The North Star, also called Polaris, is a familiar sight to stargazers. For centuries, it has served as a celestial guidepost for sailors at sea and other nighttime wayfarers. Now, astronomers have discovered that there is more to Polaris than meets the eye. The familiar nighttime object is actually three stars in one: one bright star and two faint stars.

The brightest star in the system is Polaris, which is visible to the unaided eye. In 1780, English astronomer William Herschel discovered a second, fainter star very close to Polaris. It can be seen through small telescopes.

Recently, astronomers deduced that yet another star lurked nearby, because the pull of its gravity made Polaris wobble slightly back and forth along our line of sight—a wobble detectable with telescopic instruments. However, this third star was too faint and too close to Polaris to be visible...that is, until the Hubble Space Telescope trained its lens on the North Star.

“With Hubble, we’ve pulled the North Star’s companion out of the shadows and into the spotlight,” Astronomer Nancy Evans of the Smithsonian Astrophysical Observatory explains. “The star we observed is so close to Polaris that we had to stretch Hubble to the limit of its powers.”

 The newfound star lies some 2 billion miles from Polaris. Although that gap is about the size of our solar system, the two stars are so far away from Earth that this distance seems small, and the stars appear incredibly close together. Polaris and its newfound companion orbit each other once every 30 years. To learn more about this triple-star system, Evans and her colleagues have asked for more observing time with Hubble.

In particular, they plan to sharpen their measurement of the North Star’s mass. Pinning down that number is key to understanding how the star will change over its lifetime. Astronomers want to achieve a full understanding of Polaris because it is the nearest Cepheid variable star—a type of star used to measure the distances to other galaxies and the rate of expansion of the universe.

“This discovery goes to show that even a familiar friend like the North Star can still hold surprises,” Evans says. “Thanks to Hubble, we’ve added a new page to the photographic album of Polaris.”