The image to the right is of Mizar, the larger star, and Alcor, the somewhat smaller one at the upper center. These two objects form one of the most famous double-star systems in northern skies. They are in the handle of the Big Dipper, which is part of the large constellation Ursa Major. A sharp-eyed observer may be able to see faint Alcor slightly east of Mizar. If not, a pair of low-power binoculars will do the trick.
True binary stars revolve around each other due to gravitational forces acting between them. Optical doubles, in contrast, are not gravitationally bound. Instead, they are closely aligned visually but lie at greatly differing distances. The term double star includes both types.
It has been determined that Mizar is actually two stars. Mizar A and Mizar B, and both of those are gravitational binaries, bringing the star count to four. Alcor is also a gravitational binary. As a result, the total star count in this multiple star system is actually six.
You are forgiven if you’ve become dizzy trying to imagine the complex dance of six stars in two-star pairs orbiting each other while the pairs orbit themselves, as well. These systems are not unusual. Actually, multiple star systems of main-sequence stars are far more common than single main-sequence types in the Milky Way. Binary main-sequence star systems slightly outnumber the single main-sequence variety. Triple and quadruple systems are also abundant. Most of the stars in the universe — about 90 percent — are in the main sequence portion of their life cycles. They convert hydrogen into helium in the process of nuclear fusion happening in their super-hot cores.
True binary star systems are important in astrophysics because they orbit around a common center of mass. Resultant calculations of their orbits allow masses of their component stars to be determined directly. Other stellar factors, radius and density, for example, can be indirectly estimated. Without going through the mathematics, a relationship between mass and luminosity may be derived, which can also be applied to stars by themselves.
— Kent Fairfield is a volunteer with Pine Mountain Observatory and a lifelong amateur astronomer. He can be reached at firstname.lastname@example.org . Other PMO volunteers also contributed to this article.