May 25, 1917] 



SCIENCE 



545 



stars with dark lines are found prevail- 

 ingly ill and near the G-alaxy. Pickering's 

 study of the distribution of 6,106 of the 

 brightest stars is summarized in the accom- 

 panyiog table. 



GALACTIC LATITUDES OF STARS BRIGHTER THAN 6.25 



The sky is divided into eight zones of 

 equal areas, with the boundaries of the 

 zones parallel to the galactic plane. The 

 first line of figures contains the numbers of 

 stars of the different spectral classes in the 

 one eighth of the sky around the north pole 

 of the Galaxy ; the fourth and fifth lines the 

 numbers in the zones containing the Gal- 

 axy, one zone adjoining the Milky "Way on 

 the north and the other on the south ; and 

 the last line the numbers of stars of the 

 different classes in the eighth of the sky 

 around the south pole of the Galaxy. If 

 the stars of the different classes were uni- 

 formly distributed over the sky, the eight 

 numbers in each vertical column would be 

 equal. It is seen that the Class B stars are 

 prevailingly in the Milky Way, and that 

 the red stars of Class M are about uni- 

 formly distributed over the sky, though 

 the.y are all in our stellar system. 



The Class B stars and the stars contain- 

 ing bright lines are where the jDlanetary 

 and irregular nebulte exist. Going further 

 into detail : wherever there is a great nebu- 

 lous region either in, or near, or outside of 

 the Milky Way you will find the Class B 

 and earlier types of stars abnormally plenti- 

 ful : and the chances are fairly strong that 



some of the stellar spectra will contain 

 bright lines. This is true of great regions 

 in the Milky Way, it is true of the Orion 

 and Pleiades regions, which we see at some 

 distance outside of the Milky Way struc- 

 ture, though they are doubtless within our 

 sj-stem. If you see a wisp of nebulosity 

 near a bright star, look up the star's spec- 

 trum and you will probably find it an early 

 Class B, as in the case of Gamma Cassio- 

 peia, a second magnitude star, with nebu- 

 lous structure near it (Pig. 34), whose 

 spectrum contains both bright and dark 

 lines of hydrogen and helium (Fig. 35). 

 If you see an isolated bright star appar- 

 ently enmeshed in an isolated patch of 

 nebulosity, such as the one shown in Fig. 

 37, and the books say the star (BZ>-10°- 

 4713) is j'ellow, or of Class G, communicate 

 j'our suspicions that the books are mistaken 

 about that star's spectrum to Professor 

 Pickering, and he will probably reply that 

 the star is in reality a very blue one, of 

 early Class B. That is what happened a 

 fortnight ago about this particular nebula 

 and the star near its apparent center. If 

 you find a red or yellow star of normal 

 t.ype, do not look for a nebula in apparent 

 contact with it. Nebula and red stars do 

 not coexist. You will find about the same 

 number of red stars in the Milky Way that 

 are visible in similar areas far from the 

 Milky Way. You will find an occasional 

 red star in the region of the Orion nebula 

 and of other large nebulaj, but red stars will 

 not apiDcar there in greater numbers than 

 their approximately uniform distribution 

 over the sky requires. 



The connection between the nebulse and 

 the bright-line stars, and between the nebu- 

 Iffi and the early Class B stars is close, both 

 as to their types of spectra and as to their 

 geometric distribution. 



Do the nebulse form stars or do the stars 

 form nebulse, or both? There is abundant 



