NATURE 



[May 3, 1888 



SUGGESTIONS ON THE CLASSIFICATION OF 

 THE VARIOUS SPECIES OF HEAVENLY 

 BODIES. 1 



III. 



III.— SUB-GROUPS AND SPECIES OF GROUP I. 



I. Sub-Group. Nebula. 



TTAVING, in the preceding part of this memoir, 

 -*-- » attempted to give a general idea of that grouping of 

 celestial bodies which in my opinion best accords with our 

 present knowledge, and which has been based upon the 

 assumed meteoric origin of all of them, I now proceed to 

 test the hypothesis further by showing how it bears the 

 strain put upon it when, in addition to furnishing us with 

 a general grouping, it is used to indicate how the groups 

 should be still further divided, and what specific differences 

 may be expected. 



The presence or absence of carbon will divide this 

 group into two main sub-groups. 



The first will contain the nebulae, in which only the 

 spectrum of the meteoric constituents is observed with or 

 without the spectrum of hydrogen added. 



It will also contain those bodies in which the nebula 

 spectrum gets almost masked by a continuous one, such 

 as Comets 1866 and 1867, and the great nebula in 

 Andromeda. 



In the second subgroups will be more condensed swarms 

 still, in which, one by one, new lines are added to the 

 spectra, and carbon makes its appearance ; while probably 

 the last species in this sub group would be bodies repre- 

 sented by 7 Cassiopeiae. 



Species of Nebulce. 



I have elsewhere referred to the extreme difficulty of 

 the spectroscopic discrimination in the case of the meteor- 

 swarms which are just passing from the first stage of con- 

 densation, and it may well be that we shall have to wait 

 for many years before a true spectroscopic classification 

 of the various aggregations which I have indicated, can 

 be made. 



It is clear, then, from what has gone before that in each 

 stage of evolution there will be very various surfaces and 

 loci of collisions in certain parts of all the swarms, and 

 we have already seen that even in the nebulosities dis- 

 covered by Sir Wm. Herschel, which represent possibly 

 a very inchoate condition, there are bright portions here 

 and there. 



If the conditions are such in the highly elaborated 

 swarms and in the nebulosity that the number of collisions 

 in any region per cubic million miles is identical, the 

 spectroscope will give us the same result. In the classifica- 

 tion of the nebulae, therefore, the spectroscope must cede 

 to the telescope when the dynamical laws, which must 

 influence the interior movements of meteoric swarms, have 

 been fully worked out. The spectroscope, however, is 

 certainly at one with the telescope in pointing out that 

 so-called planetary nebulae are among the very earliest 

 forms — those in which the collisions are most restricted in 

 the colliding regions. The colour of these bodies is blue 

 tinged with green ; they do not appear to have that milki- 

 ness which generally attaches to nebulae, and the bright 

 nebulous lines are seen in some cases absolutely without 

 any trace of continuous spectrum. In higher stages the 

 continuous spectrum comes in, and in higher stages still 

 possibly also the bands of carbon ; for in many cases Dr. 

 Huggins in his important observations has recorded the 

 weakness of the spectrum in the red, or in other words 

 the strengthening of the spectrum in the green and blue 

 exactly where the carbon bands lie. 



But in all the bodies of Group I. which possess forms 

 visible to us in the telescope, it would seem proper that 



1 Tht Bakerian Lecture, delivered at the Royal Society on April 12, by 

 J. Norman Lockyer, F.R.S. Continued fiom vol. xxxvii. p. 609. 



their classification should depend mainly — at present at 

 all events — upon their telescopic appearance, and there is 

 very little doubt that a few years' labour with the new 

 point of view in the mind of observers armed with suffi- 

 cient optical power, will enable us to make a tremendous 

 stride in this direction ; but it seems already that this 

 must not be done without spectroscopic aid. For instance, 

 if what I have previously suggested as to the possible 

 origin of the planetary nebulae be accepted, it is clear that 

 in those which give us the purest spectrum of lines, one 

 in which there is the minimum of continuous spectrum, 

 we find the starting-point of the combined telescopic and 

 spectroscopic classification, and the line to be followed 

 will be that in which, cceteris paribus, we get proofs of 

 more and more condensation, and therefore more and 

 more collisions, and therefore higher and higher tempera- 

 tures, and therefore greater complexity in the spectrum 

 until at length true stars are reached. 



When true stars are reached those of the cluster 

 appear nebulous in the telescope in consequence of its 

 distance ; the spectroscope must give us indications by 

 absorption. 



It is not necessary in this connection, therefore, to refer 

 to undoubted star clusters, as the presence of absorption 

 will place them in another group ; but the remark may 

 be made that it is not likely that future research will 

 indicate that new groupings of stars, such as Sir Wm. 

 Herschel suggests in his paper on the breaking up of the 

 Milky Way, will differ in any essential particular from the 

 successive groupings of meteorites which are watched in 

 the nebulae. Space and gravitation being as they are, it 

 is not necessary to assume that any difference of kind 

 need exist in the method of grouping formed stars and 

 meteoric dust ; indeed there is much evidence to the 

 contrary. 



II. Sub-Group. Bright-line Stars. 



It might appear at first sight that the distribution of 

 bright-line stars among various species should be very 

 easy, since a constant rise of temperature should bring 

 out more and more lines, so that the species might be based 

 upon complexity of spectrum merely. 



But this is not so, for the reason that the few observa- 

 tions already recorded, although they point to the existence 

 of carbon bands, do not enable us to say exactly how far 

 the masking process is valid. Hence in the present 

 communication I content myself by giving some details 

 relating to maskings, and the results of the discussions, 

 so far as they have gone, in the case of each star. I 

 shall return to the line of evolution in a later paper. 



Masking of Radiation Effects produced by Variations of 

 Interspacing. 



I have already stated that carbon bands are apt to 

 mask the appearance of other spectral phenomena in 

 the region of the spectrum in which they lie. In this way 

 we can not only account for the apparent absence of the 

 first manganese fluting, while the second one is visible, 

 but it is even possible to use this method to determine 

 which bands of carbon are actually present. There is 

 another kind of masking effect produced in a different 

 way, and this shows itself in connection with sodium. It is 

 well known that when the temperature is low, D is seen 

 alone, and if seen in connection with continuous spec- 

 trum the continuous spectrum is crossed by either dark 

 or bright D, according to the existing circumstances. 



1 showed some years ago that the green line of sodium, 

 not the red one, is really visible when sodium is burned 

 in the bunsen burner. It is, however, very much 

 brighter when higher temperatures are used, although 

 when bright it does not absorb in the way the line D 

 does. 



Now, if we imagine a swarm of meteorites such that in 

 the line of sight the areas of meteorite and interspace are 



