Suppose we are looking at some distant star through a cloud of gas. Of course, we'd like to confirm this, so we look for its characteristic spectral line pattern. No absorption or emission lines are present. In order to create spectral lines, an atom must have electrons. Suppose we were looking at a rapidly rotating star.
Stellar size can also affect line widths.
In fact, fusion doesn't occur everywhere in the Sun, only in its core. The density of the coronal gas is important.