Velocity Curves

The key observational quantity for a spectroscopic binary is the velocity curve for the components, which is a plot of the radial velocity versus time.

An Example

In the adjacent image the velocity curve for a spectroscopic binary is illustrated. The corresponding positions on the orbits for two components are labeled with numbers correlated with diagrams shown below the graph, assuming the observer to be to the right.

Blue Shifts and Red Shifts

Motion toward the observer gives a blue shift (negative velocity) while motion away from the observer gives a red shift (positive velocity). For example, at position 1 in the above figure the orange star has maximal velocity away from observer and the green star has maximal velocity toward the observer, while at position 2 the velocities are at right angles to the line of sight and the radial velocity of each star relative to the observer is zero. From a careful analysis of the amplitude, periodicity, and detailed shape of the velocity curve, it often is possible to gather considerable information about the masses and the orbits in a binary system.