Wikipedia says: Astronomy is a natural science that studies celestial objects and phenomena; Astrophysics is the branch of astronomy that employs the principles of physics and chemistry to ascertain the nature of the heavenly bodies, rather than their positions or motions in space; Astroparticle physics, also called particle astrophysics, is a branch of particle physics that studies elementary particles of astronomical origin and their relation to astrophysics and cosmology. I am not sure what I do, but I like spending my research time in thinking about atomic interactions that took place billions of years ago at cosmological distances, which produced a tiny ray of light, that eventually reached our beatiful telescopes!