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.]