136 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1942 



2. Is this galaxy in motion as a unit ? How it moves with respect to 

 nearby galaxies is not yet very clear, but certainly it rotates around 

 the Sagittarius nucleus. It does not rotate as a solid wheel, at least 

 at our distance from the nucleus. It rotates more as the planetary sys- 

 tem rotates; the planets nearer the sun go more rapidly and complete 

 their "years" in shorter times than the remoter planets. We think we 

 can very definitely measure the differential speed of stars around the 

 nucleus. The average speed in the sun's neighborhood is about 200 

 miles a second, and the direction of motion is toward the northern con- 

 stellation of Cygnus. 



3. How far are the sun and the neighboring stars from the axis of 

 rotation? Ten kiloparsecs is the approximate answer and, since a 

 parsec is 3.26 light-years or about 20 trillion miles, the distance is 

 something like 2X10" miles, or 30-odd thousand light-years. For 

 various reasons, that value of 10 kiloparsecs is not too certain, but it is 

 well established that the center of gravity of our system is between 

 8 and 12 kiloparsecs distant. Tlie direction to the center is fixed with 

 an uncertainty of only 2 or 3 degrees; this angular parameter is much 

 easier to handle than the distance. 



4. How large is the Milky Way system and how populous ? Enorm- 

 ous in size and population, if nonquantitative terms may be used. 

 There is good evidence that the total population in stars is of the order 

 (jf 200.000 million, but the evidence on over-all dimensions is as yet 

 inconclusive. Indeed it is somewhat involved with definitions. For 

 instance, how define the boundary of a galaxy? Is it at the distance 

 of the farthermost discoverable member of the system? Or is it the 

 distance to the place where the number of stars per cubic light-year has 

 decreased to a specified small quantity? Or is it, for a spiral galaxy, 

 the distance to which a spiral arm can be traced ? Or is it the distance 

 to which an escaping star can go before the* gravitational holdback is 

 exceeded by the pull from some other galaxy ? 



The diameter of the system in its plane is not less than 100,000 

 light-years if all its recognizable stars are included. There is now 

 good evidence that the wheel-shaped system is surrounded by a more 

 or less spherical haze of stars, and some of the stars in the haze are 

 50,000 light-years above the plane of the Milky Way. Probably this 

 haze extends more distantly in the plane of the system and, therefore, 

 the diameter of discoid plus haze considerably exceeds 100,000 light- 

 years. 



On the other hand, the diameter of our system in its plane might be 

 measured as only 50,000 light-years, or even less, if we had to depend 

 on photographic research equipment which, although comparab|le 

 with our own, was located in the Virgo supergalaxy, several million 

 light-years away. Our outer stars might not register. When our tele- 

 scopes are turned on the members of that group in Virgo we can trace 



