(how much wood will be consumed each hour). The length of time that the fire will burn is directly proportional to the amount of fuel and inversely proportional to the brightness of the fire. This means that a small fire with a large supply of wood will last a long time, but a large fire with a small supply of wood will burn out quickly. Exactly the same idea can be used to estimate the lifetimes of main sequence stars. In the case of stars, the amount of fuel is the mass of hydrogen that is available for fusion. Although this varies somewhat for different stellar masses, it is approximately 10% of the entire mass of the star. This means that the amount of fuel is proportional to the mass of the star. The brightness of the fire in the case of a wood fire corresponds to the luminosity of the star, which varies relatively little during its main sequence lifetime. In either case, the wood fire or the main sequence stage of the evolution of a star, the lifetime is given by