Over the years, as I've enjoyed sharing my love of astronomy with you, I’ve rattled off the distances to many stars and galaxies. Except for within our solar system, trying to express those distances in miles would be very cumbersome. The numbers would be crazy large; they would become astronomical! (horrible pun) Light years do a better job because the numbers are smaller, and they also remind us of just how long it takes for the light to reach us. The speed of light is 186,300 miles a second. A light-year is defined as the distance light can travel in one year at that speed.

Given that there are about 31.5 million seconds in a year, you’ll come up with almost 5.9 trillion miles for just one light-year! So, if you see a star tonight that’s 70 light-years away, which is relatively close for a star, that means it’s 413 billion miles away, and that light from that star has taken 70 years just to reach us! But how do astronomers determine the distances to stars? It’s certainly not an easy process, but I’ll do my best to explain their methods.

Over a hundred years ago, astronomers could get a pretty good estimate of distance using the famous Hertzsprung-Russel diagram, developed in the early 20th century by Ejnar Hertzsprung of Holland and Henry Norris Russel from the United States. They studied the spectrums of thousands of stars, which are like fingerprints.

If you take starlight and send it through a spectrograph, you can spread out the various wavelengths or components of that light and learn a lot about a star. You can see signatures of different chemical elements, the temperature, and much more from these rainbow-like displays.

Hertzsprung and Russel found a definite relationship between spectral type and luminosity, or the amount of light produced by a star. They discovered that it could be placed on a graph and fit right along a nice curve. The beauty of this is you could determine the luminosity by just getting the spectrum of a star. And once you know the luminosity, figuring out the distance is a relatively easy task using the simple inverse-square law of light.

A more direct way of estimating stellar distance is the stellar parallax method which uses basic high school trigonometry. Here’s how it works. A photo is taken of the star when the Earth is on one side of the sun. Then, another picture is taken six months later when the Earth has orbited around to the other side of the sun. If the star isn't too distant, you’ll see it shift a tiny bit against the background of far more distant stars. The shifting of the star is called the parallax angle. Using the rules of geometry and trigonometry, you can determine the star’s approximate distance.

As simple as the math is, measuring that parallax angle is very difficult because the angles are incredibly tiny. You also assume that the background stars are stationary, but in reality, they're also shifting, which is impossible to measure.

Measuring the distance to stars using stellar parallax is also extremely difficult from the Earth’s surface because you have to put up with our blurring atmosphere. That’s why satellites are used. The Hipparchos satellite launched in 1989, and the Gaia satellite in 2013. Despite its success, satellite accuracy falls off with stellar distances past 30,000 light-years.

For really distant stars, like those in other galaxies, Cepheid variable stars are used. These are stars that vary in brightness over time.

In the early 1900s, Henrietta Leavitt, an assistant in the astronomy department at Harvard University, made a huge discovery. She studied thousands of variable stars that varied in brightness over a few hours to hundreds of days. Leavitt discovered a class of variable stars that were extremely regular in brightening and dimming. She also found a near-perfect relationship between the period of variation of these stars and their average luminosity, or light output. These Cepheid variables could then be used as mile markers in very deep space.

Once you have the period of variability, you can determine luminosity, and from there, it’s simple math to determine the distance of some really far-off places!

In 1923, the famous astronomer Edwin Hubble used Cepheid variable stars to determine that the Andromeda Nebulae, as it was known back then, was, in reality, a whole other galaxy over two million light-years away. Until then, our Milky Way was thought to be the only galaxy in the universe. Since then, the distance to thousands and thousands of other galaxies has been calculated using Henrietta Leavitt’s Cepheid variables. What an unsung hero she was!

Mike Lynch is an amateur astronomer and retired broadcast meteorologist for WCCO Radio in Minneapolis/St. Paul. He is the author of "Stars: a Month by Month Tour of the Constellations," published by Adventure Publications and available at bookstores and adventurepublications.net. Contact him at mikewlynch@comcast.net.