Polaris Polaris (α UMi, α Ursae Minoris, Alpha Ursae Minoris, commonly North Star, Northern Star or Pole Star, also Lodestar, sometimes Guiding star) is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole, making it the current northern pole star.

It is a multiple star, consisting of the main star α UMi Aa, two smaller companions, α UMi B and α UMi Ab, and two distant components α UMi C and α UMi D. α UMi B was discovered in 1780 by William Herschel.

Many recent papers calculate the distance to Polaris at about 434 light-years (133 parsecs). Some suggest it may be 30% closer which, if correct, is especially notable because Polaris is the closest Cepheid variable to Earth so its physical parameters are of critical importance to the whole astronomical distance scale.

α UMi Aa is a 4.5 solar mass F7 supergiant (Ib). This is the first classical Cepheid to have a dynamical mass determined from its orbit. The two smaller companions are: α UMi B, a 1.39 solar mass F3 main sequence star orbiting at a distance of 2400 AU, and α UMi Ab (or P), a very close F6 main sequence star with an 18.8 AU radius orbit and 1.26 solar masses. There are also two distant components α UMi C and α UMi D.

Polaris B can be seen even with a modest telescope. It was found by William Herschel in 1780 using one of the most powerful telescopes at the time: a reflecting telescope that he had made. In 1929, it was discovered by examining the spectrum of Polaris A that it was a very close binary with the secondary being a dwarf (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 ternary 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 A, the supergiant primary component, 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. The variability of Polaris had been suspected since 1852; this variation was confirmed by Ejnar Hertzsprung in 1911.

Both the amplitude and period of the variations have changed since discovery. Prior to 1963 the amplitude was over 0.1 magnitude and decreasing very gradually. After 1966 it decreased very rapidly until it was less than 0.05 magnitude and since then has varied erratically near that range. It has been reported that the period is now increasing. The period increased fairly steadily by around 4 seconds per year until 1963. It then stayed constant for 3 years, but began to increase again from 1966 onwards. Current measurements show a consistent increase of 3.2 seconds per year in the period. This was originally thought to be due to secular red-ward evolution across the instability strip, but is now considered to be interference between the primary and first overtone pulsation modes. Comparison of the period luminosity relationship and the observed luminosity indicate that the main pulsations are the first overtone.

Polaris in Space Opera

  • Frontier: Elite II? (1993) and Frontier: First Encounters? (1995), computer games written by David Braben et al. Polaris is a distant (434 light-years from Earth) uninhabited system comprising many planets. In First Encounters, the player may visit Polaris in order to complete the game's major plotline.
  • FreeSpace 2? (1999), combat simulation computer game designed by Dave Baranec et al, and published by Volition, Inc.. Polaris is the capital system of the Neo-Terran Front? during their rebellion against the Galactic Terran Vasudan Alliance?.
  • Escape Velocity, computer game developed and published by Ambrosia Software. System with planet Northstar.
  • Escape Velocity Nova? (2002), computer game developed and published by Ambrosia Software. Polaris is the name of a game faction that settled planets in the Polaris system.

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