====== 1. Motion of stars and planets ======
===== 1.1. Binary star systems =====
===== 1.2. Laws of motion and gravitation =====
===== 1.3. Solution for a large central mass =====
===== 1.4. Solution for arbitrary masses =====
===== 1.5. Determining mass of stars =====
===== 1.6. Determining mass of exoplanets =====