Mizar, a Double Star, and Alcor

Middle Stars in the Big Dipper's Handle

Best time to observe: February - September

To locate:

Find the Big Dipper, which is always visible in our sky (unless obscured by something on the horizon). Look for the middle star of the three that make up the Dipper's handle. That star is Mizar. With your naked eye you should also be able to see a tiny star next to it, Alcor. This pair of stars was called the "Horse and Rider" by ancient Arabs. Mizar and Alcor are not double stars; they just happen to appear close to each other in our line of sight. Target the Telrad on Mizar, the brighter star.

What to notice:

In the telescope, look carefully -- you should be able see that Mizar is really two stars, one larger than the other. (Alcor will still be visible, but near the edge of the field of view.) The two Mizar stars orbit each other, at a distance of about 400 AU (an AU is the distance from the Earth to the Sun). So it takes several thousand years for one star to complete an orbit about the other. Alcor, the faint star you could see with your naked eye, is much farther away, and does not orbit the Mizar pair.

What you are looking at:

Mizar was the first double star to be discovered. In fact, each of the two stars you see is a multiple star itself! The brighter star of Mizar is really 2 stars so close together you cannot separate them. The fainter star of Mizar is at least 3, maybe even 4, whose blended light you see. So what appears to your naked eye as the star Mizar is really 5, or possibly 6, stars all orbiting around each other!

All the stars of the Big Dipper, including Mizar and Alcor, are members of the "Ursa Major Group", the leftover remains of an open star cluster. The stars in this group were all formed at the same time, in a nebula. But now they are slowly drifting past our solar system, and apart from each other, as they move through our galaxy. In fact, Sirius, the brightest star in our night sky, is part of this group -- it's path simply took it on the opposite side of our Sun from its traveling companions.

Sketch the multiple star Mizar on one of your observing pages.

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