[23] We refer to calcium as it occurs in the chromosphere, i. e. with one electron missing.
[24] There is an awkwardness in applying the term ‘apparent’ to something too small to be seen; but, remembering that we have armed ourselves with an imaginary telescope capable of showing the disk, the meaning will be clear.
[25] Densities below that of air have been found for some of the Algol variables by an entirely different kind of investigation, and also for some of the Cepheid variables by still another method. There are also many other examples of stars of bulk comparable with that of Betelgeuse.
[26] From a photograph taken at the Royal Observatory, Cape of Good Hope.
[27] For comparison, the nearest fixed star is distant 4 light years. Apart from clusters we rarely deal with distances above 2,000 light years.
[28] One cannot always be sure that what is true of the cluster stars will be true of stars in general; and our knowledge of the nearer stars, though lagging behind that of the stars in clusters, does not entirely agree with this association of colour and brightness.
[29] The term nebula covers a variety of objects, and it is only the nebulae classed as spirals that are likely to be outside our stellar system.
[30] This can be checked because uranium lead has a different atomic weight from lead not so derived. Ordinary lead is a mixture of several kinds of atoms (isotopes).
[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.
[32] Aston in his latest researches has been able to detect that the oxygen atom is just appreciably lighter than the four helium atoms.