746 TABLE 849.— STELLAR SYSTEMS 



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 percent of the stars known to be nearer than 16 light-years 

 are brighter than the sixth magnitude. Exploring the solar neighborhood therefore in- 

 volves a search for telescopic dwarf stars. Any body 1/100 of sun's mass within 1,000 as- 

 tronomical 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 = 11, 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 Milky Way with a radius of about 50,000 light-years. The stars within 5,000 

 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 spectro- 

 scopic 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 1,000 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 1,500 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 Supergalaxies, 1,000,000 to 500,000,000 light-years distant. Composed of clusters 

 of extragalactic nebulae. The relative diameters and brightnesses have been determined 

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

 of several hundred bright spiral, spheroidal, and irregular galaxies, about 10 7 light-years 

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

 galaxies is in Centaurus. 



TABLE 850.— STELLAR SPECTRA AND RELATED CHARACTERISTICS* 



The one-dimensional classification system. — The spectra of almost all the stars 

 can be arranged in a continuous sequence, the various types connected in a series of im- 

 perceptible gradations. With two unimportant exceptions, the sequence is linear. Accord- 

 ing to the now generally accepted Harvard (or Draper) system of classification, certain 

 principal types of spectrums are designated by letters — P, IV, O, B, A, F, G, K, M, R, N, 

 and S — and the intermediate types of suffixed numbers. A spectrum halfway between 

 B and A is denoted by 5 5 while those differing slightly from class A in the direction of 

 Class B are called B 8 or B 9. Classes R and N apparently form one side chain, and class 

 S another chain, both branching from the main series near class A'. 



The two-dimensional classification system. — In addition to the larger character- 

 istics used to determine the spectral class (temperature differences) there are smaller 

 luminosity effects that depend mainly on differences in densities in the atmospheres of the 

 stars. Thus one can distinguish between dwarfs, giants, and supergiants. At Harvard, 

 in 1897, Miss Maury was actually the first to denote certain stars by prefixing the 

 letter "c" to the spectral class. These stars are now known to be supergiants. Mount 

 Wilson observers still use this letter "c" to denote supergiants, "g" for giants, and "d" 

 for dwarfs. This dM 5 denotes a dwarf star of spectral type M 5 (see Table 874). Morgan, 

 Keenan, and Kellman have extended the classification even further." 70 Their luminosity 

 classes include not only giants (III) and dwarfs (V) but subgiants (IV) and several classes 

 of supergiants (I : la, and lb) and intermediates (II). 



Almost all the stars can be classified on the above system. In addition to individual 

 peculiar stars there are, however, groups of stars that cannot be given specific classifica- 

 tions, such as the ^4-type spectrum variables" 71 and the "metallic-line" stars. 272 



The colors of the stars, the degree to which they are concentrated into the region of the 

 sky, including the Milky Way (Table 854), and the average magnitudes of their peculiar 

 velocities in space (Tables 828 and 876) all show important correlations with spectral type. 

 In the case of colors, the correlation is so close as to indicate that both spectrum and color 

 depend almost entirely on the surface temperature of the stars. The correlation in the 

 other two cases, though statistically important, is by no mean so close. 



* Prepared by Kdith J. Teho, Harvard College Observatory. 



270 An Atlas of Stellar Spectra. University of Chicago Press. 1943. 



271 Deutsch. Astrophys. Journ., vol. 105, p. 283, 1947. 



272 Roman. Morgan, and Eggcn, Astrophys. Journ., vol. 107, p. 107, 1948. Greenstein, Astrophys. 

 Journ., vol. 107, p. 151, 1948; vol. 109, p. 121, 1949. 



SMITHSONIAN PHYSICAL TABLES 



