  GrayStar is a stellar atmospheric and spectral line modelling,
post-processing, and visualisation code, suitable for classroom demonstrations
and laboratory-style assignments, that has been developed in Java and deployed
in JavaScript and HTML. The only software needed to compute models and
post-processed observables, and to visualise the resulting atmospheric
structure and observables, is a common Web browser. Therefore, the code will
run on any common PC or related X86 (-64) computer of the type that typically
serves classroom data projectors, is found in undergraduate computer
laboratories, or that students themselves own, including those with highly
portable form-factors such as net-books and tablets. The user requires no
experience with compiling source code, reading data files, or using plotting
packages. More advanced students can view the JavaScript source code using the
developer tools provided by common Web browsers. The code is based on the
approximate gray atmospheric solution and runs quickly enough on current common
PCs to provide near-instantaneous results, allowing for real time exploration
of parameter space. I describe the user interface and its inputs and outputs
and suggest specific pedagogical applications and projects. Therefore, this
paper may serve as a GrayStar user manual for both instructors and students. In
an accompanying paper, I describe the computational strategy and methodology as
necessitated by Java and JavaScript. I have made the application itself, and
the HTML, CSS, JavaScript, and Java source files available to the community.
The Web application and source files may be found at
www.ap.smu.ca/~ishort/GrayStar.
