I'm just doing some reading on stars and I've hit a problem which I can't solve. According to a book I'm using it claims the amount of hydrogen converted in the Sun over it's lifetime so far is 5.4x10^55. I roughly agree with this figure using the method this author calculates the number. He also says that the sun had about 8.9x10^56 hydrogen atom and so concludes the star has used less than 10% of it's hydrogen. However it appears that most people say the sun has about another 5 billion years before it runs out of fuel and yet we've used less than 10& in about 4.7billion years? Can anyone explain the vast differences here? Thanks.