# Binary Star

## Study Of Orbital Motion

The first goal generally is to determine the period of revolution, then when feasible, the geometric elements (relating the apparent **orbit** on the plane of the sky to that on the true plane of the orbit) and the dynamical orbital elements of the system as far as possible, which will lead to the physical characteristics of the stellar components. The simple laws which govern the dynamics of orbital motion of double stars stem from the three Kepler laws which were originally formulated by Johann Kepler to describe the motions in our **solar system**. They had far-reaching implications of gravitational forces explained later by Newton. The laws of Kepler, as used in binary star analysis, are: 1) The orbit described by the fainter component (often called B) around the brighter (A component) is an **ellipse** with the A component at one of the foci; 2) The component sweeps over equal areas, throughout its orbital path, in equal lengths of time; 3) The sum of the masses of the two components (in units of the solar mass) is equal to the scale or semi-major axis cubed (in units of the earth-sun distance) divided by the period squared (in unit of years). The mass of the binary system from Kepler's third law is the only direct way stellar mass can be determined.

## Additional topics

Science EncyclopediaScience & Philosophy: *Bilateral symmetry* to *Boolean algebra*Binary Star - Importance, Visual Binaries, Study Of Orbital Motion, Astrometric Binaries, Spectroscopic Binaries, Eclipsing Binaries - Techniques of observation