In its youth, our Sun probably had a magnetic field that was more active than it is today, resulting in larger flares, a hotter corona and more powerful radio and X-ray emission. This field would have had a significant impact on planet formation and on the way the Sun has been spun down from its initial phase of rapid rotation. We are modelling the generation of magnetic fields deep within young stars and the way these fields are structured in the outer stellar corona. From Zeeman-Doppler images of the surface magnetic fields of young Suns we can extrapolate the coronal field to show the open field lines (where a wind might escape) and the closed field lines where stellar prominences may form (see figure). By studying the structure of these fields for a range of stars as they spin down with age, we hope to learn more about the early stages of stellar evolution.
We are also studying magnetic fields in evolved stars. The field in a planetary nebula is thought to act as a 'tube' that forces gas out along the rotation axis. This mechanism might explain why envelopes are spherical in AGB stars, but become elongated when the star evolves into the planetary nebula stage. In the first such object to be detected, NGC 7027, the polarization direction is perpendicular to the jets, suggesting the presence of a magnetic 'torus' or tube that is squeezing the gas flow.