Polaris (α UMi / α Ursae Minoris / Alpha Ursae Minoris, commonly North(ern) Star or Pole Star, and sometimes Lodestar) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole (42′ away as of 2006), making it the current northern pole star.

Polaris is about 430 light-years from Earth. Concerning the detailed physics, α UMi A is an F7 bright giant (II) or supergiant (Ib). The two smaller companions are: α UMi B, an F3V main sequence star orbiting at a distance of 2400 AU, and α UMi Ab, a very close dwarf with an 18.5 AU radius orbit. Recent observations show that Polaris may be part of a loose open cluster of type A and F stars.

Polaris B can be seen with even a modest telescope and was first noticed by William Herschel in 1780. In 1929, it was discovered by examining the spectrum of Polaris A that it had another very close dwarf companion (variously α UMi P, α UMi a or α UMi Ab), which had been theorized in earlier observations (Moore, J.H and Kholodovsky, E. A.). In January 2006, NASA released images from the Hubble telescope, directly showing all three members of the Polaris trinary system. The nearer dwarf star is in an orbit of only 18.5 AU (2.8 billion km; about the distance from our Sun to Uranus) from Polaris A, explaining why its light is swamped by its close and much brighter companion.

Polaris is a classic Population I Cepheid variable (although it was once thought to be Population II due to its high galactic latitude). Since Cepheids are an important standard candle for determining distance, Polaris (as the closest such star) is heavily studied. Around 1900, the star luminosity varied ±8% from its average (0.15 magnitudes in total) with a 3.97 day period; however, the amplitude of its variation has been quickly declining since the middle of the 20th century. The variation reached a minimum of 1% in the mid 1990s and has remained at a low level. Over the same period, the star has brightened by 15% (on average), and the period has lengthened by about 8 seconds each year.

Recent research reported in Science suggests that Polaris is 2.5 times brighter today than when Ptolemy observed it (now 2mag, antiquity 3mag). Astronomer Edward Guinan considers this to be a remarkable rate of change and is on record as saying that "If they are real, these changes are 100 times larger than 1 predicted by current theories of stellar evolution."

Pole Star

Because α UMi lies nearly in a direct line with the axis of the Earth's rotation "above" the North Pole — the north celestial pole — Polaris stands almost motionless on the sky, and all the stars of the Northern sky appear to rotate around it. Therefore, it makes an excellent fixed point from which to draw measurements for celestial navigation and for astrometry. The antiquity of its use is attested by the fact that it is found represented on the earliest known Assyrian tablets. In more recent history it was referenced in Nathaniel Bowditch's 1802 book, The American Practical Navigator, where it is listed as one of the navigational stars. At present, Polaris is 0.7° away from the pole of rotation (1.4 times the Moon disc) and hence revolves around the pole in a small circle 1½° in diameter. Only twice during every sidereal day does Polaris accurately define the true north azimuth; the rest of the time it is only an approximation and must be corrected using tables or a rough rule of thumb.