Page:Popular Science Monthly Volume 58.djvu/145

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It will now be interesting to compare the brightness of these bodies with that which the Sun would have if seen at their distance. In a former chapter we showed how this could be done. The results are:

At the distance of Procyon the apparent magnitude of the Sun would be 2m.8. At the distance of Sirius, it would be 2m.3. Supposing the Sun to be changed in size, its density remaining unchanged, until it had the same mass as the respective companions of Procyon and Sirius, its magnitudes would be:

The actual magnitudes of these companions cannot be estimated with great precision, owing to the effect of the brilliancy of the star. From the estimate of the companion of Sirius, by Professor Pickering, its magnitude was about the eighth. It is probable that the magnitude of the companion of Procyon is not very different. It will be seen that these magnitudes are very different from those which they would have were the companions models of the Sun. What is very curious is that they differ in the opposite direction from the stars in general, and especially from their primaries. Either they have a far less surface brilliancy than the Sun or their density is much greater. There can be no doubt that the former rather than the latter is the case.

This great mass of the two companions as compared with their brilliancy suggests the question whether they may not shine, in part at least, by the light of their primaries. A very little consideration will show that this cannot be the case. A simple calculation will show that, to shine as brightly as they do, the diameter of the companion of Sirius would have to be enormous, at least 1-30 its distance from Sirius. Moreover, its apparent brightness would vary so widely in different parts of its orbit that we should see it almost as well when near Sirius as when distant from it. The most likely cause of the small brightness is the low temperature of the body.

The results of the last chapter point to the conclusion that the stars, or at least the brighter among them, are masses of gas, more or less compressed in their interior by the action of gravitation upon their more superficial parts. We have now to show how this result was arrived at, at least in the case of the Sun, from different considerations, before the spectroscope had taught us anything of the constitution of these bodies.

We must accept, as one of the obvious conclusions of modern science,