out of luck.
The key is to find something called a standard candle. If you could look out at a distant object and know exactly how bright it is (in other words, you could know its luminosity) then you could compare that measurement to the brightness it appears to be. Then, with a little trigonometry, you can calculate a distance.
For example, if you knew for sure that the flashlight I had was exactly the same as the flashlight you had, then when I was far away you could measure the brightness of my flashlight compared to the brightness of your flashlight and figure from my distance.
Now all we need is a couple of flashlights.
Fortunately, nature has given us some. The first known were the Cepheids, a kind of star with different brightness. The astronomer Henrietta Swan Leavitt discovered that the longer a Cepheid takes to cycle, the brighter it is. Then, by calibrating some Cepheid with parallax, you can find any Cepheid you want and see how far away it is.
In 1998, two teams of astronomers discovered dark energy – the inexplicable accelerated expansion of the universe – by looking at another standard candle: type 1a supernovae. These types of supernovae all go off roughly the same, so it is possible to calculate their true magnitude.
Today astronomers use a variety of standard candles, from mira variables to giant red twig stars. But regardless of the method, the underlying technique is the same: know the brightness, know the distance.