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Hubble spies North Star's little brother

Actually a triple-star system

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The Hubble Space telescope has finally snapped a third member of the Polaris star group previously known only by its gravitational pull on its companion, thereby visibly demonstrating that the "North Star" is indeed a trio of stars.

The group, made up of Polaris A, B and (new family member) Ab lies around 430 light years from Earth, more-or-less at our celestial north pole, as the left-hand frame here shows:

Polaris A, B and Ab

Top right is Polaris A, and distant companion Polaris B - the latter first spotted by William Herschel in 1780. The two are separated by 240bn miles, and Polaris B can been seen through small telescopes.

Not so Polaris Ab (bottom right), which is just two billion miles from Polaris A. As Smithsonian astronomer Nancy Evans put it: "The star we observed is so close to Polaris that we needed every available bit of Hubble's resolution to see it."

Howard Bond of the Space Telescope Science Institute further explained that while Polaris is a "supergiant more than two thousand times brighter than the Sun", Polaris Ab is a main sequence star. Accordingly, the brightness difference between the two "made it even more difficult to resolve them". Bond noted: "With Hubble, we've pulled the North Star's companion out of the shadows and into the spotlight."

Rather nicely, the blurb illustrates it thus: "The companion proved to be less than two-tenths of an arcsecond from Polaris — an incredibly tiny angle equivalent to the apparent diameter of a quarter located 19 miles away."

The practical applications of the discovery relate to ultimately calculating the mass of Polaris A, considered vital because it is the nearest "Cepheid variable star" the brightness variations of which are used to "measure the distances of galaxies and the expansion rate of the universe".

Evans concluded: "Our ultimate goal is the get an accurate mass for Polaris. To do that, the next milestone is to measure the motion of the companion in its orbit." ®

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