Astrophysics is the branch of physics that studies, loosely speaking, phenomena on
large scales ? the Sun, the planets, stars, galaxies, and the Universe as a whole. But
this definition is clearly incomplete; much of astronomy1 also deals, e.g., with phenomena
at the atomic and nuclear levels. We could attempt to define astrophysics
as the physics of distant objects and phenomena, but astrophysics also includes the
formation of the Earth, and the effects of astronomical events on the emergence
and evolution of life on Earth. This semantic difficulty perhaps simply reflects the
huge variety of physical phenomena encompassed by astrophysics. Indeed, as we
will see, practically all the subjects encountered in a standard undergraduate physical
science curriculum ? classical mechanics, electromagnetism, thermodynamics,
quantum mechanics, statistical mechanics, relativity, and chemistry, to name just
some ? play a prominent role in astronomical phenomena. Seeing all of them in
action is one of the exciting aspects of studying astrophyics.