Is North Star really just a single star?
When we think of North Star, the first thing that comes to our mind is - it is the brightest star in the sky that is ALWAYS present at the North Direction. Well, in reality, that is not the case.
The North Star that we currently look up in the night sky is called Polaris, and Polaris ranks only 50th in the brightness! It is also known that Polaris remains fix in the night sky. But, if you go up to the North Pole and take a time-lapse of the night sky you can see that the North Star makes it own little circle. This happens because the North Star isn’t exactly above the celestial North Pole, the point around which the entire northern sky turns. It is actually offset a little – by about three-quarters of a degree – from the celestial north.
The reason Polaris is very easy to spot is only because it is ALMOST EXACTLY at the North. Even in a country with dark sky and a full moon, the North Star is relatively easy to find. This is what makes the Polaris the most special.
Secondly, the Polaris that we consider as the North Star, is not just one star! It is a set of three stars (though scientists believe, it might be five!!) The first indication that the North Star was more than it appeared was when an astronomer that discovered Uranus, examined it through his telescope. Instead of spotting a single star, he found two close neighbors. It was only until 2006, that the Hubble Telescope spotted the third star to this pair of stars. The primary star, Polaris A, is a supergiant with about six times the mass of our sun. A close companion, Polaris Ab, orbits 2 billion miles from Polaris. Much farther away, is the third companion Polaris B. Polaris B is located approximately 240 billion miles from Polaris A.
Three stars are believed to be an odd combination. There are almost 75% of stars that exist in pairs (those are called twin stars or binary stars). Twin stars are formed together and they orbit around each other. They are very close to each other and are practically impossible to be resolved without using instruments. But, some of them move further away from each other, expanding into different parts of the galaxy. Even the scientists have found a star, that is plausible to be the twin of our Sun!! But let’s leave it for some other day!
The Polaris has not always been the North Star! Thousands of years ago, when Egyptians were building the Pyramids, a famous star called Thuban, in the constellation Draco the Dragon was considered to be the North Star! And in some years, Vega will be our North Star. You must be wondering how this is possible? The spin axis of our Earth undergoes a motion called precession. If you have ever watched a spinning top, you know that its spin axis tends to stay pointed in the same direction. However, if you give it a slight nudge, the axis will start to change its direction, and its motion will trace out as a cone. This changing of direction of the spin axis is called precession.
So what gave the Earth the "nudge" it needed to start precessing? The Earth bulges out at its equator, and the gravitational attraction of the Moon and Sun on the bulge provided the "nudge" which made the Earth precess. Because of precession, different stars will serve as north stars; they move about one degree every 73 years. In other words, it takes 26,000 years for the axis to trace out the cone one complete time.
Polaris will remain the North Star throughout the rest of our lives and for a few centuries later. Throughout the past few centuries, Polaris has served as a North Star marker for navigators, escaping slaves and other explorers. During this time, the North Pole has appeared to be drawing closer to Polaris and continues to do so, today. By 2102, the North Pole and Polaris will attain their minimum separation distance of 27'. After 2102, the Pole will slowly move away from Polaris and within 2,000 years will pass close to Errai, a star within Cepheus the King.
In about 13,000 years, the North Pole will be close to Vega, the brightest star in Lyra the Harp, constellation. So, in the future, Vega is going to be our North Star. Now, when you look up at the Polaris, you are definitely going to have a different sense of appreciation for it!