Mizar: Double Star

Look closely at that star. On a dark night, good eyes can see that it has a companion - another dimmer star right next door.

This companion star is called Alcor.

Next, observe this star under very low power. Binoculars or the magnifying finder on your telescope are ideal. If you have only the telescope, use your lowest power eyepiece.

In this image of a 6x30 finder you can see Mizar and Alcor.

This is not a true double star - it is called an apparent double because the two stars seem close together because of our line of sight. In fact, they are quite far apart, and they are not a system - i.e. they are not gravitationally bound to each other.

However, let's go in farther. Centre your telescope on Mizar and use an eyepiece that yields about 100x magnification.

Alcor will now be well-separated from Mizar. But look at Mizar: it is beginning to "bud". A small companion is becoming visible at this magnification.

These two stars, Mizar A and Mizar B, are a true double star system - they are orbiting one another at close distance. (About 340 Astronomical Units, or about 8 times the distance from our Sun to Pluto.)

Raise the magnification even more. Here, at 200x, Alcor is so far from Mizar that it is out of the field, and Mizar A and B are now clearly separated.

Raising the magnification until the components of a double star are cleanly separated is called "splitting the double". Some doubles are so close that splitting them requires very high power, very high quality optics, and very steady air.