232 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1931 



Doctor Hubble, of the Carnegie Institution Observatory at Mount 

 Wilson, has found it to be at such a distance that its light takes about 

 800,000 years to reach us. 



There is no longer any room for reasonable doubt that, in its outer 

 parts at least, this great Nebula in Andromeda is formed of a system 

 of stars which is similar in its essential nature to our own system. 

 It is not the only such system in the sky ; millions of others can be 

 observed. 



Although these are of varied shapes and constitutions, it is found 

 that the greater number of them can be arranged in a single sequence. 

 At one end of the sequence are Nebulae consisting solely of round 

 fuzzy masses, in which no stars are visible even in the most powerful 

 telescope, while at the other extreme end we have clouds of stars 

 such as our own system. Half way along the sequence are Nebulae, 

 such as the great Nebula in Andromeda, which consist of a central 

 fuzzy mass surrounded by stars, in which both the fuzzy mass and the 

 stars are present, the former occupying the central and the latter the 

 outer regions. 



Like our own system of stars, these nebulae are generally flat in 

 shape. The comparison of the cart wheel remains quite a good one, 

 partly because many of these nebulae are known to be rotating and 

 all are believed to be so ; partly also because they often are found to 

 have a thick central projection, corresponding to the hub of the 

 wheel, while the rest of their structure is flat. The Great Nebula 

 in Andromeda is of this cart-wheel shape, but it is rather disguised 

 because we are neither looking at it full on nor edgewise on. If we 

 could look at it full on, it would appear nearly circular in shape ; if 

 we could look at it edgewise on, it would appear rather more than 

 a bright line of light; indeed it would probably look very much like 

 the nebulae. N. G. C. 891, which is seen edge-on. From the angle 

 at which we actually view it, it appears elliptical in shape. 



We know all this because the various nebulae in the sky are, of 

 course, seen at possible angles, so that we can study their structure 

 as 3-dimensional solids. AVhen we do this, we find that the sequence 

 I have already described starts with perfectly globular nebulae and 

 ends up with quite flat nebulae. The sequence is one of nebulae 

 arranged in order of flatness. 



It is easy to obtain a theoretical interpretation of this sequence. 

 We know how an increase in the speed of rotation of a body is accom- 

 panied by a flattening of its shape. Our own earth, which is rotat- 

 ing slowly, is only slightly flattened, so that we describe it as orange- 

 shaped. Jupiter rotates much more rapily (once every 10 hours), 

 and as a result is much flatter in shape. Finally, astronomical bodies 

 which are rotating very rapidly may be almost completely flat. 



