Some small nebulæ have been found in the vicinity of the globular clusters. They are described by Prof. Perrine as very small and like an “out of focus” image of a small star. “They appear to be most numerous about clusters which are farthest from the galaxy.” Prof. Perrine says, “Practically all the small nebulæ about the globular clusters are elliptical or circular. Those large enough to show structure are spirals. Doubtless the majority of these are spirals.”[354] This seems further evidence in favour of the “spiral nebular hypothesis” of Chamberlin and Moulton.
A great photographic nebula in Orion was discovered by Prof. Barnard in 1894. In a drawing he gives of the nebula,[355] it forms a long streak beginning a little south of γ Orionis (Bellatrix), passing through the star 38 Orionis north of 51 and south of 56 and 60 Orionis. Then turning south it sweeps round a little north of κ Orionis; then over 29 Orionis, and ends a little to the west of η Orionis. There is an outside patch west of Rigel. Barnard thinks that the whole forms a vast spiral structure; probably connected with the “great nebula” in the “sword of Orion,” which it surrounds.
From calculations of the brightness of surface (“intrinsic brightness”) of several “planetary” nebulæ made by the present writer in the year 1905, he finds that the luminosity is very small compared with that of the moon. The brightest of those examined (h 3365, in the southern hemisphere, near the Southern Cross) has a surface luminosity of only 1⁄400 of that of the moon.[356] The great nebulæ in Orion and Andromeda seem to have “still smaller intrinsic brightness.”
Arago says—
“The spaces which precede or which follow simple nebulæ, and a fortiori groups of nebulæ, contain generally few stars. Herschel found this rule to be invariable. Thus every time that, during a short interval, no star appeared, in virtue of the diurnal motion, to place itself in the field of his motionless telescope, he was accustomed to say to the secretary who assisted him (Miss Caroline Herschel), ‘Prepare to write; nebulæ are about to arrive.’”[357]
Commenting on this remark of Arago, the late Herbert Spencer says—
“How does this fact consist with the hypothesis that nebulæ are remote galaxies? If there were but one nebula, it would be a curious coincidence were this one nebula so placed in the distant regions of space as to agree in direction with a starless spot in our sidereal system! If there were but two nebulæ, and both were so placed, the coincidence would be excessively strange. What shall we say on finding that they are habitually so placed? (the last five words replace some that are possibly a little too strong).... When to the fact that the general mass of nebulæ are antithetical in position to the general mass of the stars, we add the fact that local regions of nebulæ are regions where stars are scarce, and the further fact that single nebulæ are habitually found in comparatively starless spots, does not the proof of a physical connection become overwhelming?”[358]
With reference to the small elongated nebula discovered by Miss Caroline Herschel in 1783 near the great nebula in Andromeda, Admiral Smyth says, “It lies between two sets of stars, consisting of four each, and each disposed like the figure 7, the preceding group being the smallest.”[359]
Speaking of the “nebula” Messier 3—a globular cluster in Canes Venatici—Admiral Smyth says, “This mass is one of those balls of compact and wedged stars whose laws of aggregation it is so impossible to assign; but the rotundity of the figure gives full indication of some general attractive bond of union.”[360] The terms “compact and wedged” are, however, too strong, for we know that in the globular clusters the component stars must be separated from each other by millions of miles!
Prof. Chamberlin suggests that the secondary nebula (as it is called) in the great spiral in Canes Venatici (Messier 51) may possibly represent the body which collided with the other (the chief nucleus) in a grazing collision, and is now escaping. He considers this secondary body to have been “a dead sun”—that is, a dark body.[361] This would be very interesting if it could be proved. But it seems to me more probable that the secondary nucleus is simply a larger portion of the ejected matter, which is now being gradually detached from the parent mass.