We are obliged to say that the places of the novae, of the

planetary and stellar nebulae, and of the Wolf-Rayets in the
evolutionary process are not certainly known
We are obliged to say that the places of the novae, of the
planetary and stellar nebulae, 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 novae, and the novae 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 novae,
planetaries and Wolf-Rayets belong to a new and second
generation: they were born under exceptional conditions. The
velocities of the planetary nebulae seem to be an insuperable
difficulty in the way of placing them between the irregular
nebulae and the helium stars. The average radial velocity of 47
planetary nebulae 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 nebulae. 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
novae are certainly known.


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