Semi-Relevant: WTF are Those Red Giants Doing up There?

When one looks at an HR diagram (ie, a Color-Magnitude diagram), one may wonder what those stars are doing, hanging out up above the general trend. Well, I did, anyway. Not right away, mind you, it took at least three years for me to wonder this, between the first time I saw this diagram and the first time I wondered what they were doing all the way up there. It turns out it's not really just the red giants that are doing weird things, and that there are a couple separate groups of stars that don't fall onto the main sequence.

In general, the full diagram would look something like this:

So, the answer to my above question is, as I understand it, that the main sequence actually represents the lifetime of a star. Depending on the star, it may break off and drift up into the realm of the red giants later on in it's life (I think around 10 billion years, ish?). If it's really massive it might not go down the main sequence at all, instead heading straight over to the supergiants. After that, more complicated things happen (ie, it could supernova, if it is a large enough star, and subsequently form a white dwarf...). So, as far as I can gather, the reason there are subsets in addition to the main sequence is that stars change as they age, and different stars age differently.

References:

NASA: Chandra X-Ray Observatory