OUR REVOLVING "ISLAND UNIVERSE" — SKILLING 129 



that counterbalances the inward pull of gravitation. If this appear- 

 ance were not sufficient proof of the rotation of the spirals, there are 

 spectroscopic observations to show it. V. M. Slipher, at the Flagstaff 

 Observatory, and more recently Dr. Hubble, at the Mount Wilson 

 Observatory, have used the spectroscope upon nebulae in much the 

 same way it is used in measuring the velocity of rotation of a planet. 

 Using spiral nebulae that do not have their flat side directly toward 

 us, the slit of the instrument was set as nearly as could be upon the 

 equator of the spiral, perpendicular to the axis of rotation. The 

 resulting spectral lines slant because of the approach of one side of 

 the light source and the recession of the other side. Slipher and 

 Hubble agree that there is motion in the direction we should expect ; 

 most of the material of the nebula is at the more condensed central 

 part, and as this turns, the two arms projecting from its opposite sides 

 are left trailing behind. 



At our great distance from the spirals the angular motion is too slow 

 to be observed with certainty on direct photographs of the nebulae, 

 even with pictures taken several years apart. Perhaps photographs 

 of the next century when compared with those taken by this genera- 

 tion of astronomers may show changes in the position of stars of these 

 galaxies. 



There can be no question as to the rotation of spiral nebulae ; what, 

 then, is the evidence that our system of stars is like them? Some of 

 the extragalactic nebulae that are not spiral show no sign of motion. 

 If our galaxy turns, how would the motion of its stars be observed to 

 show whether or not they are revolving about a common center? 

 Lindblad was unable to make any direct measurement of our galactic 

 rotation. The position of the earth within the galaxy is an unfavor- 

 able one for observation of its rotation. So Lindblad's approach to 

 the problem was to explain, insofar as possible, all observed motions of 

 the stars of the galaxy (as they appear to move with reference to our 

 sun) in terms of a great rotation. 



Lindblad interpreted the apparent motion of the so-called high- 

 velocity stars as being caused by our more rapid rate as we and they 

 revolve, all in the same direction, around the center of the stellar 

 system. These slow "high-velocity stars" seem to be headed backward 

 with respect to the true direction of all the stars. They seem rapid 

 because they lag behind so rapidly. 



But why would not stars equally distant from the center of the 

 galaxy all move equally fast? For comparison, it is well known that 

 a comet travels very slowly while going around the end of its long 

 elliptical orbit that is farthest from the sun. Then it gains great 

 speed through years spent in falling closer. Halley's comet, for 

 example, went about 35 times as fast in 1910, when it was making the 



