Giant’s new Companion: A Dwarf

Astronomers in U.S. have noticed for the first time, a dim star which seems to be a companion to the bright North Star. The North star, also known as Polaris, being a super-giant is two thousand times brighter than the sun while it’s companion being a dwarf star is hence not very bright. The observation was made through a powerful Hubble space telescope.

According to the astronomers, who have been aware of its existence from the analysis of light for about 50 years now, could not directly observe it as it was extremely dim as compared to Polaris. The newly observed companion star is known to be about 3.2 billion km from Polaris. It is estimated to be less than two-tenths of an arcsecond away from Polaris. This is an incredibly tiny angle equivalent to the apparent diameter of a quarter located 30 km away.

Being more than just a bright star used as a guide by sailors to determine the direction, the North Star i.e. Polaris, is actually a triple star system. While one companion, Polaris B, is easily viewed with small telescopes and has been known since 1780, the other, which is known as Polaris Ab, is so close to Polaris that it is had never been seen for half a century. Infact, it’s existence had only been inferred and that too because of the gravity of the little body that made the big star wobble slightly as they moved close to each other.

While in the images released by the telescope, it may simply appear to be a little blurry blob besides the big star, it has helped scientists figure out the North Star’s mass and gauge the distance between it and the North Star. By watching its motion, they have learned about the big star’s mass which they believe to be as much as four times the mass of the sun!

According to Nancy Evans, Harvard-Smithsonian Center for Astrophysics, "The star we observed is so close to Polaris that we needed every available bit of Hubble’s resolution to see it,"
By watching the motion of the companion star, astronomers expect to learn not only the stars’ orbits but also their masses.

Looking at the next set of goals to be achieved, Evans added, "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. Our ultimate goal is to get the accurate mass for Polaris, to do that, the next milestone is to measure the motion of the companion in its orbit."

What makes this ordeal of determing the exact mass of the star extremely significant is the fact that it is the nearest Cepheid variable star which is a rare cosmic object and, the Cepheids’ brightness variations are used to measure the distances of galaxies and the expansion rate of the universe.

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.