Yes, theoretical astrophysics is a branch of astrophysics.

The Indian Institute of Astrophysics is an institute designed to basically train you in, and keep you focused on, astrophysics.

William K. Rose has written:

'Astrophysics' -- subject(s): Astrophysics

'The Letters of Wyndham Lewis'

'Advanced stellar astrophysics' -- subject(s): Astrophysics, Stars

You would use the word "astrophysics" in a sentence in the role of an abstract noun. For example, "Astrophysics is the study of the physics of the universe."

Richard L. Bowers has written:

'Numerical modeling in applied physics and astrophysics' -- subject(s): Astrophysics, Data processing, Supercomputers

'Astrophysics' -- subject(s): Astrophysics