Hubble spots North Star companion.

Old drawings portray the North Star, Polaris, as a solitary beacon of light. But the star, which generations of seafarers have relied on for navigation, has two stellar companions, as indicated by Polaris' motion. One of the stars has been visible to astronomers for centuries, but the other, smaller, fainter star that tightly orbits Polaris has now been photographed for the first time by the Hubble Space Telescope.

Small telescopes can easily view the more distant partner, which English astronomer William Herschel discovered in 1780. The newfound body, dubbed Polaris Ab, lies about 6 billion kilometers from Polaris and takes about 30 years to orbit it, report Nancy Evans of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and her collaborators. The triple-star system is 430 light-years from Earth.

Evans' team plans to track the orbits of both partner stars to more accurately measure the mass of Polaris. Pinning down that number is key to understanding the star's evolution. Polaris is the nearest known Cepheid variable, a type of star used to measure the distances to other galaxies and the rate of expansion of the universe.--R.C.