Nebulae 
27 
27-29] 
points of light. The greatest angle which the diameter of any star subtends 
at the earth is (V056", which is the angle subtended by a pin-head at a 
distance of two miles, and is far too small to shew a finite disc in any tele 
scope which has yet been constructed. 
Apart from the planets, all the objects we have so far discussed in detail 
have consisted of stars and groups of stars, forming the class of objects which 
shew as points of light in the telescope. There is a second class of objects 
which appear in the telescope as areas of finite size. These are generally 
and somewhat unfortunately bundled together under the general name of 
“nebulae.” 
Planetary Nebulae. 
29 . We may mention first the comparatively unimportant class of objects 
known as “planetary nebulae.” The name is doubly unfortunate since the 
objects are not nebulae in any strict sense, and are not planetary in any sense 
at all, except that of shewing a disc of planetary size in the telescope. 
Planetary nebulae are comparatively rare, only 150 out of 15,000 nebulae 
investigated by Campbell proving to be “planetary.” In general they are of 
an apparently spheroidal or ellipsoidal shape, many shewing additional features 
and details of formation. Typical examples are shewn in Plate IV; the ring- 
shaped nebulae, such as N.G.C. 6720 (the “ring nebula” in Lyra)are probably 
ellipsoidal shells which are so transparent as only to be visible where the line 
of sight passes through a considerable thickness of the shell. 
Many of the planetary nebulae are near enough for their distances to be 
estimated by the direct trigonometrical method. That of N.G.C. 7662, for 
instance, is found by Van Maanen* to be O023", from which its diameter 
may be calculated to.be 19 times that of Neptune’s orbit. Van Maanenf and 
Newkirk^ agree in estimating that the ring nebula N.G.C. 6720 is considerably 
more distant and substantially larger in linear dimensions. Every planetary 
nebula shews a star at its geometrical centre, and when the distance of a 
nebula is known, the luminosity of its central star can at once be calculated. 
The luminosities of the central stars of the planetary nebulae prove to be 
well below that of the average star; few, for instance, are as intrinsically 
bright as our sun. , 
The planetary nebulae lie within the galactic system of stars, confirmation 
of this being found in the circumstance that they are found mainly in directions 
near the galactic plane. They appear to be rather ordinary stars which have 
in some way become surrounded by an atmosphere or shell of gas of such 
enormous dimensions as to exhibit finite discs in the telescope. Their velocities 
of motion are rather above those of average stars: Keeler found the average 
radial velocity of 13 to be 27*7 kms. a second, or 26'8 kms. a second after 
* Proc. Nat. Acad. Sci. nr. (1917), p. 133. f Publ. Astr. Soc. Pac. xxix. (1917), p. 209. 
+ Lick Obs. Bull. ix. (1917), p. 100.