Basic principles of stellar tomography 

Most stars only appear to us as blinking dots in the sky, and details on their surfaces are, in the vast majority of cases, totally out of reach of the most modern astronomical direct imaging techniques.

To access such surface details, astronomers nonetheless have one solution: they can use the rotation of stars to "scan" their surfaces, by using techniques that are very similar to those developped for medical imaging.

Thanks to the Doppler effect, spectral lines of a rotating star can indeed be viewed as  1D images of the stellar surface; while the approaching limb of the star mainly contributes to the left side of the line profile, the receding limb affects mostly the right side of the line profile.

Hence, for a star hosting a surface dark spot close to the equator, spectral line profiles are affected throughout their whole width as the spot is carried around the star by rotation, as illustrated on the top left image (click on the images to get a full animation).

If the spot is located close to the pole (top right image), spectral lines are only affected in their core regions; the difference with the previous case implies that one is able to recover information on both latitude and longitude of starspots.

Following the same principles and using the polarised signatures generated by Zeeman effect in spectral lines, one can reconstruct non only the position of magnetic regions at the surface of the star, but also the orientation of the magnetic field within the spot (circular polarisation signatures being sensitive to the field component along the observer's direction). For a magnetic spot hosting radial field (perpendicular to the stellar surface, see middle left image), the circular polarisation signature keeps the same sign as it crosses the line profile.

If the magnetic field within the spot is azimuthal (parallel to the surface of the star and oriented along parallels, middle right image), the circular polarisation signature changes sign as it crosses the line profile, demonstrating that one can recover the orientation of the magnetic field at the surface of the star with reasonable accuracy. A fully automatic software tool was developed along these lines to achieve magnetic imaging of stellar surfaces.

These are the basic principles of the techniques called Doppler Imaging and Zeeman-Doppler Imaging to image brightness and magnetic features at the surfaces of distant rotating stars. Examples of magnetic field reconstructions (and field extrapolation up to the stellar corona) are shown in the bottom animated images, in the case of the fully-convective red dwarfs V374 Peg (from Donati+2006) and EV Lac (from Donati+2024)