  We report on preliminary results of a statistical study of student
performance in more than a decade of calculus-based introductory physics
courses. Treating average homework and test grades as proxies for student
effort and comprehension respectively, we plot comprehension versus effort in
an academic version of the astronomical Hertzsprung-Russell diagram (which
plots stellar luminosity versus temperature). We study the evolution of this
diagram with time, finding that the "academic main sequence" has begun to break
down in recent years as student achievement on tests has become decoupled from
homework grades. We present evidence that this breakdown is likely related to
the emergence of easily accessible online solutions to most textbook problems,
and discuss possible responses and strategies for maintaining and enhancing
student learning in the online era.
