Stars are celestial bodies subject to strong dynamism, characterised by complex internal movements that affect their entire sphere of influence. The Sun is an example: its magnetic field influences the entire Solar System and results in surface and coronal phenomena of exceptional scientific beauty. And it is precisely the magnetic field that is one of the defining phenomena of stars that can reveal essential details about their internal, surface and coronal activities. The in-depth study of complex stellar magnetic fields has been made possible thanks to their representation and mapping, achieved by exploiting the technique of Zeeman-Doppler imaging.
A matter of plasma
The magnetic field originates from the motions of stellar plasma within the convective zone. This is the layer of stars characterised by a displacement of mass and thermal energy due to plasma circulation motions within the stars. The plasma is formed by electrical charges, whose motion generates a magnetic field that in turn drives the flow of charged particles, self-sustaining and feeding each other. The magnetic fluxes generated are sometimes so intense that they give rise to visible external effects, significantly modifying the space environment near the stars.A very important example of such effects are sunspots: stars generally have a differential type of rotation, i.e. not the same at all latitudes, which stretches and entangles the magnetic fluxes. This stretches and entangles the magnetic fluxes, which puncture the photosphere and inhibit convection in the areas themselves, which cool down to appear as dark spots.
Rings and eruptions
Another visible effect of the stellar magnetic field are coronal rings: these are very complex structures, in which the magnetic flux exiting the surface and entangled in knots reaches the corona, trapping the stellar plasma in a loop. In this way, the Sun’s energy is transferred to its corona, a process which is therefore very localised in the areas where the rings are present.
Much more violent are the flare phenomena, known as solar flares. In strongly conductive plasmas such as stellar ones, there are zones of reconnection of magnetic field lines, where different zones of uniform magnetization meet. At these points, the energy of the magnetic field spills into the particles as kinetic and thermal energy, accelerating them. This results in plasma explosions in the photosphere, which are so dangerous that they endanger the health of astronauts in certain areas where the Earth’s magnetic field is insufficient to protect them, even disrupting terrestrial radio communications and damaging some satellites.
The Zeeman-Doppler imaging technique
The possibility of mapping stellar magnetic fields is due to the work of the Dutch physicist Pieter Zeeman, winner of the Nobel Prize for physics.
The technique that allows us to represent these fields is Zeeman-Doppler imaging, which exploits the Zeeman Effect. This is a physical phenomenon involving the interaction of electrons with external magnetic fields. Normally degenerate electronic levels exist in atoms, i.e. different levels where electrons possess the same energy. When an electron passes through different energy levels it emits a photon at a certain frequency proportional to the energy jump, which is absorbed by the atoms in the stellar atmosphere. This produces a spectral line, i.e. a black line indicating the position of this absorbed photon in the possible frequency spectrum. When a magnetic field acts, the degenerate electronic levels change their energy slightly, forming new spectral lines that are very close together. In essence, the degeneration of energy levels is eliminated by the presence of the magnetic field.
Polarisation and Zeeman-Doppler imaging
The magnetic fields of stars have a further effect on photons, which is to orient their polarisation. This means that they manage to direct in a particular direction the oscillation of the electromagnetic wave with which they are associated. Given these particular physical phenomena, it is possible to measure the stellar magnetic field through the combination of spectrograph and polarimeter. Iterating throughout the rotation, it is thus possible to reconstruct the magnetic field lines of the star, knowing their direction and intensity.
Many representations would be compatible with such reconstructions, but the one shown is generally the simplest.