ζ Ursae Majoris

Zeta Ursae Majoris is called “Mizar”—girdle or waistband.

Zeta is the second star on the handle and is one of the most famous binary systems in the heavens: binoculars.

Disregarding for a moment the famous duo Mizar and Alcor, there is another reason for Mizar's fame.
      Zeta Ursae Majoris has gone down in history as the first star discovered to have a gravitationally-bound neighbour. The date of this historical discovery is usually given as 1650, by the Italian astronomer Giovanni Battista Riccioli, who single-handedly discovered the fourth magnitude neighbour.

However several decades earlier there was a similar discovery. According to the anonymous writer of the article ‘Mizar and Alcor’in Wikipedia: “Mizar was the first telescopic binary discovered—most probably by Benedetto Castelli who in 1617 asked Galileo Galilei to observe it. Galileo then produced a detailed record of the double star. Later, around 1650, Riccioli wrote of Mizar appearing as a double.”

Does Riccioli's report concern the same double as Castelli's?

The Benedictine monk Benedetto (he changed his name from Antonio when he entered the monastery) Castelli was a brilliant mathematician and hydraulics engineer, and his early support of Galileo's work was crucial to its success.

In any case, Mizar came to be known as the first binary star discovered (Struve 1744):
       AB: 2.2, 3.9; 153º, 14.5".

Then there is the binary formed with Alcor (80 UMa), long assumed optical but now thought quite possibly to be a true binary; a second companion has been added to the family:
      AC: 2.2, 4.0; 70º, 706.1" (11.8')
      AD: 2.2, 7.6; 100º, 492.5" (8.2')

In the same field of view are two other binaries, not to be neglected:
       78 UMa (BU 1082) is a little over 1º east-northeast of epsilon: 5.0, 7.9; 108º, 1.0".
       Struve 1695AB, 2º south of epsilon, a yellow primary and blue companion: 6.0, 7.8; 281º, 3.9".


From Mizar-Alcor the Messier object M101 is easily found (but not easily seen). Move just one binocular field east, keeping 81 UMa at the western edge: binoculars.

M101 is a splendid spiral galaxy seen face-on. The recorded apparent visual magnitude of 7.9 is deceptive, for the object is extremely difficult to see with binoculars; large telescopes are needed to bring out the delicate details of the spirals and the core.

There is some confusion over M102, purported to be in the same area. In fact M101 and M102 are the same object.

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