北落師門C 恆星 繞北落師門A 一圈要 2000萬年!
距離北落師門A 2.5光年
The orbit of Fomalhaut C around A is estimated to take 20 million years to complete.[16] Due to the age of the three stars being 440 million years, and the distance of 2.5 light-years, this would mean that Fomalhaut C has only completed a full orbit around Fomalhaut A 22 times.[17] The tidal radius of the Fomalhaut system is 6.2 light-years (1.9 parsecs), which makes Fomalhaut C well within it, which further proves the Fomalhaut system as trinary.
https://en.wikipedia.org/wiki/Fomalhaut_Chttp://www.pas.rochester.edu/~emamajek/fomc/