November 1, 1895.] 



KNOWLEDGE. 



2- 



Thus the bright lines at the maximum are not the effect 

 of heat, but rather of cooling, and so we comprehend 

 how it is possible for bright lines to be mostly observed in 

 the atmospheres of those stars which, according to the 

 evidence of their spectra, are to be accounted the coolest. 



Now we comprehend also why both the redness of a star 

 and its bright lines must lead to the suspicion of its vari- 

 ability, and we are not at all surprised that numerous 

 variables have been recently discovered by testing that 

 probability." It is clear, moreover, why neither red 

 stars nor stars with bright Imes are co ipm variable. Only 

 these stars will be variable where some vapours in the 

 external atmosphere are cooled to their dew-point, and sd 

 are made ready to condense, with the smallest radiation 

 of heat, in obscuring clouds. Stars which are not variable 

 (1-20 Sehj, and D.M. + 47", No. 2291) may have exactly 

 the same spectrum as variables (X Cygni and E Leoais) ; 

 for it is impossible to admit that identity of spectrum 

 proves exact identity both of chemical composition and of 

 temperature. Even a faU of temperature of 1" would be 

 sufficient, by reaching the dew point of some principal 

 atmospheric vapour, to make a star, hitherto invariable, 

 henceforth variable. 



Bright lines point, even in stars of the classes I and II, 

 to great chemical activity in the cooling atmospheres of 

 these stars, so they do also in our sun, which is also more 

 or less a bright line star ; t but in the sun, according to 

 my theory,* the condensed clouds are not yet obscuring 

 clouds, but still photospheric. Their partial evaporation 

 cannot therefore cause an increase of brightness, but 

 produces, on the contrary, dark spots, whose more or less 

 regular periodicity can be explained therefore in the same 

 way as the greater variability of the already more cooled 

 and chemically more active red stars. 



NEBULA M. 78 AND iji IV. 36 ORIONIS. 



By Isaac Eobeets, D.Sc, F.E.S. 

 E.A. oh. 41m., Decl. N. 0° 1'. 



THE photograph covers the region between E.A. 

 5h. 39m. 2l3. and E.A. 5h. 44m. 193. ; Decl., 

 between 0" 59' North, and 0' 37' South. 

 Scale, 1 millimetre to twenty-four seconds of arc. 

 Co-ordinates for the epoch a.d. 1900 of the 

 Fiducial stars marked with dots. 



Star (•)• D-M., No. 1073. Zone - 0°. E.A., 

 5h. 39m. 41-93. Decl., S. 0= 5'9'. Mag., 91. 



Star (••)• D-M., No. 1178. Zone + 0'. E.A., 

 5h. 41m. 45-4s. Decl., N. 0° .5.5-8'. Mag., 88. 



Star (vj. D.M., No. 1084. Zone -f 0^ E.A., 

 5h. 42m. 21-63. Decl., S. 0" 31-0'. Mag., 9-5. 



Star {::). D.M., No. 1184. Zone -f 0^ E.A., 

 5h. 43m. 37-8s. Decl., N. 0' 42-2'. Mag.. 7-5. 



The photograph was taken with the 20-inch reflector on 



•lanuary 28th, 1894, between sidereal time 3h. 3m. and 



6h. om., with an exposm-e of the plate during three hours. 



Eeferences. 



The nebuliE are N. G. C. Nos. 2068 and 2071, G. C. 



Nos. 1267 and 1270, h 368. Sir J. Herschel (G. C.) 



* E C. Pickering : Astrophys. Journal, I., p. 27, " Discovery of 

 Variable Stars from their Ptiotographic Spectra." M. Flemini; : 

 Astrophysic. Journal, I., p. 411, "Eleven Xew Variable Stars." 

