- July 20, 2021 - Planétarium Rio Tinto Alcan : Guided tour of the sky
The Pole Star is one of the best known stars, but it also holds its share of mysteries: is it brighter than the other stars? Is it the Shepherd’s Star? In short, what is it that makes the Pole Star special? Let’s take a closer look at its features.
The one we call the Pole Star, or Polaris, is located in the constellation Ursa Minor (Little Bear). Its official name is Alpha Ursae Minoris, because it’s the brightest star in the constellation. And yet, its apparent brightness is on the modest side, and the star is only the 48th brightest in the night sky. The star Sirius, in the Canis Major (Greater Dog) constellation, is the one that can lay claim to first place in star brightness.
The Shepherd’s Star, meanwhile, is not the Pole Star, but rather the planet Venus, which may be visible in early evening or at the end of the night, depending on the period. That planet, often very bright in the sky, indicated to shepherds just when they had to return the sheep to, or remove them from, their pen.
A triple Pole Star
Alpha Ursae Minoris is in fact a triple star system. The primary component, Alpha Aa, is a yellow supergiant whose size is 37 times that of the Sun. It’s also 5.4 times more massive than our star. This is what’s known as a variable star, whose brightness varies over a 4-day period. It’s located 448 light years away.
Two other stars, Alpha Ab and Alpha B, orbit around the primary star. Alpha B can be observed with a modest telescope.
Locating the Pole Star
The Pole Star is easily identifiable in the sky because the Little Bear region contains few bright stars. To locate it, use the stars that make up the Big Dipper in the Ursa Major constellation. Draw an imaginary upward line starting from the stars on the end of the Dipper. You’ll find a star that’s a little brighter: that’s Polaris.
Finding north with Polaris
What makes the Pole Star even more special is the fact that it points us to geographic north.
The Pole Star is aligned with the Earth’s axis of rotation. So when we observe in its direction, we’re looking in the direction of north. In addition, just like the Sun, stars rise in the east and set in the west. Except, that is, for the Pole Star, which seems fixed on the celestial sphere.
Credit:Tau’olunga, Wikipedia Commons
A celestial top
The axis of the Earth is tilted at 23.5 degrees. That’s the reason why globes are also tilted. The gravitational influence of the Moon and the Sun makes the Earth’s axis turn a bit like a top. This is what we call precession. However, the movement of precession is very slow: it takes the earth’s axis 25,800 years to complete a cycle.
Which means that Alpha Ursae Minoris has not always been the Pole Star. For example, in the period when the great pyramids of Egypt were being built, it was Thuban, in the constellation Draco, that was the Pole Star. And in 12,000 years it’ll be Vega, in the constellation Lyra.
No Pole Star in the south
If currently we’re lucky enough to have a relatively bright star to serve as Pole Star, that’s not the case with the southern hemisphere. The star closest to the south pole is Sigma, in the constellation Octans. That star is barely visible to the naked eye, and is located a little more than 1 degree from geographic south.