Around 1767 it was recognized that most close pairs are not all optical (chance alignment), but in fact many double stars are physically associated with one another. These we called Physical Doubles or Binary Stars. Figure A. shows an unequal binary system in orbit around a common center of gravity. Perhaps the most famous binary system is that of Sirius found in 1834. The bright star Sirius displayed a 'weaving' sort of proper motion through space and it was deduced that this perturbed motion was caused by the presence of an invisible companion. The companion was discovered in 1862 and is only one ten-thousandth of the luminosity of its primary (Sirius) and is now known to be a dense star called a white-dwarf. (See Figures B and C) Most binary systems can only be resolved into two distinct components through the use of a large telescope and a device called a filar micrometer.

Copyright (c) 1997-99 Michael Erlewine