Source : Webster's Revised Unabridged Dictionary (1913)

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.

   Note: Its observations are made with the spectroscope,
         bolometer, etc., usually in connection with the
         telescope.

Source : WordNet®

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