Smithsonian.com

The International Space Station passes under the Big Dipper (courtesy of flickr user Otto Phokus)

If you can only spot one constellation, it’s probably the Big Dipper. Other than being easily recognizable, the Big Dipper is special because it contains one of the first known binary star systems. The star in the crook of the handle was found to actually be two stars around 1617 by Benedetto Castelli, who then asked his teacher Galileo to take a look. The stars, named Mizar and Alcor, became known as the “Horse and Rider” and could be seen with the unaided eye.

Over time, more powerful telescopes revealed that the binary system was more complex than originally thought. Mizar is not one star but four—two pairs of binary stars that orbit around each other, around which Alcor orbits farther away.

New research published in The Astrophysical Journal, however, shows that the Mizar-Alcor system is even more complicated: Alcor has its own companion, now named Alcor B, which is likely a red dwarf about a quarter the size of our own Sun. Alcor and Alcor B, which are both about 80 lightyears away, orbit each other every 90 years or so.

The USAToday reports:

[T]he study team uncovered Alcor B using a technique pioneered by Galileo called “common parallactic motion,” which examines stars months apart in time, relying on the Earth’s motion around the sun to reveal how much the star’s apparent motion has changed. By blocking out the light from Alcor with a device called a coronagraph, the study team confirmed that, unknown for centuries, Alcor B indeed orbits the larger star.

Galileo himself attempted to study Alcor this way, [American Museum of Natural History astrophysicist Ben] Oppenheimer says, but telescopes in his day were too limited to resolve the two stars. (Galileo studied double stars this way to confirm the Earth orbited the sun.) The study team relied on the 200-inch-wide Hale Telescope at the Palomar Observatory in Palomar Mountain, Calif., to make the discovery.