Star System
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.
Read more about this topic: Polaris
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