"Astronomy" and "astrophysics" are synonyms.[6][7][8] Based on strict dictionary definitions, "astronomy" refers to "the study of objects and matter outside the Earth's atmosphere and of their physical and chemical properties,"[9] while "astrophysics" refers to the branch of astronomy dealing with "the behavior, physical properties, and dynamic processes of celestial objects and phenomena".[10] In some cases, as in the introduction of the introductory textbook The Physical Universe by Frank Shu, "astronomy" may be used to describe the qualitative study of the subject, whereas "astrophysics" is used to describe the physics-oriented version of the subject.[11] However, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics.[6] Some fields, such as astrometry, are purely astronomy rather than also astrophysics.