#150 What is Polaris?

I learned this today. Polaris is the star that is known as the North Star because it is usually over the North Pole.

Polaris is not the brightest star in the night sky. That honor belongs to Sirius, also known as the Dog Star. However, Polaris is fairly bright, and it makes up the end of the constellation called the Little Dipper.

Polaris is the largest star in a triple star system 433 light-years away from Earth. The three stars in the system are Polaris Aa, Polaris Ab, and Polaris B. Polaris Aa is the largest star and it is the one that we can see. Polaris Aa and Ab orbit each at a very close distance, about 18.5 Astronomical Units (AU) and Polaris B orbits them at a distance of 2,400 AU. One AU is about the distance from Earth to the sun, which is 150 million km. So, Polaris Aa and Ab orbit each other at 2,775,000,000 km, which is a little less than the distance from Earth to Uranus, and B orbits them at a distance of 360 billion km.

Polaris Aa (I’m just going to call it Polaris) is 5.4 times larger than our sun and 1,260 times more luminous. The other two stars are smaller, both Aa and B are 1.3 times larger than our sun.

Polaris is a yellow supergiant star. Supergiants start out as main sequence stars, like our sun, but they are much bigger so they burn through their fuel much faster. When they have exhausted all of their hydrogen fuel they expand and start to cool, becoming a supergiant. They will only be yellow supergiants for a few thousand years before they become red supergiants. Polaris is in its yellow supergiant phase now so it will become a red supergiant and will probably explode as a supernova. It may end up as a neutron star or a black hole.

Polaris is also a cepheid star. A cepheid is a type of star that pulsates. Polaris pulsates by about 10% over roughly four days. However, that rate is changing and the rate began to decrease very rapidly in the 1970s. Scientists differ as to the reason why. The star has also become much brighter. It is probably 2.5 times brighter now than it was when Ptolemy observed it in the 2^nd century BC. A cepheid star usually pulsates because of vibrations caused by energy trapped beneath its surface. These pulsations die away as the star expands and its surface cools down. Because the pulses from Polaris are slowly decreasing, the theory seems to be that it will stop pulsing altogether at some point soon. We may be very lucky because we are alive at a time when the star will stop pulsing.

Polaris is more commonly called the Pole Star because it is located above the North Pole. The Earth rotates on its axis, which means that Polaris is directly above that rotation point. This is why all of the other stars in the sky rotate throughout the night, but Polaris hardly moves. Because of this, it has been useful for navigation for centuries.

However, Polaris is not directly above that axis point and it won’t be there until March 24^th 2100. I’m pretty sure that I won’t be here to see that. But my daughter should be. And, it will take a long time, but Polaris will drift on once it has passed the point above the North Pole and one day it will no longer be the Pole Star.

Before Polaris, Thuban, was the North Star. When the Egyptians built the pyramids, Thuban would have been the star that didn’t seem to move at night. After that, Kochab was the Pole Star from 1700 BC to about 300 AD. And now it is Polaris. In about 4000 AD, a date which, unless something extremely surprising happens, I will definitely not be around for, Errai will probably be the Pole Star. Errai is three times less bright than Polaris, so it won’t be as impressive. However, in about 13,000 years, Vega will be pretty close to being our pole star and it is six times brighter than Polaris, so that will be very beautiful.

So, Polaris is the pole star because it is located almost exactly over the north pole. It will be perfectly in place by March 24^th 2100. However, because everything in the universe moves, it wasn’t always the pole star and it won’t always be. It is a yellow giant and it will eventually die, as all stars do. And this is what I learned today.

Image from: https://astrocampschool.org/north-star/

Sources:

https://www.newscientist.com/article/mg12617152-800-science-the-pole-star-stops-pulsating/

https://www.science.org/content/article/north-star-may-be-wasting-away