BEYOND THE MILKY WAY — PAGE 169 



of objects.) But other spirals and the elliptical nebulae are found to 

 have diameters much smaller than this ; from 2,000 light-years to 10,000 

 light-years. We are thus forced to the conclusion that our galaxy is a 

 giant among the spiral nebulae. 



How can we "weigh" a nebula consisting of billions of stars? The 

 method is similar to that for determining the mass of the sun ; it de- 

 pends upon measuring the motions of other masses in the vicinity. 

 Since the outer stars in a nebula are attracted by the rest of the nebula, 

 just as the planets are attracted by the sun, they would "fall" into the 

 center were they not in motion in an orbit around that center. The 

 motion is a measure of the gravitational pull of the nebula, which in 

 turn is a measure of its mass. We can scarcely hope to see — or photo- 

 graph — such motion as a change of position in the sky ; at a distance of 

 a million light-years a star would have to be moving at about 9,000 

 miles per second across the line of sight to change its position by even 1 

 second of arc in a century ! Luckily there is a more sensitive method 

 of measuring motion along the line of sight (toward or away from us) 

 by its effect on the spectrum. This "Doppler effect," which may be de- 

 duced from basic notions of the nature of light, space, and time, con- 

 sists of a slight change in color, toward the red for recession and 

 toward the blue for approach, which can easily be detected with a spec- 

 trograph, if the light source has readily identified original colors in its 

 spectrum. Early work at the Lowell Observatory and more recent 

 work at the Lick Observatory have established from this effect that the 

 spirals are rotating — at least, the ones viewed edge-on show more ap- 

 proach at one end than at the other. The amount of rotation of the 

 outermost parts indicates a mass about 100 billion times that of 

 the sun for the Andromeda nebula, and smaller masses, about 1 bil- 

 lion to 10 billion suns, for other nebulae. Similar reasoning leads us 

 to expect that the Milky Way system is also rotating; this has been 

 measured, and leads to a mass of over 200 billion suns. Again we 

 find our own galaxy larger than the rest, which seems to leave us 

 in a "preferred" position in the universe. Moreover, there are other 

 reasons to feel dissatisfied with these small measured values of nebular 

 masses, as will become apparent later on. 



The form and content of the extragalactic nebulae have contributed 

 largely to their classification. Although there are others, the most 

 meaningful classification seems to be one proposed by Hubble. He 

 recognized three broad classes, based on form alone: the elliptical 

 nebulae, the spirals, and the barred spirals. Within each of these 

 classes there is a continuous sequence from "early" to "late" types— 

 from smaller, more compact forms to larger, looser forms in the 

 spirals, and from circular to more elliptical forms in the elliptical 

 nebulae. The terms "early" and "late" seem to have been unhappily 



