Polaris, the North Star, is a time-honored symbol of constancy and dependability. Boy scouts and other travelers know that — from locations in the Northern Hemisphere — this star can always be found in the northern sky, at a height above the horizon that corresponds to your latitude. Thus Polaris has guided travelers, no doubt for millennia. But Polaris is not constant in brightness. The single star we see as Polaris, which actually consists of a main star and two smaller companion stars in a triple star system, is a variable star. The main star is a classic Cepheid variable, whose minute variations in brightness have been known and watched by astronomers since the early 20th century. In the early 1990s, Polaris was waning in brightness. Beginning around 2000, astronomers studying the star found that the brightness of Polaris was on the rise again. Looking back over centuries, Polaris’ brightness increase may be “rather dramatic.”
That’s according to astronomer Scott Engle of Villanova University in Pennsylvania. He explained to Nola Taylor Redd of Space.com that he and his team have been searching back through historical records of Polaris’ brightness, plus observing the star with the Hubble Space Telescope, to conclude that Polaris has grown brighter over the past 100 years. Redd wrote :
The next step was to determine just how far back the increasing brightness went. Engle pursued observations by Danish astronomer Tycho Brahe in the 16th century and Persian astronomer Abd al-Rahaman al-Sufi in the 10th century, using information from historical texts to determine just how bright Polaris was in the ancient sky.
According to Engle … the North Star has brightened by about two-and-a-half times over the last two centuries. Modern interpretations of the historical data indicate that it could be as much as 4.6 times brighter than it was in ancient times.
The waxing and waning in brightness of Cepheid variable stars is important to astronomers, because these stars are used as standard candles for determining the distance scale of our universe. That is, after watching many Cepheid variable stars change in brightness — and knowing distances to them via parallax or other means — astronomers have learned how bright a Cepheid variable of a particular intrinsic brightness should look at a given distance from Earth. Armed with this knowledge, they can watch the pulsations of Cepheid variables in distant space. They can deduce the stars’ intrinsic brightnesses because of their rates of pulsation. Then they can see how bright these stars look — to learn their distances.
The variations in the brightness of Polaris are small, but, because Polaris is the closest Cepheid variable stars known at 325-425 light-years away, astronomers routinely study it.
Notice the range of distances given above? In fact, the distance to Polaris has been a source of puzzlement among astronomers with various studies using different measuring techniques getting different results. Read more about astronomers’ clash over the distance to the North Star here.
And, in other ways as well, Polaris is not as “dependable” as its long-held reputation suggests. It is not exactly at the sky’s north pole of course, but an apparent width of about 1.5 full moons from it. Thus, although we commonly say that all stars in the northern sky wheel around Polaris, the North Star itself makes a tiny circle around the exact northern axis of the sky.
Also, because of the wobbling motion of the Earth’s axis (precession), Polaris will not always be a pole star. It will be closest to the sky’s north pole on March 24, 2100 AD. As time passes, it will then be farther and farther from the celestial pole. In about 12,000 years, our descendants will have the beautiful blue-white star Vega as their Polaris.
For more information, go to www.earthsky.org