BEYOND THE MILKY WAY — PAGE 167 



are so powerful that they can be used to measure distances up to a 

 million light-years. Using the distances and directions of the globu- 

 lar clusters, Shapley plotted the positions of about 100 of them and 

 found that they are distributed in a large spherical volume of space, 

 the center of this array being at a point in the brightest part of the 

 JVIilky Way, in the direction of the constellation Sagittarius. It was 

 reasonable to assume, as Shapley did, that this point is the center of 

 the Milky Way system. 



At this juncture it is necessary to examine one of those refinements 

 of method with which astronomers continually busy themselves. In 

 using the brightness method of measuring distance, Shapley assumed 

 that interstellar space is transparent ; that the only reason one globular 

 cluster appears fainter than another is because of its greater distance. 

 For reasons too lengtliy to indicate here, astronomers now know that 

 dust and gas between the stars also dim the light of distant stars, and 

 this smokiness of interstellar space, neglected by Shapley, made his dis- 

 tances somewhat too large. Correcting for interstellar absorption, 

 we find the distance to the center of the Milky Way system to be 

 about 30,000 liglit-years, a figure corroborated, incidentally, by studies 

 of the dynamics of the galaxy — how it is held together without col- 

 lapsing, and how it moves. 



Our present picture of the Milky Way, then, is that of a flat pancake 

 of stars, gas, and dust, some hundred million light-years across, in 

 which the sun and planets are located about two-thirds of the way 

 from the center to the edge, and which is partly embedded in a spher- 

 ical array of globular clusters extending out some 30,000 light-years 

 from the center. Now we can discuss what is beyond it. 



Because they are extended surfaces, in contrast to the stars which 

 appear as mere points of light in a telescope, the nebulae are some of 

 the most interesting objects in the sky. From their form, their spectra, 

 their positions, and, when established, their distances, two classes of 

 nebulae can be distinguished : the galactic nebulae and the extragalac- 

 tic nebulae. The former are, by and large, irregular in shape; they 

 show the spectra (colors of light) emitted by low-density gases, and, 

 as their name implies, they are found mostly near the Milky Way in 

 the sky, at distances which place them well within that system. The 

 extragalactic nebulae, on the other hand, have circular, elliptical, and 

 spiral shapes, their spectra are like a mixture of star spectra, they are 

 found predominantly in parts of the sky other than the Milky Way, 

 and their distances have been found to be enormously larger than any- 

 thing in the galaxy. This last point is the crux of the matter; how 

 can we prove that these spiral and elliptical nebulae are beyond the 

 confines of the Milky Way system ? 



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