Caption: An animation of a gravitationally-bound gavitational two-body system with two spherically symmetric, equal-mass astro-bodies orbiting in elliptical orbits around their common center of mass (called the barycenter for orbits) which marked by a red cross.

Features:

1. The star sizes are vastly exaggerated relative to the inter-star distances for most binaries---but NOT all---there are some very close binary systems.

2. The elliptical orbits of this two-body system are determined by Newtonian physics (what is universal about the physical system) and initial conditions (what is peculiar or individual about the physical system).

3. Gravity is, of course, the force that pulls the astro-bodies in orbits.

4. The periapsis (AKA pericenter) is the arrangement of closest separation and the term is also used for the closest separation distance.

5. The apoapsis (AKA apocenter) is the arrangement of farthest separation and the term is also used for the farthest separation distance.

6. The apse line (AKA line of apsides is drawn through the periapsis and apoapsis.

7. The relative mean orbital radius (AKA relative semi-major axis) is given by

           a=(1/2)*( r_periapsis +r_apoapsis ) , 

   where r_periapsis is the periapsis separation and r_apoapsis is the apoapsis separation.

8. In most real orbits, perturbations cause noticeable deviations from exact two-body system behavior.