One common misconception of beginner sky watchers is that we can tell the distances of the stars by just looking at them.
It seems reasonable, after all, that brighter stars must be closer than fainter ones.
Unfortunately, this is completely wrong.
Sure, this would be true if all stars were alike. In that case, if star A appeared four times fainter than star B, for example, it must lie twice as distant.
In reality, however, stars are quite different from each other.
Some are hot, while others are relatively cool. Some pump out tremendous radiation, while others do not. And some stars might be huge and others quite small.
For these reasons, we can’t assume a star that appears fainter must be farther away.
In fact, you might be surprised to learn that the opposite is often true. Stars that radiate the least can be seen only if they’re nearby, while those that are most luminous can be seen at much greater distances.
So, if we can’t gauge stellar distances by just looking, how do astronomers do it?
There are several techniques that, in principle, are rather simple but in practice require large telescopes and sophisticated instrumentation.
One way we can measure stellar distances is by triangulation. We all learned to do this in elementary school; hold your thumb out at arm’s length and alternately blink your eyes back and forth.
You’ll see your thumb seems to shift its position relative to background objects, even though you’re holding it perfectly still. Since your brain knows the separation of your eyes, it can estimate the distance of your thumb from this shift.
Our binocular vision provides the “depth perception” that allows us to move safely about our world, but it’s completely useless when viewing a star.
Instead, we measure a star’s exact position against the background stars at six-month intervals — when the Earth lies on opposite sides of its orbit around the sun.
From the shift we observe between these two measurements, we can calculate the star’s distance.
Unfortunately, this works for only the nearest stars. Thankfully, astronomers are a clever lot and have discovered a unique type of star that can help us plumb the depths of the cosmos.
The first of these to be discovered is named Delta Cephei, and we can see this faint star low in the northeastern sky after dark this week.
Use the nearby illustration to find it in the constellation Cepheus, the King. This star oscillates in size and temperature, but to us it just appears to brighten and dim every 5.3 days.
It was in 1912 that Harvard College astronomer Henrietta Swan Leavitt noticed that the longer it takes one of these “Cepheid” variable stars to complete one cycle, the more luminous the star must be.
In other words, by measuring a Cepheid’s variability anywhere in our galaxy (or beyond), we can know its actual luminosity.
And by comparing this with how faint it appears in our sky, we can estimate its distance.
In this way, Cepheid variables have become one of astronomers’ most valuable “cosmic yardsticks” and help us to measure distances across the universe.
