Polaris is our Star for December

This month look for the star Polaris, the lucida of Ursa Minor or the Little Bear. Astronomers know Polaris as Alpha Ursae Minoris. In popular culture, we call this star the North Star and the Lode Star. Polaris gets these names because it is the guide to true north (as opposed to magnetic north). This is because Polaris appears almost straight up to anyone standing on the North Pole. The star is not the brightest star in the sky nor is it exactly true north. Polaris is the 40th brightest star in the sky and ¾ of a degree (1-1/2 moon diameters) away from the point of true north in the sky. In long duration photographs, Polaris makes a tiny little circle around the true North Pole while the other stars make larger and larger circles.

Astrophysicists classify Polaris as an F star, which means it’s a bit hotter than our sun (which is a G star). However, Polaris is an old F star and has begun fusing helium in its core (hydrogen is still being fused within Polaris, but this takes place within a shell surrounding the helium-fusing core). Strongly ionized helium in the star’s hot outer layer is opaque to light emitted by the star because helium absorbs much of it. This dims the star’s brightness since less light escapes the star. However, because of this absorption, the star’s outer layer gets hotter and starts expanding. Expansion of the outer layer eventually cools the helium in the outer layer, which then lets more of the star’s light escape. This then allows the star’s outer layer to shrink back to the point where it will get hot and begin expanding again. The process of becoming opaque and expanding followed by becoming transparent and contracting repeats itself over and over again. Since the star’s mass and energy generation is constant, the rate at which the star fluctuates in size and brightness also remains constant. Once astronomers know how long it takes Polaris to go through a cycle of brightening and dimming (this is the star’s period), they can determine the star’s true maximum brightness. By comparing Polaris’ apparent maximum brightness (how bright it looks from Earth) to its true maximum brightness, it is possible to calculate the distance to the star. Stars like Polaris are Cepheid variables, named after the Delta Cephei, the first one that astronomers discovered. Cepheid variables are bright and astronomers can use them to measure the distance to nearby galaxies.  


Polaris is an easy star to find since most people can locate the Big Dipper. The two stars at the end of the Big Dipper’s bowl are called the Pointers. A line drawn up from the Pointers just about runs into Polaris. Since Polaris is the star that marks the end of the Little Dipper’s handle, the rest of the constellation is located to the lower left of Polaris.