Alf UMi - Polaris
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How often have you been looking at Polaris, the North Star? Well, at least as many times as you aligned your scope equatorial north, but have you ever mused about this so useful star? How far out there? How big is it?

Polaris, also designated α UMi and 1 UMi, is located in the northernmost constellation of Ursa Minor, the Little Bear, and is the brightest naked eye star nearest to the celestial north pole. Its current position is 42.5 arc minutes displaced from the exact north pole (about 1.5 times the moon's disc).

This distance changes over time as a result of pertubations to Earth's rotation axis, namely the effect of precession and nutation by which Earth's axis spins around drawing out a circle. In several thousands of years, the northern tip of Earth's axis will point at another region in the sky eventually at a new polestar such as Thuban in Draco and Vega in Lyra. The year 2100 will mark Polaris' closest approach to the north pole assuming a minimum distance of less than 30 arc minutes, the diameter of the full moon.

Because of its vicinity to the pole Polaris appears motionless on the sky, while all the stars of the Northern sky appear to rotate around Polaris. For sailors in the northern hemisphere, it has therefore always been a reliable beacon for celestial navigation.

Polaris is not a single star, it is a triple star system composed of Polaris Aa and two dwarf stars Polaris Ab in a close 18.5AU distance and yellowish Polaris B much further out in 2400AUs. The system is approximately 430 light-years away and shines at 2 visual magnitudes, bright enough to serve for polar alignment of telescopes. Unfortunately, our friends in the southern hemisphere are not blessed with a bright star in the vicinity of the celestial south pole.

The extreme proximity of Ab required the Hubble Space Telescope to resolve the system. The B component has been first discovered by William Herschel in 1870. Credit: NASA, ESA, N. Evans (Harvard-Smithsonian CfA), and H. Bond (STScI)

Compared with our sun Polaris is a giant star, 2400 times intrinsically luminous and probably 50 times larger. The sun as seen from the distance of Polaris would be 10.4 maginitudes faint, hence invisible to the unaided eye. If Polaris would be placed at a distance of 32.6 light-years (the distance for measuring absolute brightnesses of stars) it would outshine all bright stars and solar system planets except Venus. The light spectrum emitted by Polaris is similar to the sun, as is the surface temperature. However, with its much wider surface area, Polaris is the stronger heater.

As for all "fixstars" Polaris, too is in constant space motion relative to us. Its radial (line of sight) velocity is -16.4 km/sec, while negative values are assigned to stars which are approaching us. Its true velocity in three-dimensional space is 33 km/sec, in other words, no cosmic collision with Earth sometime in eternity.

Polaris is a so-called Cepheid variable star and the closest of its kind. Professional astronomers are striving to determine the precise mass of Polaris. Known variations in star brightness in conjunction with a precisely measured mass allow calulation of the distances of galaxies and the expansion rate of the universe, as well as determination of a probably more accurate distance of Polaris itself and accurate orbits models of its companion stars.

The importance of Polaris is manifold:
• Helps align telescopes
• Navigation aid from ancient to modern times
• Distance measuring in the universe

After all, please do not forget about completing your scope alignment whilst musing about the pole star.