[Footnote 31: You may wonder why, having said that the sun contains 2,000 quadrillion tons of energy _at the most_, I now assume that it contains just this amount. It is really only a verbal point depending on the scientific definition of energy. All mass is mass of _something_, and we now call that something ‘energy’ whether it is one of the familiar forms of energy or not. You will see in the next sentence that we do not assume that the energy is convertible into known forms, so that it is a terminology which commits us to nothing.] [Footnote 32: Aston in his latest researches has been able to detect that the oxygen atom is just appreciably lighter than the four helium atoms.] [Footnote 33: A measurement of the heat observed to flow from a continuous fountain of heat is a measurement of the output of the fountain, unless there is a storing of energy between the output and the outflow. The breakdown of the Kelvin time-scale indicates that the storing in the stars (positive or negative) and consequent expansion or contraction is negligible compared to the output or outflow.] [Footnote 34: The stars all put together cover an area of the sky much less than the apparent disk of the sun, so that unless their surface-layers are generating this radiation very much more abundantly than the sun does, they cannot be responsible for it.] [Footnote 35: The term ‘dwarf stars’ is not meant to include _white dwarfs_.] [Footnote 36: We can scarcely suppose that all stars after reaching the main series pass through _precisely_ the same stages. For example, Algol, when it has become reduced to the mass of the Sun, may have slightly different density and temperature. But the observational evidence indicates that these individual differences are small. The main series is nearly a linear sequence; it must have some ‘breadth’ as well as ‘length’, but at present the scatter of the individual stars away from the central line of the sequence seems to be due chiefly to the probable errors of the observational data and the true breadth has not been determined.] [Footnote 37: Exhaustion of supply without change of mass would cause the star to contract to higher density; it would thus have a combination of density and mass which (according to observation) is not found in any actual stars.] [Footnote 38: This increase was assumed in our detailed description of the automatic adjustment of the star, and it will be seen that it was essential to assume it.] APPENDIX _Further Remarks on the Companion of Sirius_ I HAVE preferred not to complicate the Story of the Companion of Sirius with details of a technical kind; some further information may, therefore, be welcome to those readers who are curious to learn as much as possible about this remarkable star. I am also able to add a further instalment of the ‘detective story’ which has just come to hand, the sleuth this time being Mr. R. H. Fowler.