In the 10-plus year history of the starwatch column, I’ve rattled off distances to many stars and galaxies. The distances are so far that the miles are too cumbersome to express. Light years do a better job because the numbers are smaller and you’re reminded of just how long it takes for the light from the stars to reach your eyes.

All light travels at the speed of 186,300 miles per second, and a light year is defined as the distance that light travels at that speed in one year. There is close to 31.5 million seconds in a year, making it approximately 5.8 trillion miles for one light year. So if you see a star tonight that’s 70 light years away, which is a typical distance for a star you can see with the naked eye, that would equate to about 406 trillion miles.

Also by definition, since it’s 70 light years away the light we see from that star tonight left that star 70 years ago, so what we see is that star as it looked in 1944.

So how do astronomers know how far away these stars are?

For stars that are less than about 2,000 light years from Earth, you use the stellar parallax method for determining distance. You take a picture of a star when the Earth is on one side of the sun in its orbit and take another picture six months later when the Earth is on the other side of the sun. If the star is not too distant, you’ll see it shift a tiny bit against the background stars.

This process comes down to simple high school trigonometry. The shifting of the star against the background stars creates what’s called a parallax angle. Using the simple geometry rule that opposite angles are equal, you can make a triangle between the Earth, sun and the star. You take the parallax angle and cut it in half. Since you know what that angle is and you know the length of one side of the triangle it’s simple trig. The distance x (to the star) = 93,000,000 miles divided by the tangent of the parallax angle.

As simple as the math is the practice of measuring that parallax angle is very difficult and you’re also making assumptions. You’re assuming that the background stars you are using to measure the stellar parallax angle are stationary. In reality they may be shifting as well.

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 the Hipparchos satellite was launched in 1989 to measure the stellar parallax and distances to hundreds of stars. Despite its success, the satellite’s accuracy falls off with smaller parallax angles and larger stellar distances past 500 light years. Stars beyond that require another method.

That method uses the famous Hertzsprung-Russel diagram. It was developed in the early 1900s by Ejnar Hertzsprung of Holland and Henry Norris Russel of the United States. They studied the spectrums of thousands of stars, which are like fingerprints. If you take star light and send it through a spectrograph, you can spread out the various wavelengths that make up that light and learn much about a star. From these rainbow-like displays you can see signatures of different chemical elements, temperature and much more.

Hertzsprung and Russel found a definite relationship between the spectral type of a star and its luminosity, which is the amount of light a star produces. They found that most stars could be put on a graph and fit along a curve. The beauty of this is that by just getting the spectrum of a star you could determine its luminosity. Once you know the luminosity, figuring out the distance is an easy math equation using the very simple inverse-square law of light.

For really distant stars, Cepheid variable stars are used. This was a huge discovery made by Henrietta Leavitt early in the last century at Harvard University. She studied thousands of variable stars, stars that vary in brightness over a period of a few hours to hundreds of days. In all of her observations she discovered that the variable stars called Cepheids were extremely regular and extremely bright, shining 500 to 10,000 times the sun’s luminosity. They varied in brightness due to cycle changes within the star.

Mike Lynch is an amateur astronomer and professional broadcast meteorologist and is author of the book, “Stars, a Month by Month Tour of the Constellations” published by Adventure Publications available at bookstores at http://www.adventurepublications.net.