# Homework Help: Radial Velocity of the centre of mass of spectroscopic binary system

1. Mar 4, 2013

### JordanGo

My book says (Foundations of Astrophysics):
" The radial velocity ... can be found from the Doppler shift of the star's absorption lines:
$\nu_{r}=\frac{\Delta\lambda}{\lambda}c$
If the star you are observing is part of a spectroscopic binary system, you can separate the radial velocity of the star relative to the center of mass and the radial velocity of the center of mass itself. This can be done by averaging the radial velocity of the star over an entire period."

Now I am confused how to find the radial velocity of the center of mass. Can someone show me how?