NEBULAR HYPOTHESIS 



NEBULAR HYPOTHESIS 



logucd by Messier in 1771, to en 



al. ! linn ti> ilMnr'ii! -li tin-in from 

 possible new comet*. Afterwards 

 tin- llorsohels catalogued 5,000 of 

 tin-in. I. ut n many "f the so-called 

 nebulae- were rem>lved into star 

 i-lu.sii-rs by the greater telescope* 

 that it was believed all nebulae were 

 assemblages of very distant stars. 

 8pectroscopy disproved tin* 

 .i>.-iiiii|)tii>n in the year of the pub- 

 lieatuxi of the Herochcls' catalogue, 

 \vlien Hii'j'jm^ shuwed, in 1864, 

 that tin- I raco nebula was a mass 

 of ineamlescent gases which ap- 

 peared to him to be hydrogen 

 ami nitrogen. By 1868 he had 

 examined the spectra of about 

 7u nebulae, of which a third 

 displayed a gaseous character. 

 Sinci- then opinion has oscillated, 

 and the application of the spectro- 

 M-t.pe to nebulae, by revealing 

 many differences in the character 

 of their spectra, has evoked the 

 spei -illation that some of them 

 whic-h have a doubtfully gaseous 

 character may in fact be groups of 

 stars outside the stellar universe 

 external universes. All the known 

 nelitiliie are too far away to 

 enable any estimate to be made 

 of their distance, a circumstance 

 which has favoured the foregoing 

 speculation. The belief that all 

 toe nebulae are 

 comprised within 

 the boundaries of 

 the Milky Way is 

 founded on three 

 considerations. The 

 first is on the 

 nature of nebulae 

 as disclosed by the 

 spectroscope ; the 

 second upon their 

 associations with 

 neighbouring 

 stars ; the third on 

 their systematic 

 arrangement as 

 compared with the 

 systematic arrange- 

 ment of the stars. 



Nebula. Top, right, spiral nebula in Ursa Major, from 

 ida - a telescopic photograph by 0. W. Ritchey ; below, the 

 t ions, from formless great irregular nebula in Orion, from a photograph 

 masses of gas many taken at the Lick Observatory 



hundreds of millions of miles in ceeded in recent years through 

 linear dimension, to what are the aid of photography; and this 

 called planetary nebulae, and to progress began with the remarkable 

 the nebulae (or nebulosity) sur- photographs which were made by 

 rounding stars, as in the familiar J. E. Keeler at the Lack Observa- 

 example of the Pleiades. There are 

 irregular nebulae, ring nebulae, 

 and elliptical nebulae, spiral nebu- 

 lae, and planetary nebulae. Their 

 names suggest their variety of 

 shapes the ring nebulae in Lyra, 



a star aa a nucleus, 

 and one such 

 nebula has a spiral 

 form. Almost 

 every kind of 

 nebula suggests at 

 some stage of de- 

 velopment an 

 evolution passing 

 through the stages 

 of a mass of gas, a 

 whirlpool million, 

 a spiral, to a con- 

 densation into a 

 planetary or solar 

 system. 



Some hundreds 

 of thousands of 

 nebulae have been 

 discovered, and 

 they may be 

 grouped into two 

 great classes, 

 elliptical (to which 

 spirals belong) and 

 irregular nebulae. 

 The irregular 

 nebulae are un- 

 mistakably gas- 

 eous ; and most 

 gaseous nebulae 

 are distinguished 

 by a blue or green- 

 ish tinge. Investi- 

 gation has pro- 



the crab, the well-known dumb-bell 

 nebula in Vulpecula, the key-hole, 

 the fish-mouth, the spider, and 

 the whirlpool in Canes Venatici. 

 Planetary nebulae have generally 



tory, from 1898 to 1900. Keeler's 

 photographs led him to the opinion 

 that half the nebulae of the sky 

 were spiral in form. Of these 

 spirals the greatest is the Andro- 

 meda nebula, as the nebula in Orion 

 is the greatest of the irregular 

 nebulae. 



In Jan., 1921, Dr. V. M, Slipher, 

 of the Lowell Observatory, at 

 Flagstaff, Arizona, announced as 



the result of his spectographic ob- 

 servations there, that the nebula 

 Dreyer, No. 584, in the constella- 

 tion of Cetus, was dashing through 

 space away from the earth at a 

 velocity of 1,240 m. a second, the 

 highest speed ever attained by a 

 celestial object. See Andromeda ; 

 Astronomy ; Milky Way ; Stars. 



Nebular Hypothesis. In astro- 

 nomy, a theory to account for the 

 origin of the planetary system. 

 First suggested by Immanuel Kant, 

 in 1755, and placed on a more 

 definite basis by Laplace, the 

 latter supposed that the matter 

 which now fonns the sun, planets, 

 and satellites, existed once in the 

 state of gas, and that this gaseous 

 mass formed a vast globe which 

 extended from the sun's present 

 position as a centre out to, or 

 beyond, the orbit of Neptune, and 

 that this gaseous mass was rotat- 

 ing. As it rotated it gradually 

 flattened, its particles consolidated, 

 and its speed of rotation increased. 



Under the action of increasing 

 speed of rotation and flattening, 

 some of the gaseous matter would 

 be detached from the present mass 

 in the form of a ring. This ring 

 would break up into separate 

 globular masses which would ulti- 

 mately coalesce in the largest of 

 them and thus form the first, and 

 outermost, planet, and so for other 

 planets. See Planet. 



