Shallow Sky Object of the Month: Mizar and Alcor

Original article appears in GuideStar June, 2012.

Alcor and Mizar finder. From TheSkyXObject: Mizar and Alcor
RA: 13 h, 23 m, 56 s
Distance: 83 ly
Class: A double (and triple) star
Constellation: Ursa Major
Magnitude: 2.2 Size/Spectral: with Alcor — 11.8’; close double with Mizar at 14.4”
Optics needed: This is an easy but very nice double (triple) star group that is a showpiece in small telescopes.

Why this is interesting:

Alcor and Mizar. From TheSkyXThere’s more to this system than what meets the eye. But, first things first, and let’s take a look at the three stars that you’ll see in a small telescope (and you don’t have to be at a dark site to see them). The primary star is simply called Mizar and it is in the handle of the Big Dipper (asterism). It’s the second star, away from the bowl, and is at the bend of the dipper handle.

It’s easy to see and easy to find in a telescope. This beauty shines brightly and is of spectral type A1B. The sun is a G star, so Mizar is bluer, and bigger, and has a higher luminosity than the sun. The luminosity of a star is a measure of the total power output of the star and Mizar is about 2.5 solar masses and about 30 times as luminous as the sun.

The second star in the group is called Alcor. It shines at magnitude 4 and it is of spectral type A5V, so it’s only slightly redder than Mizar. Many references talk about separating Mizar and Alcor with the unaided eye — being able to do so is, supposedly, a test of good eyesight. But is it?

I found many resources that say that the magnification required to resolve a double star is equal to 480/d, where d is the angular separation of the double star (in seconds) and 480 arc seclonds is equal to 8 arc minutes (60 x 8).

Double star (arcsec) magnif req'd
0.5 960
1 480
2 240
5 96
10 48
20 24
50 9.6
100 4.8
200 2.4
500 0.96
1000 0.48

So, for the magnification to be one… the separation must be 8 arc minutes — meaning that the resolving power of the unaided eye is about 8 arc minutes.

If you need to calculate the magnification required for various separations of equal magnitude stars the 480/d rule applies. When the magnitudes are different, more magnification is required. Here is a simple table showing the magnification required for various double stars but this assumes about equal brightness of the stars and doesn’t take into account limitations of the equipment. The magnification of the telescope is simply the focal length of the telescope divided by the focal length of the eyepiece.

So, the Mizar / Alcor pair should be relatively easy to see with the unaided eye. Try it.

There is yet another star in this system, Mizar B, at 14.4 arc seconds from Mizar. Even low magnification will separate this star from Mizar A. Mizar B is an A class (bluish) star shining at magnitude 4, so it’s easy to see.

Each of these three stars has another companion which can’t be seen in amateur telescopes. So, all-in-all it’s a six star system.

Historically, Mizar was the first telescopic binary found in 1617 and it was later observed by Galileo. And, Mizar was the first spectroscopic binary discovered in 1889.

HAS Online Store

Get Connected!

HAS has begun using RainedOut, a text message service, to communicate late-breaking news about events. Click here to learn more and subscribe!

Night Sky Network