Mizar and Alcor

If you look at the Big Dipper, have a look at the center star of the “handle,” called Mizar. If you have good eyesight or wear corrective lenses you should be able to make out a star very close to it, called Alcor. In ancient times and even through the medieval period the ability to distinguish these two stars was a common vision test. These two stars are loosely gravitationally bound and form a double star.

However, in the early 1600s Benedetto Castelli made the first suggestive observation that Mizar itself was a double star, and asked his friend Galileo to observe it. Galileo confirmed it was a binary system in 1617, now consisting of Mizar A and B. We call such a system a visual binary since it is readily apparently through simple telescopic observation. The story doesn’t stop here, however…

In 1889, Edward Pickering discovered that Mizar A was itself a binary system, specifically what we call a spectroscopic binaries since the stars could not be individually resolved and their existence and motion is inferred from spectroscopic data (the spectral lines of each star are red-shifted and blue-shifted and then vice versa, respectively). In 1908, Mizar B was also found to be a spectroscopic binary, meaning the Mizar system was actually a quadruple star system, and then there was Alcor orbiting about it.

Fast forward a century, and in 2009 Alcor was found to be a binary system as well, consisting of Alcor A and B. That Alcor and Mizar are gravitationally bound was confirmed in 2007. Therefore, the entire set of six stars (two from Alcor and four from Mizar) form a sextuple star system.

Oh, and it’s not even the closest sextuple star system. That “honor” belongs to Castor, which went through the same historical dance of binary discovery as Mizar-Alcor.

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