A star begins life with a large proportion of short-lived atoms, and so at first generates energy furiously. As it ages, the shortest lived atoms disappear first, and in so doing reduce the average energy-generation of the star per ton so that, as a star’s weight decreases, so also must its rate of generation of energy per ton. Finally all the atoms with much energy-generating capacity have disappeared, and the star is left, a shrunken and diminished mass of atoms which have very little capacity for generating radiation. To put the same thing in another way, the rate at which a star generates energy per ton is proportional to the death-rate in its population of atoms. To say that Sirius generates 16 times as much energy per ton as the sun is only another way of saying that the average atom in Sirius has only a sixteenth of the expectation of life of the solar atoms; their death-rate is 16 times as high. As those types of atoms which have the highest death-rate gradually die off in any star, the average death-rate of the population decreases, or, in other words, as a star ages its capacity for energy-generation per ton decreases. This agrees with the findings of observational astronomy. The most massive stars not only generate more energy than less massive stars, as is in any case to be expected; they also generate enormously more energy per ton. This is illustrated by the following list of main-sequence stars: +-----------------+-----------------+--------------------+ | Star | Weight |Generation of energy| | |(in terms of sun)| (ergs per gramme) | +-----------------+-----------------+--------------------+ |Pearce’s Star _A_| 36·3 | 15,000 | |_V_ Puppis _A_ | 19·2 | 1,000 | |Sirius _A_ | 2·45 | 29 | |Sun | 1·00 | 1·90 | |ε Eridani | (0·45) | 0·26 | |Kruger 60 _B_ | 0·20 | 0·021 | +-----------------+-----------------+--------------------+ Seares’ diagram of stellar weights (p. 282) shews that this is a general property of the stars. To repeat our former metaphor, the stars squander their substance lavishly in their youth, while they have plenty left to spend, but parsimony comes over them with old age. Theoretical considerations have now given us an explanation of this phenomenon.