This Month's Star is Mizar

This month look for the star Mizar and its companion star Alcor. Mizar is the star located in the bend of the handle of the Big Dipper. Look carefully at this star and you’ll probably detect its faint companion star, Alcor. Use your binoculars if you’re not certain that you can see this fainter star. The position of Alcor, relative the Mizar places it at an angle pointing away from the bend in the Dipper’s handle. I get the impression that the distance between the two stars is the same as 1/8th of an inch appears when viewed at arm’s distance. It may be hard to believe, but Mizar and Alcor have an angular separation close to half the diameter of the moon. When you look at Mizar with even slight optical aid, you’ll notice something else; it’s a double star (so there are three stars visible in a telescope).

Mizar was the first double star astronomers discovered (in 1650). Mizar is over twice as massive as our sun and Alcor is over 50% heavier than our sun. The pair is between 78 and 81 light years from the earth (the distance is uncertain). The actual distance between Mizar and Alcor may be anywhere between 1/4 to over 3 light years. So it’s possible that these two stars are not actually in orbit around each other. However, if they are in orbit around each other, their orbital period may be in the hundreds of thousands of years!