Stellar variability (on timescales of days or longer) results predominantly from surface manifestations of magnetic activity such as starspots and faculae, and is one of the most important limiting factors in the detection and characterisation of exoplanets. If variability amplitudes in time series photometry are sufficiently large, the periodic transit signature of an orbiting planet may go undetected. Spots and faculae occulted in-transit introduce line profile distortions to transit lightcurves, and the spectral contributions of unocculted features contaminate measured transmission spectra. Realistic models for magnetic features are required to understand and accurately constrain their effects. The implementation of faculae in lightcurve models for non-solar spectral types is an open problem, with scaling typically based on spectra equivalent to hot stellar atmospheres or assuming a solar-derived facular contrast.
In this thesis, I introduce the stellar variability code actress, a software tool enabling geometrically-accurate rotational and transit lightcurve modelling. For the first time, synthetic spectra calculated from magneto-convection simulations for stellar spectral types other than solar are used as model inputs to represent the brightness contributions of the quiet stellar surface and facular regions. I use actress to simulate variability in Kepler and TESS observational passbands, examining the characteristic lightcurve signatures of individual features. Rotational lightcurves are generated with realistic Sun-like feature distributions to investigate how variability amplitudes are affected by changes in stellar parameters such as feature coverage level, emergence latitude, stellar inclination, facular region field strength, spot temperature contrast and activity level. The investigations are extended to wavelengths 240 nm − 10 μm, and both rotational variability and transit contamination are simulated throughout the full spectral range. Finally, I summarise the thesis findings and propose potential future investigations that complement and extend this line of research into stellar variability and transit lightcurve contamination.