The Collaborative International Dictionary of English v.0.48:

astrophysics \astrophysics\ n.
   1. the concerned with the physical and chemical properties of
      celestial bodies.
      [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Astrophysics \As`tro*phys"ics\, n. [Astro- + physics.] (Astron.)
   The science treating of the physical characteristics of the
   stars and other heavenly bodies, their chemical constitution,
   light, heat, atmospheres, etc. It is a branch of astronomy.
   [1913 Webster]

   Note: Its observations were traditionally made with the
         spectroscope, bolometer, etc., usually in connection
         with the telescope. In modern times, radiotelescopes,
         infrared telescopes, X-ray and gamma ray detectors and
         neutrino detection equipment located deep underground
         are all put into service to observe and test theories
         about astronomical phenomena.
         [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

astrophysics
    n 1: the branch of astronomy concerned with the physical and
         chemical properties of celestial bodies

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "astrophysics":
   Newtonian physics, acoustics, aerophysics, applied physics,
   astrogeology, astrognosy, astrography, astrolithology, astronomy,
   astrophotography, basic conductor physics, biophysics,
   celestial mechanics, chemical physics, cryogenics, crystallography,
   cytophysics, electron physics, electronics, electrophysics,
   geophysics, gravitational astronomy, macrophysics,
   mathematical physics, mechanics, medicophysics, meteoritics,
   microphysics, natural philosophy, natural science, nuclear physics,
   optics, philosophy, physic, physical chemistry, physical science,
   physicochemistry, physicomathematics, physics, psychophysics,
   radar astronomy, radiation physics, radio astronomy, radionics,
   solar physics, solid-state physics, spectrography, spectroscopy,
   stargazing, statics, stellar photometry, stereophysics,
   theoretical physics, thermodynamics, uranography, uranology,
   uranometry, zoophysics