b) The lifetime of a star is basically mass divided by luminosity (M/L). This is because
L = rate of fuel usage and M = mass of fuel; divide the two to get a fuel lifetime.
example: If I have 10 kg of gas in my tank and use gas at the rate of 5 kg per hour
then I can drive for 2 hours before I have exhausted the fuel.
If the main sequence lifetime of a one solar mass star is 10 billion years show,
using the above scaling, that the main sequence lifetime of a 4 solar mass star would
be about a few hundred million years.
c) What is the lifetime of a 0.1 Solar mass star?