Polaris, designated Ursae Minoris, commonly the North Star or Pole Star, is the brighest star in the constellation of Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star. The revised Hipparcos parallax gives a distance to Polaris of about 433 light-years (133 parsecs), while calculations by other methods derive distances around 30% closer.
Polaris is a triple star system, composed of the primary star, Polaris Aa (a yellow supergiant), in orbit with a smaller companion (Polaris Ab); the pair in orbit with Polaris B (discovered in August 1779 by William Herschel). There were once thought to be two more distant component, Polaris C and Polaris D, but these have been shown not to be physically associated with the Polaris system.
Polaris Aa is a 5.4 solar mass F7 yellow supergiant of spectral type IB. It is the first classical Cepheid to have a mass determined from its orbit. The two smaller companions are Polaris B, a 1.39 F3 main-sequence star orbiting at a distance of 2400 astronomical units (AU), and Polaris Ab (or P), a very close F6 main-sequence star with an 18.8 AU radius orbit and 1.26.
Polaris B can be seen even with a modest telescope. William Herschel discovered the star in August 1779 using a reflecting telescope of his own, one of the best telescopes of the time. By examining the spectrum of Polaris A, it was also discovered in 1929 that it was a very close binary, with the secondary being a dwarf, which had been theorized in earlier observations. In January 2006, NASA, released images, from the Hubble telescope, that showed the three members of the Polaris ternary system.
Ab, the nearby dwarf star, is in an orbit of 18.5 AU (2.8 billion km) from Polaris Aa, about the distance between the Sun and Uranus, which explains why its light is swamped by its close and much brigher companion. Polaris Aa, the supergiant primary component, is a low-amplitude Population I classical Cepheid variable, although it was once thought to be a type II Cepheid due to its high galactic latitude. Cepheids constitute an important standard candle for determining distance, so Polaris, as the closest such star, is heavily studied.
