Hello, all, I am an adult college student and I'm having a terrible time "understanding" a particular astronomy question. I do not ask that any of you answer the question, but I am asking that maybe some of you can re-structure the original question so that I may have a clue as to what's expected? Here is the question: "Tell what single stellar physical property the Main Sequence is a sequence of." There are several properties like spectral types and luminosities -- so I'm not sure what the intent of the professor with this inquiry. Again, please don't answer the question. I want to "think it through" myself -- I've been stumpped by this for the last 2 days. Any help would be most graciously appreciated. J.