Using the Hubble space telescope, U.S. astronomers have for the first time spotted a little, dim companion to Polaris, which is better known as the North Star.

The newly observed companion star is about 3.2 billion km from Polaris, astronomers reported on Monday. They have known its existence for about 50 years from analysis of light coming from the system, but it was so dim compared to Polaris that direct observation was impossible.

But Hubble's pointing capabilities, combined with the performance of its instruments, now allow scientists to see the universe in finer detail than ever before, said Michael Moore, NASA 's Hubble program executive.

"It is that clear vision that makes these types of images possible," he said.

Light from the North Star has helped humans find their way for thousands of years, but there is more to this star than meets the eye, astronomers said.

The North Star is actually a triple star system. While one companion is easily viewed with small telescopes, the other hugs Polaris so tightly that it has never been seen for half a century.

"The star we observed is so close to Polaris that we needed every available bit of Hubble's resolution to see it," said astronomer Nancy Evans of the Harvard-Smithsonian Center for Astrophysics.

The companion proved to be less than two-tenths of an arcsecond away from Polaris. That is an incredibly tiny angle equivalent to the apparent diameter of a quarter located 30 km away.

The brightness difference between the two stars made it even more difficult to resolve them, according to the astronomers.

Polaris is a super-giant more than two thousand times brighter than the sun, while its companion is a dwarf star. But with the Hubble telescope, astronomers have pulled the North Star's companion out of the shadows.

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

Astronomers want to determine the mass of Polaris, because it is the nearest Cepheid variable star. Cepheids' brightness variations are used to measure the distances of galaxies and the expansion rate of the universe.

It is essential to understand their intrinsic physics makeup and evolution. Knowing their mass is the most important ingredient in this understanding.

Presenting their data during the 207th meeting of the American Astronomical Society, the researchers said they plan to continue observing the Polaris system for several years to detect the movement of the small companion.

Source: Xinhua