Glossary term: Polaris (alpha Ursae Minoris North Star Pole Star)

Description: Polaris is the nearest bright star (within one degree) to the northern celestial pole. It is commonly known as the North Star. Polaris is a triple star system, composed of the primary, a yellow supergiant designated Polaris Aa, in orbit with a smaller companion, Polaris Ab, a very close main-sequence star; the pair has a wide companion, Polaris B, a main-sequence star orbiting at a distance of 2,400 astronomical units (the orbital period is about 29.3 years). Polaris B can be resolved with a modest telescope. The Hubble Space Telescope was able to resolve all three members of the Polaris ternary system. The apparent visual magnitude of Polaris fluctuates because Polaris Aa is a Cepheid variable. The Polaris system is about 447 light years from Earth.

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