Thus, v = - 0.00166 x c = - 0.00166 x 3 x 105 = - 498 km/s. The value is negative because this is a blue shift.
2. The lines will merge when the two stars emitting the light have the same radial velocity relative to the observer. This happens twice for each orbit, when the stars are moving at right angles to the line of sight.
3. The period corresponds to the time for the velocity curve to repeat itself. From the spectrum below for HD 80715 we can see, for example, that the lines merge at the times marked 0.061 days and 2.038 days. Since, from the preceding question, this time difference must correspond to half of an orbit, the period is approximately 2 x (2.038 - 0.061) = 3.95 days. This is the period in which the velocity curve will repeat itself.