Basic spectrometry: Line broadening by rotation

When a telescope–spectrometer combination is applied to a stellar radiation, the resultant spectrum results from the light received from the whole of the presented stellar disc.
For rotating stars, this means that the generated absorption lines from each location on the disc are subject to Doppler displacements according to their position and line-of-sight velocity. Radiation from the approaching limb will be blue shifted, while that from the receding limb will be red shifted. Integration of these effects over the presented disc produces an overall absorption line profile which is saucer-shaped. From observations of the line profile spread, the projected equatorial velocity, V sin i , of the star can be determined.

Figure 1. A section of simple electromagnetic disturbance.