September 8, 1923J 



NA TURE 



175 



The Gaseous Nebulae.^ 

 By J. H. Reynolds. 



I 



T has been recognised for many years that the 

 nebulae fall into two great divisions — the spirals 

 and the gaseous and diffuse. The distinction between 

 the two is fundamental, for there can be no doubt 

 now that the spirals are extra-galactic, and the 

 gaseous inter-galactic formations, although it is im- 

 possible yet to define the scale and distances of the 

 spirals with any certainty. Since Huggins's great 

 discovery of the gaseous nature of certain nebulae, 

 the principal work on these objects has been done in 

 America, especially at the Lick Observatory and at 

 Mt. Wilson ; and coupling up these investigations with 

 recent advances in physics in which Great Britain has 

 played no mean part, we are able to form a good 

 general idea of the meaning and origin of the gaseous 

 nebulae. 



The first important fact which emerges from the 

 physical work of Fowler, Saha and others, is that 

 only the elements of simplest constitution, such as 

 hydrogen and helium, are known to be present, and 

 that ionisation of known elements is probably re- 

 sponsible for all the unknown lines in the nebular 

 spectrum. Then, again, the gaseous nebulae are only 

 associated with stars of the highest temperature, as 

 the stars in the centre of the planetaries are usually 

 " O " type stars of the Harvard scale, which yield 

 an emission or absorption spectrum of ionised helium 

 on a continuous background, and the " B " type 

 stars, which are next in order of temperature, and are 

 associated with the irregitlar gaseous nebulae in Orion. 

 Until recently it was supposed that the so-called 

 " nebulium " identified with the nebular radiations 

 at XX5007 and 4959, and other well-known lines such 

 as XX4363, 4686 and 3727, represented unknown gases 

 in the same sense that helium was unknown until 

 identified in the laboratory by Sir William Ramsay, 

 but the work of Moseley and Aston and the formation 

 of the scale of atomic numbers has taught us that 

 there is no room for any more unknown elements in 

 the atomic scale of the lighter gases, and we must 

 look in the direction of ionisation of the known 

 elements in the first ten numbers of the scale for 

 these unknown nebular lines. 



These considerations and the progressive spectra of 

 Novae lead to the conclusion that in all gaseous 

 nebulae we are dealing with the same material, and 

 that the differences found in the spectra are to be 

 assigned to differences only in physical conditions, 

 iiid the key to these conditions is to be found in the 

 Itective radiation of the star or stars involved in the 

 nebula. Only the first two numbers of the atomic 

 scale are definitely established as appearing in nebular 

 spectra, the helium appearing often near the nuclear 

 star in its ionised form. But Wright in Lick Observa- 

 tory Pub., vol. xiii., provisionally identified certain 

 nebular lines with carbon and nitrogen, and the 

 presence of the former at any rate is now accepted. 



The principal nebular lines of unknown origin are 

 the following : 



x^g^^l'^Mssa. 4340. 3868, 3727-9. 



The first two are invariably found in all the gaseous 

 nebulae in the same relative strength, and they 

 evidently form a doublet. X5oo7 was the first gaseous 

 radiation to be discovered in a nebula, and with its 

 ompanion it exists in a very marked form in nearly 

 all the objects of this class yet observed. The other 



' Substance of an address delivered before the Binningham University 



Physical Society on March 14. 1923. 



NO. 2810, VOL. 112] 



radiations vary much in relative strength, and some- 

 times seem to be absent altogether. Perhaps the 

 most remarkable is that usually described as X3727, 

 which has been found by W^right to be in reality a 

 doublet separated by only two Angstrom units. It 

 is astonishingly brilliant in the Orion Nebula, and 

 with the hydrogen radiations it is responsible for the 

 great photographic effect of this object. It is always 

 of maximum extension, and is found in regions far 

 removed from the involved stars, where the hydrogen 

 radiations are comparatively faint. An investigation 

 by means of screened direct photographs was under- 

 taken by the writer some years ago as to the relative 

 distribution of the doublet X(5007, 4959), the hydrogen 

 series, and X3727 in the Orion Nebula. The principal 

 results were : 



(i) To establish Keeler's conclusion that the doublet 

 X(5oo7, 4959) was conspicuous only in the central 

 region surrounding B Orionis, where it was very 

 brilliant. 



(2) To demonstrate that the hydrogen radiations 

 extended faintly to the most remote regions of the 

 nebula, and 



(3) To show that the radiation X3727 was much 

 stronger than the hydrogen in these outer regions, 

 besides giving certain differences in detail. 



We have then in the Orion Nebula a bright central 

 portion where the radiations X(5oo7, 4959), the 

 hydrogen and helium radiations and X3727 are in- 

 tegrated together, an intermediate region where 

 hydrogen and X3727 appear of about the same 

 strength, and an outer region where X3727 pre- 

 dominates. There is no difficulty in ascribing X(5oo7, 

 4959) probably to an ionised form of one of the 

 elements heavier than helium, as we should expect 

 the heavier elements to predominate in the central 

 regions surrounding the star Q and the other involved 

 stars. There is, however, very great difficulty in 

 explaining the distribution of X3727. It must be 

 remembered that the " B " type stars involved in the 

 nebula are not of the highest stellar temperature, and 

 it is almost impossible to imagine the existence of 

 ionisation at all in these remote regions. There is, 

 indeed, good reason for thinking that X3727 is a 

 comparatively low temperature radiation, as it is 

 either faint or absent altogether in the planetaries 

 containing " O " type stars, and it does not occur so 

 far as can be ascertained in the nebular stages of 

 Novae. 



An interesting point bearing on the distribution of 

 this radiation is brought out by an examination of 

 the nebulosity surrounding the star Bond 734 to the 

 north of the principal nebula. In the screened ex- 

 posure of the radiations X(5oo7, 4959) no nebulosity 

 appears round this star, and the absence of these 

 radiations is confirmed in Lick Observatory Pub., 

 vol. xiii. The hydrogen image is quite strong, and 

 is accompanied by an equally strong image in X3727. 

 It is quite evident, therefore, that there is no direct 

 relation between the radiations X(5oo7, 4959) and 

 X3727, and it is known that the hydrogen radiations 

 occur without either. But the question arises as to 

 whether X3727 can exist independently of hydrogen. 

 The differences of detail visible in the photographic 

 images given by hydrogen and X3727 certainly suggest 

 an independent origin, but a complete answer to the 

 question is impossible, as hydrogen is found in varying 

 strength in all the gaseous nebulae and nova;. If it 

 were not an impossibility, one would naturally look 

 for the radiation as some element actually lighter 



