Fig. 2. Planetary nebula N.G.C. 7009 (composite drawing, from Curtis’s photographs of the nebula made with the Crossley reflecting telescope. The scale is in seconds of arc).
From Proceedings of the National Academy of Sciences of U. S. A.
Interesting observations have been presented recently also with reference to the largest among the irregular nebulæ, namely the Orion nebula. Three astronomers in Marseilles, Bourget, Fabry, and Buisson, found that parts of this nebula, in the neighbourhood of the so-called trapeze and very close to each other, moved with different velocities and that this difference might amount to 10 km. (6.2 miles) per second. The south-easterly part approaches us while the north-easterly recedes from us. Consequently a violent whirl-motion undoubtedly takes place in this region. This observation has been verified by the well-known Chicago astronomer, Frost, who employed a different method of investigation than his predecessors. He noted differences in velocity amounting to 11 km. (6.8 miles) per second between points not over two seconds of arc distant from the trapeze.
If therefore we say that the irregular nebulæ on the average possess no motion, this statement does not preclude important local deviations from the rule within the nebulæ, intimating a transformation which probably leads to the concentration of the nebulous matter toward the centre of the whirl.
Leaving out, to begin with, the planetary nebulæ, it appears that the original matter of the stars stands still in space, that their average velocity increases with increasing age and approaches a mean value of about 18 km. per second or roughly 1000 times the speed of the ordinary passenger train. Our Sun, in particular, moves with a velocity of 20 km. (12.4 miles) per second toward a point in the constellation Hercules 30 degrees north of the equator.
What force then shall we say it is that causes the motion of the stars? As far as we know none but gravitation. It appears therefore as if the gaseous primeval substance of the stars were not governed by this force. It might prove hazardous, however, to make this assumption as gases also possess weight and even the most rarefied strata of the Earth’s atmosphere exert barometric pressure by virtue of their attraction to the mass of the earth. Rather the immobility of the nebulæ is due to the frequent collisions between the molecules in any quantity of gas even if it be attenuated to such a high degree as in the nebulæ. Thus, the molecules strike a balance, as it were, against each other so that the different parts of the gas accumulations shortly are brought to rest relative to each other. The irregular gas mists around the Milky Way form therefore a continuous whole. A different condition obtains in regard to condensed stellar bodies such as the stars. They may in the densest throng move during billions of years before they collide; but they might on the other hand enter nebulous masses and thereby suffer gradual retardation. We now refer to stars moving outside of the vapour clouds. They are therefore unrestricted and the longer they have obeyed gravitation without impeding encounters with nebulous matter, in other words the longer the time elapsed since they emerged from the gas accumulations which gave them birth, the swifter is their motion. Their (average) velocity can of course not exceed a certain limit which in our parts of the universe appears to be about 18 km. (11.2 miles) per second. Campbell’s measurements show that for the youngest stars (all except the red) the velocity is greatest in the plane of the Milky Way, a natural enough condition as the attracting matter here is most abundant.
The planetary nebulæ possess a greater velocity although they, as consisting of mist vapours, are in the first stage of evolution. Faster yet do the spiral nebulæ move according to measurements by Wolf of Heidelberg. This shows that they are of a different nature from the irregular nebulæ, which form the matrix of the Milky Way. A closer examination of the few—thirteen in all—planetary nebulæ, determined by the American astronomer Keeler, convinced me that they approach the Galaxy from its poles with a moderate speed, and subsequently under the influence of its attraction curve their orbit, rapidly gain in velocity, and finally rush into the nearest part of the Milky Way with a very high speed.
A great number of them are no doubt caught in the mists or star-throngs of the Milky Way after exposure to numerous collisions and sweeping away all matter in their course. Such clean-swept traces are very common in the area of the Milky Way. One of the most beautiful examples is the so-called Cocoon nebula in the constellation Cygnus (the Swan). It has left in its wake a dark rift, in whose bottom, however, exceedingly small and evidently very remote stars are visible according to the German astronomer Wolf (see Worlds in the Making, page 172, Fig. 55).
The great mean velocity of the planetary nebulæ indicates that they originally did not belong to the Galactic system, a conclusion also reached by Bohlin, but for other reasons. They are nevertheless more abundant in the neighbourhood of the Milky Way than in other parts of the heavens. This fact, if viewed superficially, might lead to the belief that they are indigenous to the Galactic system, but is explained by their concentration in obedience to gravitation toward the Milky Way.
Quite recently (1917) Van Maanen determined the distance of one of these highly interesting celestial bodies, tabulated in the New General Catalogue as No. 7662. Its distance was found to be only about 140 light years. This is about sixteen times the distance of Sirius and the mean distance of a star of the fifth magnitude. This circumstance agrees very well with the idea that this nebula is captured by the Galactic system to which it has approached from very distant parts of the space outside of the Galactic system.