I am having difficulty understanding the concept of Doppler imaging as used to detect and study starspots. With regards to the conventional Doppler imaging, so far what i have understood is that a spectral absorption line from a star is observed over time, and if the star is rotating sufficiently fast then the spectral line will be broad enough to observe any distortion caused by the existence of spots. These spots cause a distortion because they are of lower temperature, therefore reducing the line absorption at the spectral line under observation. The reason for this is that particular absorption lines can be observed only for a certain range of temperatures and if the temperature drops then less absorption occurs. The above is what i have understood as the explanation for the below diagram: http://helene.ethz.ch/research/stars/starspots/results/DI_atom_anim.gif [Broken] Moving onto Zeeman doppler imaging...well i cant seem to understand this. What i have picked up is that the spectral (absorption) lines are split due to the strong magnetic field at starspots (Zeeman effect), the size of the splitting reveals the magnitude of the magnetic field at that point. As seen below: As to the direction of the field, i dont know how we can tell this. Also, what do the horizontal lines below represent? And i am drawing a blank as to what the second profile below represents: I appreciate that from this we can tell that the two spots are of opposite polarity but i cant quite tell what i am looking at. Is my explanation so far is correct? Why is Zeeman Doppler imaging used rather than just plain Doppler imaging?