What two things determine how long a star will live?

There is a diagram called the Hertzsrpung Russell Diagram originated in 1919 that contains star relationships. There is a line of stars going from the top left to the bottom right called the Main Sequence. This indicates or is said to be the most stable part of a star's life. The diagram is organized by colour, surface temp. and brightness. Otherwise you would know how long a star its mass. This then relates to the 3 different masses of stars, low mass (100 billion years), medium mass (100 billion years), and High mass (7 billion years).  People may argue about the years on the star mass. 
Hope this helps!
Source: Cramming for Science Exam! 
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They are like other investigations? say there was a murder u have to do blood test and finger prints which run through a lab.