The main sequence lifetime of a star is the length of time that it spends consuming hydrogen in its core. Thus the main sequence lifetime of a star depends on its internal structure and its evolution. Main sequence lifetime are difficult to calculate accurately, but they can be estimated reasonably well using the masses and luminosities of main sequence stars. The method used to estimate main sequence lifetime is essentially the same as the one that someone would use to estimate how long a fire can be kept going with a specified supply of wood.