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Deepsky Top-100 (7): Mizar and Alcor

There is a couple of stars that I observed 25 years ago for the very first time, but still is one of my favorites: the naked eye pair Mizar and Alcor in the big dipper, a part of the constellation Ursa Major. At the beginning of spring Ursa Major culminates around midnight, a good time to observe Mizar and Alcor. This naked eye double is very easy to locate. As you can see on the map below, Mizar is the centre star in the handle of the Big Dipper. Alcor lies only 12' to the east-northeast of Mizar.


Finder chart for Alcor and Mizar. Limiting magnitude 6.5.
Credit and copyright SkyTools 2 by Capellasoft

Click here to download a printable finder chart (PDF)

Mizar, or Zeta (ζ) Ursae Majoris, is probably the best-known star in the constellation of Ursa Major. According to James Kaler (The hundred greatest stars) "Mizar" is derived from the Arabic word meaning the "Groin" of the bear. Alcor (80 Ursae Majoris) comes from the Arabic word for "bull". The pair of stars is also called "the horse (Mizar) and rider (Alcor)".

Mizar and Alcor are the only double star with each member having its own proper name. They are both a member of the Ursa Major Moving Cluster (Collinder 285), but it is unknown if they are a couple that is gravitationally bound.

Mizar shines at magnitude 2.3 while Alcor is of the 4th magnitude. As I said earlier, they can be seen with the naked eye, though you should have a limiting magnitude of 4 (naked eye). Alcor lies about 12' to the east-northeast of Mizar in a position angle 72degrees. Between Mizar and Alcor lies an 8th magnitude star (SAO 28748), forming a triangle with the pair. Mizar itself is a true double star. The 4th magnitude B component lies only 14" to the south-southeast of the A component, at a position angle of 150 degrees. Alcor and both Mizar A and B are spectroscopic binaries, so in fact the whole system consists of 6 stars.


Eyepiece impression of Mizar and Alcor. Fov 46'. Limiting magnitude 12.5. North is up, east is to the left.
Credit and copyright SkyTools 2 by Capellasoft

On the image above you cannot see the B component of Mizar clearly split. Through your telescope you will be able to see it.

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