Date: August 21, 2012
Title: The North Star
Podcaster: Christopher Crockett
Description: The North Star is arguably the most famous star in the sky and one of the most useful for finding one’s way around the heavens. In this podcast, I’ll introduce you to this jewel of the night. You will learn how you can easily locate the North Star, why it is useful, why we won’t always have its guidance, and what is the nature of the star itself. Come discover this beacon to astronomers and navigators around the world and take your first step in charting the night sky!
This podcast originally aired on January 11, 2009 (one of our first podcasts ever!)
Bio: Dr. Christopher Crockett is an astronomer at the United States Naval Observatory in Flagstaff, Arizona. His research involves searching for planets around very young stars (“only” a few million years old). It is hoped that the results from this research will help constrain models of planet formation and lead to a better understanding of where, when, and how often planets form. Chris is also passionate about astronomy outreach and education and will talk for hours about the Universe if you let him.
Hello, this is Christopher Crockett from Lowell Observatory.
The North Star, Polaris, has long served as a beacon to navigators and astronomers. Its apparent fixed position in the sky acts as a guidepost – a reference from which we can chart a path over an ocean or across the heavens. Owing to its lighthouse legacy, it is commonly assumed that Polaris is the brightest star in the sky when, in fact, it’s the 48th brightest as seen from Earth. It is, however, the brightest star in the constellation Ursa Minor (known to many as The Little Dipper). To find Polaris you first have to find the Big Dipper whose seven stars stand out brilliantly in northern skies making it easily recognizable. In January, the Big Dipper sits low on the northern horizon in early evening. The two stars on the edge of the dipper’s “cup” furthest from the dipper’s handle are often called the “pointer stars” because if you draw a line through them up out of the top of the cup, you’ll find that they point to a third star roughly three hand widths away. They point right at the North Star!
Having a star that so thoughtfully points us north is clearly very helpful. But how does it work? How does this one star manage to stay put while every other star glides across the sky in great celestial arcs? Well, remember that the apparent motion of the stars during the night is due to the rotating Earth. As the Earth spins on its axis, our view of the heavens changes over the course of a night. Polaris, however, happens to be nearly aligned with the axis of our planet. It’s not exact, but it’s pretty close. If you were to travel to the North Pole and look straight up, Polaris would be right over your head. Because of that, it’s near one of only two positions in the sky that doesn’t appear to move over the course of a night (see if you can guess where the second position is located). This star’s unique location relative to the Earth means it always appears in the same place in the Northern Sky.
But Polaris has another trick up its sleeve. If you were to measure the angle between the North Star and the horizon, you’d find that it perfectly matches your latitude! Go ahead and try it! If you live in, say, Boston, which sits roughly at 42 degrees north latitude, you’d find that Polaris sits 42 degrees above the horizon. Here in Flagstaff, the North Star hovers closer to 35 degrees off the horizon. The further south you travel, the lower in the sky Polaris will appear. At the equator, it sits right on the horizon. Travel any further south, and Polaris disappears behind the curvature of the Earth.
But as steady as it seems, Polaris hasn’t always been, and won’t always be, our North Star. Which star we call the “North Star” changes over the millennia and there are times when there is no North Star at all. This has less to do with the stars themselves and more to do with the motion of the Earth.
If you’ve ever spun a top, what happens? Sure it spins around, but what else? Try it, and you’ll notice that the top not only spins, but it also wobbles. And just like a spinning top, the Earth wobbles. Astronomers use a fancy word for this: we call it precession. However the precession of the Earth is painfully slow to human observers: it takes 26,000 years for the Earth to complete one wobble. What this means is that over the course of these 26,000 years, where the North Pole points in the sky changes. In fact, it slowly traces out a circle in the sky, returning to the same point every 26,000 years. Over the course of the millennia, the North Pole points towards different stars, and sometimes to none at all. When the Egyptians were building the Great Pyramid, they didn’t look to Polaris to find north; they used a star in the constellation Draco called Thuban. 12,000 years from now, Vega, one of the brilliant stars in the Summer Triangle, will take the mantel of North Star.
Polaris is more than just a useful tool; it’s also a fascinating place in its own right. Located 430 light years from the Earth, Polaris is actually the brightest star in a triple star system – three stars gravitationally bound together, circling one another in an endless cosmic ballet. The star closest to Polaris in this system is about as far from Polaris as the planet Uranus is from our own Sun; the other star is about 130 times further out still!
Polaris itself is a bright hot supergiant, about 30 times larger than our Sun while putting out as much energy as over 2000 Suns every second. And while you may never have noticed, its brightness fluctuates over a four-day period, earning Polaris a classification as a “variable star”. These fluctuations, typical of aging stars, are driven by instabilities deep in the stellar interior that cause the star’s outer layers to repeatedly expand and contract as if it were breathing heavily. What’s more, Polaris is of a special family of variable stars called “Cepheid variables”. Cepheids are stars whose frequency of variability is determined by their brightness. This simple fact has a rather remarkable consequence: if you can find a Cepheid variable and measure the period of its fluctuations, you can calculate how bright it should be. Then you can measure how bright it appears to us here on Earth and use the difference to determine the distance to the star. Using this technique, Edwin Hubble, in the 1920s, was able to make the first accurate measurements to the distances of other galaxies and in turn revealed the expanding Universe. Stars like Polaris led to one of the most unexpected and profound discoveries of the past century!
So be sure to go out tonight and see if you can find Polaris. It’s relatively straightforward to do and a great way to start familiarizing yourself with the night sky. Once you’ve found it, you’ll be continuing in a tradition that helped our ancestors chart their way across treacherous oceans and continues to light the way through an ever-surprising Universe.
365 Days of Astronomy
The 365 Days of Astronomy Podcast is produced by the New Media Working Group of the International Year of Astronomy 2009. Audio post-production by Preston Gibson. Bandwidth donated by libsyn.com and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at 365DaysOfAstronomy.org or email us at info@365DaysOfAstronomy.org. Until tomorrow…goodbye.