I want to make an order-of-magnitude estimate of the Sun's present-day luminosity. For this, I want to use just the following two pieces of information in a simple idealised model of (the mechanism of energy production in a star): 1) the mass of a helium nucleus is about 1 percent less than the mass of four hydrogen nuclei. 2) the Sun’s hydrogen-burning lifetime is believed to be 10 Gyr. This is my attempt at the problem: The Sun is passing through its mid-life at the moment. So, I am assuming that half of the hydrogen nuclei have to converted to helium. Therefore, (0.5)(mass of all H nuclie) + (0.5)(mass of all He nuclie) = (present) Mass of the Sun "Mass of a helium nuclues = (0.99)(4*Mass of a hydrogen nucleus)" implies that the mass of all helium nuclei in the Sun is 0.8*1030 kg. So, the intial mass of the Sun = (2*Present mass of the hydrogen nuclie) = 2.481030 kg. So, total number of reactions (that will have occurred by the time the Sun dies) = initial mass of Sun / 4 = 0.6*1030. Energy release per reaction = (delta-m)(c2) = (4*(1-0.99))(c2) = 6.0*10-12 J. So, (average) luminosity = Total energy released / energy release per reaction = 3.3*1026 W. I would be grateful if anyone could point out any flaws in the argument. Also, I am not sure whether I have arrived at a reasonably good answer with incorrect methods.