Polaris is only the 46th brightest star in the night sky but it's an important one that's been of immense value to navigators. What makes it special is that it's positioned almost exactly at the North Celestial Pole making it the current Northern Pole Star. The star is about three quarters of a degree from the pole and is edging closer; on March 24, 2100 it will be less than half a degree distant but after that will slowly start to move away.
Polaris is a multiple star system located 434 light-years from Earth that shines with a combined apparent magnitude of +1.97. The main component (α UMi Aa) usually referred to as simply Polaris is a type F7Ib yellow-white supergiant that dominates the system. It's a classical Cepheid variable with a period of slightly more than 4 days. There is evidence that the brightness variations have decreased and then increased over the past few years. Before 1960, the change was about 0.15 magnitudes but a few years later it was down to 0.05. Now it appears the range is on the increase and with modern imaging and magnitude measuring techniques, it will be interesting to see how it changes over the coming years. Since one of the nearest Cepheid's - important stars for measuring astronomical distances - Polaris is heavily studied by astronomers. It's 2,300 more luminous than the Sun and 46 times larger than our star.
The other system components are two nearby yellow-white dwarf companions, Polaris B (α UMi B) and α UMi Ab and two more distant dwarfs, α UMi C and α UMi D. Through amateur telescopes only Polaris itself and the B star are visible forming a notable double. The B star shines at magnitude +8.7 with a generous separation of 18 arc seconds from its much brighter companion. Polaris B was discovered by Sir William Herschel in 1780 and is an easy object when viewed through a scope of 80mm (3.1 inches) aperture or more. A magnification of 75x easily splits the pair. In space the two stars are a long way apart with a revolution period amounting to many thousands of years.
In 1929, α UMi Ab was discovered spectrospically and subsequently imaged by the Hubble telescope in 2006. The nearer dwarf star orbits only 18.5 AU (2.8 billion km) from the main star which is equivalent to about the distance between our Sun and Uranus, explaining why its light is swamped by its close and much brighter companion.
Finding Polaris is easy. An imaginary line extending northwards from the Big Dipper pointer stars Dubhe and Merak heads straight to the pole star. In addition to its navigational importance, Polaris is a wonderful marker for Northern Hemisphere astronomers setting up equatorial mountings. It's a pity there is no southern counterpart.
The system is also known by numerous names. One ancient name for Polaris was Cynosure from the Greek meaning "dogs tail". Also commonly used names are the "North Star", the "Steadfast Star" and the "Guiding Star". It's always interesting to take a time exposure photograph or image of the region centered on Polaris with the trails very evident. Since it's not exactly at the pole, even Polaris leaves a short trail.
Polaris Data Table
|Bayer||alpha (α) UMi|
|RA (J2000)||02h 31m 49s|
|DEC (J2000)||+89d 15m 51s|
|Surface Temp (K).||6,015|
|Age (million years)||70|
|Other designations||HR 424, HD 8890, HIP 11767, North Star|
|Notable feature||Closest bright star to the North Celestial Pole|