Spectroscopic binary stars are double stars that appear single in even the largest of telescopes, but whose double-ness becomes apparent from periodic changes in their spectra. The brighter component of the double star Mizar (Zeta Ursa Major) was the first star to be recognized as a spectroscopic binary. Some spectroscopic binaries have orbits oriented to our perspective such that they pass in front of one another or eclipse each other. These are termed eclipsing binaries and are illustrated in Figure D. Algol or Beta Persei, which undergoes eclipse every few days, was the first known eclipsing binary. Astrometric Binaries are a group of double stars in which the presence of the unseen companion is determined by its gravitational action on the motion of the visible primary, much like Neptune was discovered through the perturbations of Uranus. Both Sirius and Procyon are examples of astrometric binary systems.

Figure A (above): Binary System. The two components revolve around the center of gravity of the system. The intersecting lines represent the center of gravity of an unequal system. The more massive component has the smaller orbit (a,b.c ... h), while the less massive component has the larger orbit (A,B,C ... H). If these were equal components, the center of gravity would be midway between the stars.

Copyright (c) 1997-99 Michael Erlewine