DIFFUSE MATTER IN SPACE 
389 
Planetary Nebulae. 
264. Planetary nebulae are similar to diffuse nebulae in many respects, 
but they are of more regular shape, they surround a single star, and they 
are of much smaller absolute dimensions. They give bright line spectra 
accompanied by a certain amount of continuous spectrum. There is not 
much difference between the spectra of diffuse and planetary nebulae, 
the latter differing among themselves quite as much as they differ from 
the diffuse nebulae. 
The central star is always faint, a great deal of its light being doubtless 
lost through the continuous absorption of the nebula. The spectrum is 
very rich in ultra-violet light, and the stars capable of supporting this 
nebulous appendage are probably limited to type 0. 
The annular nebulae are a characteristic and interesting type of 
planetary nebulae. 
Spectroscopic measurements of radial velocity have demonstrated the 
rotation of several planetary nebulae including the Ring Nebula in Lyra. 
It is not a rigid body rotation, but diminishes outwards as orbital revolu 
tion would do. It may be assumed that rotation plays an essential part 
in the phenomenon. It looks as though the nebulous matter cannot have 
been expelled from the central star (by radiation pressure or otherwise) 
since this would not possess enough angular momentum. But on the other 
hand, in the Novae we observe matter, which certainly seems to be ex 
pelled from a star, acquiring rotation or at any rate large transverse 
velocity. The fact is that rotation of celestial objects is altogether mys 
terious, and we really know no adequate cause for the almost universal 
prevalence of rapid rotation*. 
The observed spectrum points to a high degree of ionisation. The 
elements detected are H, He, He + and probably C +++ ; in addition, there 
are numerous unidentified lines including the prominent doublet of 
nebulium. Ca + is not shown so that presumably calcium is triply ionised. 
On the other hand, the occurrence of the He spectrum shows that helium 
is not doubly ionised. As the second ionisation potential of helium is 
* [The view that the nebula consists of matter left behind in the course of stellar 
condensation seems to be inadmissible, because a tenuous structure of this kind 
could not be propelled through the interstellar cloud at 100 km. per sec. without 
suffering rapid change and dissipation. We feel bound therefore to admit replenish 
ment both of the material and of the angular momentum by emission from the 
central star. Failing any other explanation we may perhaps invoke the magnetic 
field of the star. If ionised material is streaming out in the equatorial plane the ions 
will acquire angular momentum of one sign and the electrons angular momentum 
of the opposite sign about the magnetic axis of the star. A weak field of the same 
order as the sun’s general magnetic field is quite sufficient to produce the required 
transverse velocity. This explanation might apply also to the transverse velocities 
observed in Novae. It is difficult to develop a detailed explanation on these lines 
and the idea is at present a vague conjecture.]