This was originally thought to be due to secular redward (lower temperature) evolution across the Cepheid instability strip, but it may be due to interference between the primary and the first- overtone pulsation modes. It has steadily increased by around 4.5 seconds per year except for a hiatus in 1963–1965. The period, roughly 4 days, has also changed over time. Polaris and its surrounding integrated flux nebula. It has been reported that the amplitude is now increasing again, a reversal not seen in any other Cepheid. After 1966, it very rapidly decreased until it was less than 0.05 magnitude since then, it has erratically varied near that range. Prior to 1963, the amplitude was over 0.1 magnitude and was very gradually decreasing. The range of brightness of Polaris is given as 1.86–2.13, but the amplitude has changed since discovery. The variability of Polaris had been suspected since 1852 this variation was confirmed by Ejnar Hertzsprung in 1911. Cepheids constitute an important standard candle for determining distance, so Polaris, as the closest such star, is heavily studied. 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. There were once thought to be two more widely separated components-Polaris C and Polaris D-but these have been shown not to be physically associated with the Polaris system.
Anderson gave a period of 29.32 ☐.11 years with an eccentricity of 0.620 ☐.008. Kamper in 1996 produced refined elements with a period of 29.59 ☐.02 years and an eccentricity of 0.608 ☐.005. Roemer used radial velocity data to derive an orbital period of 30.46 y for the Polaris A system, with an eccentricity of 0.64. Īs part of her doctoral thesis, in 1955 E. This period was confirmed by proper motion studies performed by B. Moore published preliminary orbital elements of the system in 1929, giving an orbital period of about 29.7 years with an eccentricity of 0.63. Moore in 1927 demonstrated that the changes in velocity along the line of sight was due to a combination of the four-day pulsation period combined with a much longer orbital period and a large eccentricity of around 0.6. Since Polaris A is a known cepheid variable, J. Campbell in 1899, which suggested this star is a binary system. The variable radial velocity of Polaris A was reported by W. In January 2006, NASA released images, from the Hubble telescope, that showed the three members of the Polaris ternary system. William Herschel discovered the star in August 1779 using a reflecting telescope of his own, one of the best telescopes of the time. Polaris B can be resolved with a modest telescope. The two smaller companions are Polaris B, a 1.39 M ☉ F3 main-sequence star orbiting at a distance of 2,400 astronomical units (AU), and Polaris Ab (or P), a very close F6 main-sequence star with a mass of 1.26 M ☉. It is the first classical Cepheid to have a mass determined from its orbit. Polaris Aa is an evolved yellow supergiant of spectral type F7Ib with 5.4 solar masses ( M ☉). Polaris components as seen by the Hubble Space Telescope The outer pair AB were discovered in August 1779 by William Herschel. Although appearing to the naked eye as a single point of light, Polaris is a triple star system, composed of the primary, a yellow supergiant designated Polaris Aa, in orbit with a smaller companion, Polaris Ab the pair is in a wider orbit with Polaris B. The revised Hipparcos parallax gives a distance to Polaris of about 433 light-years (133 parsecs), while calculations by some other methods derive distances up to 35% closer. Historically, the stable position of the star in the northern sky has made it useful for navigation. The position of the star lies less than a degree away from the north celestial pole, making it the current northern pole star. With an apparent visual magnitude that fluctuates around 1.98, it is the brightest star in the constellation and is readily visible to the naked eye at night. It is designated α Ursae Minoris ( Latinized to Alpha Ursae Minoris) and is commonly called the North Star or Pole Star. Polaris is a star in the northern circumpolar constellation of Ursa Minor. Polaris (α Ursae Minoris, circled in red) in the constellation Ursa Minor (white area)