538 



SCIENCE 



[N. S. Vol. XLV. No. 1169 



the proof of this ; but the chances would ap- 

 pear to be strong that the ancestor of our 

 solar system was a mass, nebulous or other- 

 wise, comparatively diminutive in size. 



There seems to be no reason to doubt the 

 value of Herschel's opinion that a plane- 

 tary nebula will develop into a star. La- 

 place 's hypothesis that our solar system has 



Fig. 27. Visual Image (right end) and Part of 

 Visual Spectrum (left end) of the Planetary Neb- 

 ula N.G.C., 418. 



[The horizontal line in the left half of the fig- 

 ure is a part of the continuous spectrum of the 

 stellar nucleus of the nebula at the right end. Of 

 the three white circles in the spectrum the left one 

 is the hydrogen Beta image of the nebula 14 sec. 

 of arc in diameter, the middle circle is the ' ' second 

 nebulium" green image of the nebula 9 sec. of 

 are in diameter; and the right one is the "first 

 nebulium" green image of the nebula, 11 sec. of 

 arc in diameter.] 



developed from a small rotating nebula is 

 still exceedingly valuable, but it has been 

 so buffeted bj^ the winds and waves of criti- 

 cism that many of the details have had to 

 be thrown overboard. I think we should 

 give ourselves some assurance that certain 

 of the planetary nebulae may develop into 

 systems bearing many resemblances to our 

 solar system. Campbell and Moore have 

 been able in the past year and a half to 

 prove, by means of the spectrograph (see 

 Figs. 26a and 26h), that these bodies are in 

 rotation — just as we should expect them to 

 be from their more or less symmetrical 

 forms — around axes passing through their 

 centers; these axes, in general, at right 

 angles to their longest dimensions. The ro- 

 tation of our sun and the revolution of our 

 planets about the sun, all in one and the 

 same direction and very nearly, in the prin- 



cipal plane of the system, afford a close 

 analogy. 



The rotational velocities of the gases 

 composing the principal rings in the plane- 

 tary nebulfe are comparable with the orbital 

 speeds of our great planets, Jupiter, Sat- 

 urn, Uranus and Neptune; and the rota- 

 tional speeds of the gases are slower and 

 slower as we go out from the principal 

 rings, which is true of the orbital speeds of 

 the planets of our system. 



It appears that there is very little ma- 

 terial between the stellar nuclei and the 

 principal rings of the planetaries. The 

 material which we should normally expect 

 to find there has apparently been drawn 

 into the central stars. Very little material 

 is left in that space to condense into plan- 

 ets, just as in our solar system the four 

 inner planets, Mercury, Venus, earth and 

 Mars, and the many asteroids, are of almost 

 negligible mass. Jupiter and the other 

 three outer planets contain 225 times as 

 much matter as the earth and the other 

 three inner planets and the more than 800 

 asteroids. 



If we assume that the rotational speeds 

 of the particles composing the rings and 

 other outer structure of the planetary 

 nebulas are controlled by Newton's law of 

 gravitation, we have means of estimating 

 the masses of their central nuclei, or stars. 

 The indications are extremely strong that 

 the planetary nebuls are in general at least 

 as massive as our solar system : many times 

 as massive in some cases, possibly less mas- 

 sive in others. In them we seem to have 

 enough materials to develop into systems 

 comparable in mass with our solar system. 



More than twenty years ago I observed 

 that the different gases composing several 

 of the nebulas are not uniformly distributed 

 throughout the nebular structure. The hy- 

 drogen in some of the nebulas that were 



