You know the old gag where the woman is in labor, and then gives birth, and asks the doctor “What is it, a boy or a girl?” and the doctor says “Both! You have twins!”

Well, astronomers have been playing this gag for some time now with a particular star system and the latest surprise just happened.

It has long been known that one of the stars of Ursa Major is a binary system, with two stars very close to each other. This was apparently discovered in ancient times. They are known as Alcor and Mizar, and are thus the longest-identified binaries.

With telescopes, Mizar was then discovered to be a pair of binaries, and thus constitutes a four-star system, and many consider Alcor to not be part of this system (while others have).

The latest development is that Alcor is a binary system … two stars … and it’s gravitational linkage with Mizar is confirmed. So now there are six stars all interacting in a great orgy of gravitational interaction. Considering that Alcor is one of the most studied stars in the sky, this revelation is somewhat surprising.

“Finding that Alcor had a stellar companion was a bit of serendipity,” says Eric Mamajek, assistant professor of physics and astronomy at the University of Rochester, and leader of the team that found the star. “We were trying a new method of planet hunting and instead of finding a planet orbiting Alcor, we found a star.”

Mamajek says that a separate group of scientists, led by Ben Oppenheimer of the American Natural History Museum, has also just found that the Alcor companion is physically associated with the star.

…

“It’s pretty exciting to have found a companion to this particular star,” says Mamajek. “Alcor and Mizar weren’t just the first known binaries — the four stars that were once thought to be the single Mizar were discovered in lots of ‘firsts’ throughout history.”

There are many more interesting details here.