Over the 25 years I’ve written this column, I’ve often marveled at the mind-boggling distances to many celestial objects. Astronomers don’t often use miles to express stellar and galactic distances because the numbers would quickly become unwieldy. Instead, light-years are used because the numbers are more manageable, and they’re also a testament to the unfathomable scale of the universe. All light travels at the speed of 186,300 miles a second in the vacuum of space. A light-year is defined as the distance that light travels at that speed in one year.
Given that there are about 31.5 million seconds in a year, you can calculate that a single light-year equals around 5.8 trillion miles. So, saying a star is 70 light-years away, which is pretty typical for stars we see with the naked eye, means that the star is about 406 trillion miles away. That’s 406, followed by 15 zeros! Also, by definition, the light we see from that star tonight left that star 70 years ago. We actually see the star the way it looked in 1954. If we see a star tonight that’s 3,000 light-years away, we see it as it was in 976 B.C.! Whenever you look at the stars, you’re looking back into the past, sometimes the very distant past!
So how do astronomers know how far away these stars are? Unfortunately, it’s not a short and easy answer. You use the stellar parallax method to determine the distance for stars less than 2,000 to 3,000 light-years away. This method is like measuring the distance to an object by looking at it with one eye, then shutting that eye and looking at it with the other eye and noticing how its position seems to shift. You take a picture of a star when the Earth is on one side of the sun in its orbit, and then you 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 slightly against the background stars. This process comes down to simple high school trigonometry. The shifting of the star against the background stars creates a parallax angle. You can draw a triangle between the Earth, the sun, and the star using basic geometry rules that say opposite angles are equal. You take the parallax angle and cut it in half. Since you know what that angle is and the length of one side of the triangle (the distance between the Earth and the sun), the distance x (to the star) = 93,000,000 miles divided by the tangent of the parallax angle.
As simple as the math is, measuring that parallax angle is challenging because it’s such a tiny angle. You must also assume that the background stars you’re using to measure the stellar parallax angle are stationary. In reality, they’re shifting as well!
Measuring the distance to stars using stellar parallax is also extremely difficult from the Earth’s surface because you have to endure the blurring effects of our atmosphere. That’s why satellites have been launched into orbit to measure the stellar parallax more precisely and calculate distances to thousands of stars.
Stars farther than 3,000 light-years away require other methods to determine stellar distance. One method is the famous Hertzsprung-Russel diagram, developed in the early 1900s 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 that make up that light and learn much about a star. These rainbow-like displays show signatures of different chemical elements, temperature, and 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 discovered a distinct pattern when plotting a spectral type vs. luminosity graph. Most stars fit right along a nice curve. The beauty of this is that by just getting the spectrum of a star, you can determine its luminosity. Once you know the luminosity, figuring out the distance is an easy math equation using the straightforward 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 that varied regularly in brightness over several hours to hundreds of days. In her observations, she discovered that the variable stars called Cepheids were extraordinarily regular and extremely bright, shining 500 to 10,000 times the sun’s luminosity. They varied in brightness because of cycle changes within the star. Leavitt found a near-perfect relationship between a star’s period of variation and its average luminosity, or light output. Cepheid variables could then be used as mile markers in deep space because of their brightness. If you spot a Cepheid variable, you can determine how far away it is just by observing its period. Once you have the period, you can get its luminosity, and from there, relatively simple math can be used to determine the distance of extremely far away places!
The famous astronomer Edwin Hubble used observations of Cepheid variable stars in what was then known as the Andromeda Nebulae to determine that Andromeda was a whole other galaxy, over 2 million light-years away. Until then, our Milky Way was thought to be the only galaxy in the universe. This is Hubble’s discovery, but he could not have done it without Henrietta Leavitt and her Cepheid variables. What a tremendous celestial yardstick!
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. Mike is available for private star parties. You can contact him at [email protected].
