For questions regarding the application of the laws of physics to orbital mechanics, stellar evolution, and other astronomical processes and celestial bodies.
Astrophysics is a hybrid discipline, applying traditional physics to celestial bodies. It can be used, for instance, to study how nuclear fusion works in stars, how black holes bend space and time, and how interstellar clouds absorb light. It's closely tied to astronomy, and the boundary between the two is often fuzzy.
The astrophysics tag is good for questions which require the use of physics in space - see the examples above. It can also be used (or instead of) with a number of related tags, including astronomy, space, planets, stars, orbital-mechanics, black-holes, and others.