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Concept
Polaris
Summary
Polaris is a star in the northern circumpolar constellation of Ursa Minor. It is designated α Ursae Minoris (Latinized to Alpha Ursae Minoris) and is commonly called the North Star or Pole Star. With an apparent magnitude that fluctuates around 1.98, it is the brightest star in the constellation and is readily visible to the naked eye at night. The position of the star lies less than 1° away from the north celestial pole, making it the current northern pole star. The stable position of the star in the Northern Sky makes it useful for navigation.
As the closest Cepheid variable its distance is used as part of the cosmic distance ladder. The revised Hipparcos stellar parallax gives a distance to Polaris of about , while the successor mission Gaia gives a distance of about . Calculations by other methods vary widely.
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 outer pair AB were discovered in August 1779 by William Herschel, where the 'A' refers to what is now known to be the Aa/Ab pair.
Polaris Aa is an evolved yellow supergiant of spectral type F7Ib with 5.4 solar masses (). It is the first classical Cepheid to have a mass determined from its orbit. The two smaller companions are Polaris B, a F3 main-sequence star orbiting at a distance of 2400astronomical units (AU), and Polaris Ab (or P), a very close F6 main-sequence star with a mass of . Polaris B can be resolved 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. In January 2006, NASA released images, from the Hubble telescope, that showed the three members of the Polaris ternary system.
The variable radial velocity of Polaris A was reported by W. W. Campbell in 1899, which suggested this star is a binary system. Since Polaris A is a known cepheid variable, J.
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