In a theoretical paper, the primary investigator which is my advisor, is first author, and he also writes the first draft. Me, the second author, and another graduate student have contributed substantially to the numerical simulation. We have another external collaborator which has not contributed that much, and my advisor selected him/her to be the last author. Do you think that this is a fair order, regarding the position of the second and last authors? In other words, where should the one who had the second major contribution be positioned in the author list?