Just
contemplating this list inspires some observations:
The different subfields
relate to each other – there’s a hierarchy of scale (e.g. the
Milky Way is made up of stars and gas);
Many interesting questions
that are currently being debated involve these interrelations
(e.g. Is the Milky Way an isolated object or should we think of
it as being physically connected to its neighbors – galaxy
collisions and mergers, infalling gas?)
The Universe is dynamic –
objects form, evolve, and sometimes disappear – but usually on
very long timescales