198 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1928 



has been used by Opik and gives for the Andromeda nebida a mnss 

 nearly 2,000 million times the mass of an average star like our sim. 

 The second method depends upon the spectroscopic determination of 

 the line-of-sight velocity of opposite edges of the nebula. If one 

 side be found to be approaching and the other side receding, the only 

 logical conclusion is that the whole nebula is rotating. Now, a rotat- 

 ing mass will fly asunder by centrifugal force unless some equal 

 and oi:)posite force holds its members together. If gravitation, 

 acting toward the center of the nebula, provides this balancing force, 

 it is possible to calculate the total mass necessary to give rise to the 

 restraining force required. The period of rotation of the Andromeda 

 galaxy was found to be 17,000,000 years. From this its mass was 

 calculated, giving just over 3,000 million suns. The agreement with 

 Opik's result is satisfactory. 



Our picture of this best-known island galaxy can be briefly summed 

 up in a few w^ords : A thousand million stars — like those in our own 

 galaxy, some larger and some smaller than our sun — and much uncon- 

 densed gas, all forming the giant spiral nebula traveling through 

 space at least 300 kilometers per second, and as it travels, slowly ex- 

 panding and unwinding its spiral arms, while as a whole it is turn- 

 ing round with solemn, majestic deliberation. 



THE EINSTEIN UNIVERSE 



Men of science throughout all the ages have been obsessed with 

 the idea that there is order in the universe. When the great wave of 

 agnosticism passed over Europe, threatening to sweep the thoughts 

 of men from all moorings, this fundamental tenet of scientific faith 

 w^as the sheet anchor which saved mankind. So deeply implanted 

 is this belief in natural law and order, that when some facts of 

 astronomy and physics appeared to be incompatible with the current 

 conception of the Universe, based as it is on the stately geometry of 

 Euclid and the Newtonian mechanics, men of science were willing to 

 consider throwing over the old conception and adopting a nev/ con- 

 ception suggested by Einstein. This willingness is the more remark- 

 able when it is remembered that, to the nonmathematical mind, the 

 four-dimensional space-time universe of Einstein seems mysterious, 

 fantastic, and unreal. Yet there is alreadj^ considerable evidence in 

 its favor, and so, generalizing from his detailed study of 400 galaxies, 

 Hubble proceeds to evaluate the radius, volume, and mass of the 

 Einstein universe. 



He calculates first the average density of space. If the matter 

 forming all the stars and gaseous nebulae in our galaxy and in the 

 400 other galaxies studied by him were to be spread evenly through- 

 out the space occupied by these galaxies, there would be a density of 

 matter equivalent to one atom of hydrogen in every 300 cubic feet. 



