top of page
  • Writer's pictureKGB

Mizar & Alcor.

Updated: Dec 31, 2023

Hello again, and I hope you are all enjoying yourself at this wonderful Christmas time (haha). I have not done a blog post for a while, so I thought I better hit the keys again. It has been a very poor December as regards imaging and observing, indeed the last clear night was the 5th & 6th evening of this month. However, it was clearish for a bit last night (29th Dec), so I managed to get a few images in. One of them was IC 405 (The Flaming Star Nebula) in the constellation of Auriga. It will be a while coming though as it has not been processed yet. Another object I imaged was the title of this blog. The famous double star pairing of Mizar & Alcor in the constellation of Ursa Major.

Graphic showing the main stars of Ursa Major including Mizar & Alcor.

Mizar and Alcor are two stars forming a naked eye double star in the handle of the Plough, the 7 star asterism in Ursa Major (see above graphic). This system consists of two stars with magnitudes 2.2 (Mizar) and 3.9 (Alcor), that can be seen easily without the aid of a telescope. Mizar is not only a double star, but a four-star binary, comprising two pairs of double stars that are gravitationally bound to each other. Alcor itself is a binary star, it has a faint red dwarf companion separated by 1 second of arc. This pair together with Mizar and it's companions forming a sextuple system. In fact, it was the first known binary star system, discovered by Italian astronomer Giovanni Battista Riccioli in 1650. This entire system lies about 83 light-years away from the Sun, as measured by the Hipparcos astrometry satellite.

Mizar and Alcor's proper motions show that they move together, along with most of the other stars of the The Plough except Alpha Ursae Majoris and Eta Ursae Majoris, as members of the Ursa Major Moving Group (great name for a metal band), a mostly dispersed group of stars sharing a common birth. However, it has yet to be demonstrated conclusively that they are gravitationally bound. Gaia parallax measurements indicate that the Alcor binary and Mizar quadruple are somewhat closer together than previously thought. This is due to our uncertainty about the exact distances from us. If they are exactly the same distance from us then the distance between them is only 17800 AU (0.281 ly). If not, then their orbital period must be hundreds of thousands of years long!

Mizar has been honoured by several scientific firsts: it was the first double star to be found by a telescope (by Giovanni Battista Riccioli in 1662), the first star to be photographed ( by William Cranch Bond in 1857), and the first spectroscopic binary detected (by Edward Charles Pickering in 1889). The co-ordinates for Mizar are: RA 13h 23.55m Dec +54° 55'. For those of you with a go-to telescope the SAO number is 28737. This system is also known as Zeta Ursae Majoris and STF 1744 AB. Incidentally, the other bright star in the middle, just off to the right in my image is called "Sidus Ludoviciana" or Ludwig's Star. Also known as HD 116798, this is an 8th-magnitude giant star.

MIzar (top star) & Alcor (below) imaged by me, the author on December 29th 2023.

Now before I end this blog post, here is a little test for you. On a clear night seek out the Plough asterism in the Northern sky and see if you can see this double with your own eye. The ability to resolve Mizar and Alcor with the naked eye is often quoted as a test of eyesight and that has been confirmed by modern experimental research. Arabic literature says that only those with the sharpest eyesight can see the companion of Mizar. The 14th century Arabian lexicographer Firuzabodi called it "Our Riddle", while the 13th century Persian astronomical writer Zakariya al-qazwini said that "people tested their eyesight by this star." In Japan, Alcor is sometimes referred to as the 'Lifespan Star' 寿命星 (Jumyōboshi), and it was rumoured that being unable to see Alcor with the naked eye was a sign of impending death from old age! Resolve it and this is proof that astronomy can prolong your life!!

I for one, have always been able to see this double easily with my eyes, until about a year ago, when I discovered I had cataracts. However, having surgery has removed them and normal service is now resumed, easy!! Go and try this test for yourself, I am sure you will resolve them, as to be honest they are not that hard, not to me anyway. I hope you have enjoyed this post and please let me know if you have or not by commenting. All comments welcome, good or bad. Happy New Year to you all and see you next time...







19 views1 comment

Recent Posts

See All

1 Comment

Rated 0 out of 5 stars.
No ratings yet

Add a rating
Kez Bloor
Kez Bloor
Dec 31, 2023
Rated 5 out of 5 stars.

Excellent article with really interesting information and even the suggestion of some very literal speculating that anyone can do (ie The Eyesight Test haha). Also, really great photo of Mizar & Alcor 👍

bottom of page