THE UNIVERSE OF STARS. 547 



a Centauri, has a parallax of three fourths of a second. To bring 

 within easier comprehension the enormous distance this parallax 

 involves, let the rapidity of light be considered. Light travels at 

 the rate of over one hundred and eighty thousand miles a second, 

 and a year of such travel may be taken as a unit for star-distances. 

 Thus, the distance of a Centauri would be measured by nearly 4^ 

 " light-years." The Polar Star is forty light-years, Sirius one hun- 

 dred and twenty-one light-years, distant from our globe ; while 

 stars of the sixteenth magnitude may be so remote that it would 

 take a wave of light thirty -six thousand years to reach the solar 

 system. The parallax of Sirius is only about one thirty-third of 

 a second — a striking example of the dependence of the most pro- 

 digious measurements of astronomy upon the minutest readings 

 of apparatus, necessitating the utmost perfection of workman- 

 ship, as well as consummate skill and knowledge on the part of 

 the observer. 



Over eight thousand nebulae have now been subjected to ex- 

 amination. The great nebulae in Andromeda and Orion are, of 

 course, familiar to every one. The telescopic nebulae are of all 

 sizes and shapes, and scattered over the whole heavens. Many 

 stars have nebulous wisps and whirls, tails and helices, attached 

 to them. The nature of nebulae is still more or less of a mystery. 

 But it is certain that they are initial, or at least early, phases of 

 the life-history of stars. That life-history may be shortly stated 

 in Miss Gierke's own words : 



By the ceaseless advance of condensation nebulae are transformed first into 

 gaseous stars (showing bright lines in the spectrnm, and therefore shrouded in 

 glowing atmospheres, chiefly of hydrogen and helium), then into stars with banded 

 spectra (showing outer atmospheric strata below incandescence over inner strata 

 at glowing heat), from which (by further condensation and increase of inner heat 

 below irregular outer clouds of metallic vapor) solar stare, and from these again 

 Sirian stars, gradually emerge. Here the ascent ends ; the maximum of temper- 

 ature is reached, and a descent begins, the initial stage of which is marked by a 

 second group of objects like our sun and Capella, distinguished from the first by 

 the circumstance that they are losing instead of gaining heat ; while, lower still, 

 the condition immediately antecedent to solidification and obscurity (dark stars) 

 is represented by Father Secchi's " carbon stars." 



The nebula in Orion is of a very irregular shape ; imbedded 

 in it lies the stellar group 0^ of the constellation, and some other 

 stars, all of which together seem to form an enormous system 

 whose dimensions can scarcely even be guessed at. Examined by 

 the spectroscope, the nebula is found to consist of glowing gas, 

 which the spectrum indicates to be a mixture of hydrogen and 

 nitrogen. The Andromeda nebula, on the other hand, presents a 

 well-defined oval, and gives a continuous spectrum in which no 

 bright lines have been certainly distinguished. It may, therefore, 



