January 9, 2006

Hubble spies North Star’s dim little companion

WASHINGTON (Reuters) - The Hubble Space Telescope has captured the first image of a cosmic object that has been known but never before seen: a small, faint companion of the North Star.

While the image released on Monday may not seem spectacular -- it's a little blurry blob beside the big star -- it helped scientists figure out the North Star's mass.

"This is the final piece of the jigsaw puzzle which will let us get the most prized piece of information about a star, that is, its mass," said Nancy Evans of the Harvard-Smithsonian Center for Astrophysics.

The North Star, also known as Polaris, has been used as a navigation beacon throughout human history because it appears in the sky close the celestial pole.

The image of the companion star known as Polaris Ab allowed scientists to gauge the distance between it and the North Star, and by watching its motion, they learned the big star's mass. It turns out to have about four times the mass of our sun.

Polaris is a rare type of cosmic object known as a Cepheid variable star. These kinds of stars are used by astronomers to measure the distance to various galaxies and to chart the expansion rate of the universe, so it is essential to know about their physics and evolution, including their mass.

Astronomers knew Polaris had a small companion because the gravity of the little body made the big star wobble just a bit as they waltzed around each other.

Images are available online at http://hubblesite.org/news/2006/02.