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.


  1. #1 Jared
    December 12, 2009

    Where, exactly, is “here” for there is no link.

  2. #2 gruebait
    December 12, 2009

    There is a USAToday article at

    A PDF of the paper by Eric E. Mamajek at the University of Rochester
    is here:

  3. #3 Mike Haubrich, FCD
    December 12, 2009

    “What is it, a boy or a girl?” and the doctor says “Both! You have twins!”

    It’s not always a gag, though. I am a fraternal twin. Dad tells us that as Mom was waking up from anesthesia when my sister and I were born, he had to tell her times before she had comprehensions.

  4. #4 Eamon
    December 13, 2009

    Hmmm… reminds me of an old Asimov story.