Two-body Kepler period and mass inference
An isolated Newtonian gravitational binary has relative semimajor axis a=a₁+a₂ and period T satisfying T²=4π²a³/[G(m₁+m₂)].
Key knowledge
An isolated Newtonian gravitational binary has relative semimajor axis a=a₁+a₂ and period T satisfying T²=4π²a³/[G(m₁+m₂)]. Thus an independently measured physical relative orbit and period determine the total mass; barycentric radii obey m₁a₁=m₂a₂.
Values and equations
T²=4π²a³/[G(m₁+m₂)]; a=a₁+a₂; m₁a₁=m₂a₂
- T
- Orbital period s
- a,a₁,a₂
- Relative/barycentric semimajor axes m
- m₁,m₂
- Component masses kg
- G
- Newtonian gravitational constant m³ kg⁻¹ s⁻²
Conditions and scope
- Bound nonradial inverse-square two-body orbit, positive constant masses, physical semimajor axis of relative separation and negligible perturbations/relativistic corrections.
- An angular orbit requires distance, and a projected orbit requires inclination/orbital reconstruction. A single radial-velocity amplitude usually gives a mass function, not both masses directly.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Richard Fitzpatrick / University of Texas at Austin2 sources
Evidence and review
Newtonian Dynamics: Reduced mass: Equations 324–328: barycentric relation and relative equation; combine the gravitational force with reduced mass. Newtonian Dynamics: Kepler's third law: Equations 257–265: elliptical period and semimajor-axis definition; central GM becomes G(m₁+m₂) in the relative equation.
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