Double star in Ursa Major
Mizar is the middle star in the handle of the Big Dipper, shining at magnitude 2.2. Just 1/5 of a degree to its northeast is Alcor, a dimmer star of magnitude 4. This pair of stars is quite easy to see with the naked eye, and they are one of the best-known double stars. Mizar and Alcor are sometimes called the "horse and rider." Their location in the Big Dipper is shown in this image.
With these very short exposures, I tried to approximate how Mizar and Alcor appear when observed through a telescope at medium power. Alcor is the bright star on the left, and Mizar (on the right) is found to be a double by itself, with a magnitude 4 companion 14.5 arcseconds away. These two stars (named Mizar A and Mizar B) are about 345 astronomical units (UA) apart. (An AU is the average distance between the Sun and the Earth, about 93 million miles). Alcor is about 3 light years away from the Mizar system, and all of these stars are about 86 light years away from Earth.
Image details: 10 exposures, each 1/4 second at ISO 1600, taken with a Canon 400D camera through a Meade 12” telescope at f/5.4. Image processing with ImagesPlus and Photoshop.
March 2009