 Backhouse : Observatory, Febr., 95. 



t G. E. Uale : A. and A., 1894. p. 58 (Xote on the Spectroscopy 

 at Stonyhurst College Observatory). Miss A. M. Clerke': 

 KyowLEDOE, Aug., 93, " The Sun as a Bright-line Star." 



X Theorie du Soleil. Verh. d. Koniukl. Akad. v. "Wet. le 

 Amsterdam Decl., I., Xo. 3, pp., 1—168. A. and A., 1893, p. 914 ; 

 1894, pp. 218—237, pp. 849—856. 



describes M. 78 as bright, large, wispish, very gradually 

 much brighter in the middle, three stars involved, barely 

 resolvable. In the Pliil. Trans., 1833, PI. XII., Fig. 36, 

 a drawing of it is given which, in outline, resembles the 

 central part of the nebulosity. The nebula y IV. 36 

 (N. G. C. No. 2071) he describes as a star with very faint 

 large chi'i-i'Iurc. 



Lord Eosse (Ohs. of AV/-. awl CI. of Stars, p. .51) gives 

 the result of nine observations made between the years 

 1850 and 1877, and states that a " spiral arrangement is 

 sufficiently seen to confirm former observations." A 

 marginal sketch is given, and a drawing on PI. I., Fig. 5 ; 

 but they are not like the photograph, and they fail to 

 indicate the form of the nebula as it is there shown. 



The photograph shows the central part of the nebula as 

 a dense cloud, with a well-deSned margin on the north 

 prececUwi side, and the three stars referred to by Herschel 

 and Eosse are shown involved in the nebulosity. The star 

 on the northern edge has a faint companion, and faint 

 star-like condensations are also seen involved in the 

 nebulosity. There are floceulent clouds of nebulosity 

 surrounding the dense central part, with wide dark spaces 

 between them, which the descriptive matter and the 

 drawings by Sir J. Herschel and Lord Eosse do not indicate. 



The nebula Ijl IV. 36 (G. C. 1270) has a stellar nucleus, 

 with a small companion star close to it on the south side, 

 and this double nucleus is surrounded by cloud-like streaky 

 nebulosity. 



The distance between the centres of the nebulse is about 

 fifteen minutes of arc, and there are indications that the 

 iwo nebulfe are connected together with very faint 

 nebulosity ; but there is no spiral form (as suggested by 

 Lord Eosse) visible in either of them. 



The sky within a radius of about half a degree of these 

 nebulns is remarkably void of stars, but beyond this dis- 

 tance, both precediwi and foUoifiii;/, the stars are crowded 

 on the plate. 



WHAT IS A NEBULA? 



By E. Walter M.\under, F.E.A.S. 



THIS question was asked by the late Editor of 

 Knowledge, some three years ago, and was 

 suilered by him to go without any definite answer. 

 The time that has since elapsed has brought us 

 some details of further information, but still 

 leaves us without any full clue to the solution of the 

 problem. Indeed, if anything, our difficulties are in- 

 creased, for new and widely extended nebulfe have been 

 discovered where none were previously known, and old 

 nebulae prove to have a greater extension than had been 

 hitherto suspected. 



The difficulty attaching to our understanding the nature 

 of a widely-extended nebula, is very obvious. Such an 

 object as the great nebula in Orion, or the new nebula 

 near .\ntares, recently discovered by Mr. E. E. Barnard, 

 or the faint wisps that the same observer has found 

 sweeping round the Pleiades, or the still vaster nebulosities 

 in the Cepheus and Cygnus region of the Milky Way, must 

 either be exceedingly close to us, or must have real 

 dimensions of a vastness that baffles the imagination. The 

 first alternative may at once be set aside. Except in a 

 very few cases, we know that the stars are not nearer to 

 us than is implied by a parallax of one-tenth of a second 

 of arc, or a distance more than two million times greater 

 than that which separates the earth from the sun. And 

 the distances of the nebulse are of the same order as 

 those of the stars. We know this, not only from the 

 numerous examples of evident connection between nebulse 



