Observations : detecting astrophysical sources of radiation, from radio to very high energy gamma rays, studying their morphology, spectra, variability, polarisation. We observe light but also other "messengers" : gravitational waves, neutrinos, cosmic rays (charged particles). Telescopes collect ever increasing amounts of data that constantly challenge our ability to process, analyse, archive, and share them.
Instrumentation : is always at the edge of what can be done, pushing back the limits of feasibility, using the latest technologies. These instruments need to be conceived, designed, modelled, tested, integrated within the allocated time and budget, delivered with the appropriate analysis tools, upgraded, etc.
Models/Theory : aim to construct a representation of reality, limited to the aspects thought to be the most important to explain or predict observations. Numerical simulations play a major role by exploring complex models. Models rely on theory, which connects to the basic principles of physics. Observations and models can thus open up new fields in theory.
Experiments : some astrophysical processes can be reproduced in laboratory conditions, or measured in situ by space probes. Experimental data bring in key input to understand the synthesis of molecules on ice grains ; the reaction cross-section between atoms or molecules ; the composition of dust grains or meteorites ; etc.