Nebula M. 101 Ursæ Majoris—after Ritchey.
The form of the spiral nebulæ proclaims their motion, but one of its particular features discloses more. For it implies the past cause which set this motion going. A distinctive detail of these spirals, which so far as we know is shared by all of them, are the two arms which leave the centre from diametrically opposite sides. This indicates that the outward driving force acted only in two places, the one the antipodes of the other. Now what kind of force is capable of this peculiar effect? If we think of the matter, we shall realize that tidal action would produce just this result. We see it daily in the case of the Moon; when it is high tide in the open ocean hereabouts, it is high tide also at the opposite end of the Earth. The reason is that the tideraising body pulls the fluid nearest it more strongly than it pulls the Earth as a whole, and pulls the Earth as a whole more than it pulls the fluid at the opposite extremity.
Suppose, now, a stranger to approach a body in space near enough; it will inevitably raise tides in the other’s mass, and if the approach be very close, the tides will be so great as to tear the body in pieces along the line due to their action; that is, parts of the body will be separated from the main mass in two antipodal directions. This is precisely what we see in the spiral nebula. Nor is there any other action that we know of which would thus handle the body. If it were to disintegrate under increased speed of rotation due to contraction upon itself, parts of its periphery should be shed continually and a pin-wheel of matter, not a two-armed spiral, be thrown off. If explosion were the disintegrating cause, disruption would occur unsymmetrically in one or more directions, not symmetrically as here.
REPRESENTATIVE STELLAR SPECTRA
Photographed, in 1907 and 1908, by V. M. SLIPHER, at LOWELL OBSERVATORY
Flagstaff, Arizona, with prism spectrograph.
As the stranger passed on, his effect would diminish until his attraction no longer overbalanced that of the body for its disrupted portions. These might then be controlled and forced to move in elliptic orbits about the mass of which they had originally made part. Thence would come into being a solar system, the knots in the nebula going to form the planets that were to be.
Before proceeding to what proof we have that it actually did occur in this way we may pause to consider some consequences of what we have already learned. Thus what brought about the beginning of the system may also compass its end. If one random encounter took place in the past, a second is as likely to occur in the future. Another celestial body may any day run into the Sun, and it is to a dark body that we must look for such destruction, because they are so much more numerous in space.
That any of the lucent stars, the stars commonly so called, could collide with the Sun, or come near enough to amount to the same thing, is demonstrably impossible for æons of years. But this is far from the case for a dark star. Such a body might well be within a hundredth of the distance of the nearest of our known neighbors, Alpha Centauri, at the present moment without our being aware of it at all. Our senses could only be cognizant of its proximity by the borrowed light it reflected from our own Sun. Dark in itself, our own head-lights alone would show it up when close upon us. It would loom out of the void thus suddenly before the crash.