Polaris A (Pole Star) with its two stellar companions, Polaris Ab and Polaris B. Artists impression. Credit: NASA
Polaris, also known as the Northern Star, Pole Star, and the Lodestar, is the brightest star in the Ursa Minor constellation. It is called the pole star because it is very close to the celestial north pole. The star is about 430 light years from Earth. It is a multiple star. Multiple stars contain more than two stars in a close system.
The main star, Umi A, is an F7 bright giant or, possibly, a supergiant that is six times the mass of our Sun. UMi B is a F3V main sequence star with 1.5 solar mass that orbits about 2,400 astronomical units(AU) from the primary. UMi Ab is a dwarf that is orbits a mere 18.5 AU away. There are two more components, UMi C and UMi D, that we have very little information about.
While Polaris B can be seen with a modest telescope, it took images from the Hubble Space Telescope, in 2006, to clearly show all three major components in the ternary system. The main reason for this is that the very brightness of the primary and the close orbit of the tertiary make it difficult to see UMi Ab except with spectroscopic examination.
Polaris is a classic Population I Cepheid variable. Cepheids are considered to be an important standard candle(objects with known luminosity) for determining distance, Polaris is the closest star in this group and is heavily studied. Some research has shown that Polaris is 2.5 times brighter today than when observed in ancient times, changing from third to second magnitude. If this is true, this rate of change is 100 times greater than current theories of stellar evolution.
Polaris is a Population I Cepheid variable, what that means is that it contains stars that are high in heavy metals, very young, and are pulsating variable stars. These are considered to be the newest stars in the universe. Our Sun would fall into the Population I class because of its metal content and age.
Polaris is one of the stars that Ptolemy gazed upon and is often used to gauge the distance to other stars. As its light continues to brighten, we will continue to study it.
We have written many articles about Polaris for Universe Today. Here’s an article about famous stars, and here’s an article about the North Star.
If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.
We’ve done many episodes of Astronomy Cast about stars. Listen here, Episode 12: Where Do Baby Stars Come From?
Comments on this entry are closed.