TABLES 538-639 * j ^ 



ASTRONOMICAL DATA- 



TABLE 538. Brightness of the Stars. 



Stellar magnitudes give the apparent brightness of the stars on a logarithmic scale, a numerical increase of one 

 magnitude corresponding to a decrease of the common logarithm of the light by 0.400, and a change of five magnitudes 

 to a factor of 100. The brightest objects have negative stellar magnitude ! magnitude of the 



of the mean full Moon, 12.5; of Venus at her brightest, 4.3; of Jupiter, at opposition, rius, 1.6; of 



Vega, +0.2; of Polaris, +2.1. (The stellar magnitude of a standard candle i m distant is -14.18.) The faintest stars 

 visible with the naked eye on a clear dark night are of about the sixth magnitude (though a single luminous point as 

 faint as the eighth magnitude can be seen on a perfectly black background). The faint- .Me with a telescope 



of aperture A in. are approximately of magnitude 9 + 5 logic A. The faintest photographed with the 6o-inch reflector 

 at Mt. Wilson are of about the 2ist magnitude, A standard candle, of the same color as the stars, would appear of 

 magnitude +0.8 at a distance of one kilometer. 



The actual luminosity of a star is expressed by means of its absolute magnitude, which (Kapteyn's definition) is 

 the stellar magnitude which the star would appear to have if placed at a distance of ten parsecs. The absolute mag- 

 nitude of the sun is +4.8 (equal to that of 0,2 Centauri); of Sirius is +1.3; of Arcturus, 0.4. The faintest star at 

 present known (Innes), a distant companion to a Centauri, has the (visual) absolute magnitude +15.4, and a luminosity 

 0.00006 that of the sun. The brightest so far definitely measured, ft Orionis, has (Kapteyn) the abs. mag. 5.5 and 

 a luminosity 13,000 times the sun's. Canopus, and some other stars, may be still brighter. 



Intrinsic brightness of sun's surface = 57,000 candles per cm 2 of surface. (Abbot-Fowle, 1920) 



The absolute magnitudes of 6 planetary nebulae average 9. i; average diameter, 4000 astronomical units (Solar 

 system to Neptune = 60 astr. units), van Maanen, Pr. Nat. Acad. 4, p. 394, 1918. 



Giant and Dwarf Stars. 



The stars of Class B are all bright, and nearly all above the absolute magnitude zero. Stars of comparable bright- 

 ness occur in all the other spectral classes, but the inferior limit of brightness diminishes steadily for the "later or 

 redder types. The distribution of absolute magnitudes conforms to the superposition of two series, in each of which 

 the individual stars of each spectral class range through one or two magnitudes on each side of the mean absolute 

 magnitude. In one, the "giant stars," this mean brightness is nearly the same for all spectral classes, and not 

 far from absolute magnitude zero. In the other, the "dwarf stars," it diminishes steadily from about abs. mag. 

 2 for Class Bp to +10 for Class M. The two series overlap in Classes A and F, are fairly well separated in Class K, 

 and sharply so in Class M. Two very faint stars of Classes A and F fall into neither series. 



The majority of the stars visible to the naked eye are giants, since these, being brighter, can be seen at much greater 

 distances. The greatest percentage of dwarf stars among those visible to the eye is found in Classes F and G. The 

 dwarf stars of Classes K and M are actually much more numerous per unit of volume, but are so faint that few of the 

 former, and none of the latter, are visible to the naked eye. 



Adams and Stromberg have shown that the mean peculiar velocities of the giant stars are all small, increasing 

 only from about 6 km/sec, for Class B to 12 for Class M, while those of the dwarf stars are much greater, increas- 

 ing within each spectral class by about 1.5 km per unit of absolute magnitude, and reaching fully 30 km for stars of 

 Class M and abs. mag. 10. Both giant and dwarf stars show the phenomenon of preferential motion. 



TABLE 539. Masses and Densities. 



The stars differ much less in mass than in any other characteristic. The greatest definitely determined mass is 

 that of the brighter component of the spectroscopic binary /3 Scorpii, which is of 13 times the sun's mass, 400 times 

 its luminosity, and spectrum Bi. The smallest known mass is that of the faint component of the visual binary Krueger 

 60, whose mass is 0.15, and luminosity 0.0004 of the sun's, and spectrum M. 



The giant stars are in general more massive than the dwarfs. According to Russell (Publ. Astron. Soc. America, 

 3, 327, 1917) the mean values are: 



Spectrum. ^ m - Mass " 



62 12 X Sun Fa dwarf 3.0 X Sun 



Ao 6.5 " G2 " 1.2 



F S giant 8 K8 " 0.9 " 



KS " 10 



The densities of stars can be determined only if they are eclipsing variables. It appears that the stars of Classes 

 B and A have densities averaging about one tenth that of the sun and showing a relatively small range about this value, 

 while those of Classes F to K show a wide range in density, from 1.8 times that of the sun (W Urs. Maj.) to 0.000002 



The surface brightness of the stars probably diminishes by at least one magnitude for each stop ;ilmg the Harvard 

 scale from B to M. It follows that the dwarf stars are, in general, closely comparable with the s in in diameter, while 

 the stars of Classes B and A, though larger, rarely exceed ten times the sun's diameter. The rol.U-r k'hnt stars, how- 

 ever, must be much larger, and a few, such as An tares, may have diameters exceeding that of the earth's orbit. The 

 densities of these stars must be exceedingly low. 



If arranged in order of increasing density, the giant and dwarf stars form a single sequence Stirling with the giant 

 stars of Class M, proceeding up that series to Class B, and then down the dwarf seri^ M It is belu 



Russell and others that this sequence indicates the order of stellar evolution, a star at fir-t nperature as 



it contracts and then cooling off again. The older theory, however, regards the evolutionary sequence as proceeding 

 in all cases from Class B to Class M. 



SMITHSONIAN TABLES. 



