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. This star is the current Northern Pole Star since it's positioned only three quarters of a degree from the North Celestial Pole. Polaris is edging closer still and on March 24, 2100 it will be less than half a degree away, before starting to slowly drift away.
Polaris is a multiple system located 434 light-years away that shines with a combined magnitude of +1.97. The dominant main component, α UMi Aa, is usually referred to as simply "Polaris" and is a type F7Ib yellow-white supergiant. It's a classical Cepheid variable with a period of slightly more than 4 days. There is evidence that the brightness variation has changed over recent times. Before 1960, it was about 0.15 magnitudes but a few years later it was down to 0.05 magnitudes. Now it appears to be back on the rise and with modern imaging and measuring techniques, it will be interesting to see how it changes in the coming years. Since one of the nearest Cepheid's, Polaris has been heavily studied by astronomers. The main star is 2,300 more luminous than the Sun and 46 times larger.
The other system components are two 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 the main star and the B star are visible and they form 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 for scopes of 80mm (3.1 inches) aperture or more. A magnification of 75x will split the pair. Spatially, the two stars are a long way apart and have an orbital period of the order of many thousands of years. In 1929, α UMi Ab was spectroscopically determined to be a binary and later imaged by the Hubble telescope. This dwarf star orbits only 18.5 AU (2.8 billion kilometres) from the main star, which is equivalent to about the distance between the Sun and Uranus.
Finding Polaris is easy. An imaginary line extending northwards from the pointer stars Dubhe and Merak of the "Big Dipper" asterism of Ursa Major leads directly to the star. In addition, Polaris is an almost perfect marker for Northern Hemisphere astronomers setting up equatorial mountings. It's a pity there is no equivalent bright southern counterpart.
This system has numerous names. One ancient name for Polaris was Cynosure from the Greek meaning "dogs tail". Also commonly used are the "North Star", the "Steadfast Star" and the "Guiding Star". A time exposure photograph or image of the region centred on Polaris reveals many star trails and since it's not exactly over the pole, Polaris also 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 (years)||70 Million|
|Other designations||HR 424, HD 8890, HIP 11767, North Star|
|Notable feature||Closest bright star to the North Celestial Pole|