High School Senior
Your first mission is to concentrate on getting a good set of results from high school. That really means not spending your time chasing astronomy and astrophysics yet.
Yes, this is what your parents and teachers would say, but you know, if you ever want to have the time, resources and maybe the chance to study and/or work in these fields you absolutely are going to need the very best high school results you can manage. And even if you don't weant to study a B.Sc. or higher then you still need the resources and that means a decent job and career prospects. Money is important, or as the saying goes "No bucks, no Buck Rogers”.
So focus on that first.
a systematic order
For self study this is not needed. I'd suggest not being systematic outside of a specific goal, like having to pass exams set by other people.
You need to differentiate between Astronomy and Astrophyics.
Astronomy is a kind of "catch-all" term for every possible thing you could study or observe about anything that goes on outside of our atsmosphere and planet. You can include geology, biology, chemistry, physics and just about anything else that can be termed a science, as well as engineering (you have to build stuff to do stuff - rocket science and telescopes of many types, etc.). Astrophysics is then a subset of Astronomy focusing on the application of physics to astronomy.
Now you want graduate level or graduate level "foundation" as a target. That means you want a B.Sc. level course. As you want Astrophysics you basically need a B.Sc. level course in physics. That's a lot of study. You'll cover topics like:
- basic quantum theory
- basic special and general relativity
- basic thermodynamics
Keep in mind a systematic B.Sc. course on basic physics for astrophysics typically takes four full years. You have to learn to think about physics problems and get a feel for how things are done. That's arguably more important than the actual details of any particular theory.
Don't be distracted by glamor subjects like quantum theory and relativity. Mechanics, Electromagnetism and Thermodynamics are very, very important.
So let's focus on astrophysics and understand that you have to start with physics and what resources will help.
Great things, not loved or used enough. One set I personally really useful was the Schaum Outline series as a launching point and if you can get anything from your library, borrow them. You don't need to swallow books whole - read what you need and borrow again when you need more. There are lots of books out there and for physics and we all have favorites (and ones we'd avoid ever reading again). Boring old textbooks like e.g. Halliday and Resnick are as good a place to start as any. My first exposure to quantum theory proper was by a book borrowed from my local library.
There are multiple courses on physics online. A couple to consider :
There's also excellent YouTube channels like :
By no means the only ones.