Binary star systems

In a binary star system, the gravitational force that the first star exerts on the second is

(4.108) |

(4.109) |

(4.111) |

Equation (4.110) is identical to Equation (4.2), which we have already solved. Hence, we can immediately write down the solution:

where(4.113) |

(4.114) |

(4.115) |

In the inertial frame of reference whose origin always coincides with the center of mass—the so-called *center of mass frame*—the position vectors of the two stars are

Binary star systems are very useful to astronomers, because it is possible to determine the masses of both stars in such a system by careful observation. The sum of the masses of the two stars, , can be found from Equation (4.116) after a measurement of the major radius, (which is the mean of the greatest and smallest distance apart of the two stars during their orbit), and the orbital period, . The ratio of the masses of the two stars, , can be determined from Equations (4.118) and (4.119) by observing the fixed ratio of the relative distances of the two stars from the common center of mass about which they both appear to rotate. Obviously, given the sum of the masses, and the ratio of the masses, the individual masses themselves can then be calculated.

Incidentally, an obvious corollary of the material presented in this section is that we can correct the two-body solar-system orbit theory outlined in Sections 4.1–4.15 in order to take into account the finite mass, , of the orbiting body, compared to the solar mass, , by replacing by wherever it appears in the analysis. For instance, Equation (4.40) generalizes to give .