1. The problem statement, all variables and given/known data I've been working through coursework set from a module called Sun & Stars. Basically the concept I understand is that there is a theoretical way of estimating the lifetime of a class II protostar as 2x10^6 years. Using this we can estimate the lifetime of class 0 and class I protostars based on their abundance in comparison to class II (if there are 4x as many class II than class I we assume that class II lifetime is 4x longer also). Just come across the last question which asks: How is the lifetime of a class II star estimated? I've been scouring through my notes and the internet but can't seem to find the theory/derivation/whatever it is they used to figure this out! I can add that a class II protostar is when the nebula/gas cloud collapses into a protostar with a massive optically thick disc which is correlated with the Classical T Tauri phase Any help/pointers would be great!