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RELATION OF NOVÆ, PLANETARY NEBULÆ AND WOLF-RAYET STARS

Although all recent novæ have been observed to become planetary or stellar nebulæ, they seem not to remain nebular for any length of time; they have gone further and become Wolf-Rayet stars. Whether any or all of the planetary nebulæ that have been known since Herschel's day, and have remained apparently unchanged in form, have developed from new stars, is uncertain and doubtful. If they have, the disturbances which gave them their character must have been violent, such as would result from full or glancing collisions of two stars, in order to produce deep-seated effects which change slowly, rather than surface effects which change rapidly.

Whether the Wolf-Rayet stars have in general been formed from planetary nebulæ is a different question: some of them certainly have. Wright has recently shown that the stellar nuclei of planetary nebulæ are Wolf-Rayet stars, and he has formulated several steps in the process whereby the nebulosity in a planetary eventually condenses into the central star. The distribution of the planetaries and the Wolf-Rayet stars on the sphere affords further evidence of a connection. We saw. that the novæ are nearly all in the Milky Way. The irregular, ring, planetary and stellar nebulæ, plotted in Fig. 27, prefer the Milky Way, but not so markedly. The Wolf-Rayets, without exception, are located in the Milky Way and in the Magellanic Clouds, and those in

the Milky Way are remarkably near to its central plane. 107 of these objects are known, 1 is in the Lesser Magellanic Cloud, and 21 are in the Greater Magellanic Cloud. The remaining 85 average less than 2 3/4° from the central plane of the Milky Way.

We are obliged to say that the places of the novæ, of the planetary and stellar nebulæ, and of the Wolf-Rayets in the evolutionary process

are not certainly known. If the Wolf-Rayet stars have developed from the planetaries, the planetaries from the novæ, and the novæ have resulted from the close approach or collision of two stars, or from the rushing of a dark or faint star through a resisting medium, then the novæ, planetaries and Wolf-Rayets belong to a new and second generation: they were born under exceptional conditions. The velocities of the planetary nebulæ seem to be an insuperable difficulty in the way of placing them between the irregular nebulæ and the helium stars. The average radial velocity of 47 planetary nebulæ is about 45 km. per second; and, if the motions of the planetaries are somewhat at random, their average velocities in space are twice as great, or 90 km. per second. This is fully seven times the average velocity of the helium stars, and the helium stars in general, therefore, could not have come from planetary nebulæ. The radial velocities of only three Wolf-Rayet stars have been observed, and this number is too small to have statistical value, but the average for the three is several times as high as the average for the helium stars. We can not say, I think, that the velocities of any novæ are certainly known.

If the planetaries have been formed from novæ, especially the novæ which encountered the fiercest resistance, the high velocities are in a sense not surprising, for those stars which travel with abnormally high speeds are the ones whose chances for collisions with resisting media are best; and, further, the higher the speeds of collision the more violent the disturbance. This line of argument also leads to the conclusion that the novæ, planetaries and Wolf-Rayets belong not in general before the helium stars, but to another generation of stars. They may, and I think will, develop into a small class of helium stars having special characteristics; for example, high velocities.