North Star's Unseen Companion Photographed

http://www.space.com/scienceastronom...companion.html

Washington, DC-Light from the North Star, Polaris, has helped humans find
their way for thousands of years. Yet its gravity has guided the movements
of two lesser known companion stars for much longer.

One of its stellar companions is clearly visible with a telescope, but the
other hugs Polaris so tightly that it has never been directly observed
until now. Using the Hubble Space Telescope, astronomers have photographed
this close neighbor for the first time.

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

The finding was announced here today in a press conference at the 207th
meeting of the American Astronomical Society in Washington, DC.

The newly observed companion star is about 2 billion miles from Polaris.
Astronomers have known about it for about 50 years from analysis of light
coming from the triple star system, but it was so dim compared to Polaris
that direct observation was impossible.

"With Hubble, we've pulled the North Star's companion out of the shadows
and into the spotlight," said astronomer Howard Bond from the Space
Telescope Science Institute, which operates Hubble for NASA and the
European Space Agency.

Polaris is the brightest star in the constellation Ursa Minor and located
about 431 light-years away. It is situated almost directly above the
celestial north pole, making it Earth's current northern polestar. It
appears as a fixed point in the night sky around which all the other stars
revolve, and sailors have long used it to orient themselves.

Polaris belongs to a special class of massive, pulsating stars known as
Cepheids, which dim and brighten at regular intervals. Scientists use
Cepheids to measure the distance to faraway galaxies and star clusters and
to calculate the expansion rate of the universe. Knowing a Cepheid's mass
is important to this understanding, but calculating the mass for most stars
is difficult.

Although Polaris is a triple star system, it can be broken down into a
binary system and a single star located farther away. Binary systems are
important because their stars are among the few whose masses can be
accurately determined. But calculating the mass for each star in a binary
setup requires knowledge of their complete orbits. This in turn requires
visual observation of their movements-something that was impossible with
the Polaris binary system until now.

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