New Stars and Planetary Neulæ.

19 The mass of gas produced by such an impact will obviously expand temporarily into a hollow shell of gas. Herschel tells us this is the condition of planetary nebulas.

20 A partial impact of stars will therefore generally produce in less than an hour an intensely heated body that will expand enormously without, for a time, much diminution of heat.

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It will consequently become very bright indeed, after that it will continue to expand with di-minishing intensity until it becomes a planetary nebula, and will then often disappear by the molecules dissipating completely into space.

21. Therefore in this time a new star has been born that increases in intensity until the general parallelism of motion of the molecules causes a lessening number of impacts between the molecules. As Sir William Crook's experiments in radiant prove that molecules only radiate immediately after encounters, the luminosity will diminish, and will go on diminishing until the body disappears. In special cases the planetary nebula may be fairly permanent. In other cases a permanent star may appear in the centre of the nebula.

22. The graze occupies less than an hour and as with bodies of equal density the velocity acquired is proportional to the diameter, all grazing impacts of true stars or dead ones will occupy about one hour.

23. The molecules on the far side of the nebula (or third body formed by the two colliding stars) will be retreating from us; those on the near side will be advancing towards us.

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The spectrum of such a body will consequently be crossed by broad bright bands with a maximum in the centre and gradually dying imperceptibly away. If this body has any motion in space, as it probably will have when the two colliding stars are unequal, the line of maximum intensity, though in the centre of the band, may be displaced from its true position.

24. Soon after impact the escaping molecular velocity will be greater than the motion of recession of the two cut stars, consequently the displaced lines of these bodies will be on either side of the centre of the broad band, but on its surface.