616 Table 785 



STELLAR SYSTEMS 



(See Shapley, Harvard Reprint 68, 1931, Harvard Explorations, Science, 74, 207, 1931.) 



The solar neighborhood distance of 50 light-years, explored chiefly through the motions 

 of nearby stars. A large majority are of less than solar luminosity, most below naked eye 

 visibility. Only 40% of the stars known to be nearer than 16 light-years are brighter than 

 the 6th magnitude. Exploring the solar neighborhood therefore involves a search for 

 telescopic dwarf stars. Any body 1/100 of sun's mass within 1,000 astronomical units 

 (.015 light year) would be detected by its disturbance on Neptune and Uranus even if 

 invisible (Russell). Nearest known star is 4 light-years distant (Proxima centauri, 

 m= ii, M= 15.5). 



Region of brighter stars extending 500 light-years. The great majority of naked-eye 

 stars lie in this region, though some of unusually high intrinsic luminosity are farther 

 away. It includes probably 500,000 telescopic stars. Studied by proper motions, trigono- 

 metric and spectroscopic parallaxes, and photometry. 



The local system. — Its diameter is several thousand light-years. There is good but not 

 incontrovertible evidence of a localized star cloud in our part of the galaxy. Its popula- 

 tion is in the tens or hundreds of millions of stars. Shapley considers it may be comparable 

 in dimensions and composition with Magellanic clouds or a typical spiral nebula. In- 

 vestigated principally statistically by spectra, magnitudes, and positions, and explored by 

 spectroscopic parallaxes, star counts, and structure of variable stars and galactic clusters. 



The Milky Way with a radius much greater than 5000 light-years. The stars within 

 5000 light-years of the sun are a trifling part of the galactic system outlined by the 

 globular clusters and Milky Way clouds. The stars are so remote that proper motions 

 and spectroscopic analyses hopelessly fail. Statistical counts are of some help in the 

 nearer parts. But most of our knowledge comes from eclipsing binaries, long-period 

 variables, and Cepheids. The period-luminosity relation for Cepheid variables is the key 

 to practically all distances > a few 1000 light-years. 



The Clouds of Magellan, nearly 100,000 light-years distant, nearest of all external 

 galaxies and the most easily studied. Great advantage ; all of its varied manifestations 

 are seen at practically the same distance. These phenomena include gaseous nebulae, star 

 clusters, giant and supergiant stars, some 1500 known Cepheids in the Larger Cloud. In 

 this cloud 750 stars brighter than — 5.0 abs. mag. and over 200,000 brighter than the 0.0 

 have been estimated. The following gives an indication of the classes of stars measured in 

 and in front of the Larger Cloud and adjacent field. 



The Super-Galaxies, 1,000,000 to 100,000,000 light-years distant. Composed of clusters 

 of extra-galactic nebulae. The relative diameters and brightnesses have been determined 

 for some of the super-galaxies. The most conspicuous is the Coma- Virgo cloud A, a 

 stream of several hundred bright spiral, spheroidal, and irregular galaxies, about io 7 light- 

 years distant ; its greatest length about \ this. One of the richest and most distinct super- 

 galaxies is in Centaurus. 



The Meta-Galaxy. — Great irregularity is found in the distribution of the objects 

 exterior to our galaxy — perhaps partly due to obscuring clouds in our system but much 

 attributable to aggregation of galaxies into super-systems and large indefinite streams. 

 We find no evidence that we have approached the limits of a populated universe — no 

 fallin°" off in the number per cubic million light-years. The red-shift in the spectra of 

 distant galaxies may be taken as an observational, relativistic indication of an expanding 

 finite universe, " but so far as the present census of the meta-galaxy goes, the total number 

 of galaxies and the radius of space may both be infinite" (Shapley). 



Smithsonian Tables 



