Astronomers have discovered that the North Star has a second smaller star next to it. Researchers at the Space Telescope Science Institute in Baltimore found the star, which they called Polaris Ab, by using the orbiting Hubble Space Telescope.

"With Hubble, we've pulled the North Star's companion out of the shadows and into the spotlight," said Howard Bond, a member of the research team.

The scientists described Polaris Ab and how they detected it on Monday at the 207th meeting of the American Astronomical Society in Washington, D.C.

While the North Star, also known as Polaris, is a super-giant more than 2,000 times brighter than the sun, its newly discovered second close companion is a dwarf star 3.2 billion kilometres away from it.

The first known companion can be seen with a small telescope. The three-star system is 430 light years away from Earth. A light-year is about 9.5 trillion kilometres, the distance light travels in a year.

By observing the movements of the new companion star, researchers hope to determine the mass of the North Star more accurately.

Knowing the mass is important for astronomers and cosmologists, who rely on measuring differences in the brightness of the North Star to determine the distance of galaxies and the expansion rate of the universe.