302 POPULAR SCIENCE MONTHLY. 



of the nebulae was solved. The answer, which had come to us in the 

 light itself, read : Not an aggregation of stars, but a luminous gas." 



With this advance a new era of progress began. The power of the 

 spectroscope to distinguish between a glowing gas and a mass of partially 

 condensed vapors like a star established it at once in its place as the chief 

 instrument of the student of stellar evolution. It became apparent that 

 the unformed nebula might furnish the stuff from which stars are 

 made. Observations tending to this conclusion were not long in pre- 

 senting themselves. In the heart of the Orion nebula are four small 

 stars which constitute the well-known Trapezium. Situated as they 

 are in the midst of this far-reaching mass of gas, it is not hard to 

 picture them as centers of condensation, toward which the play of 

 gravitational forces tends to concentrate the gases of the nebula. It 

 might therefore be expected that stars in this early stage of growth 

 should show through the spectroscopic analysis of their light some 

 evidence of relationship with the surrounding nebula. Now this is pre- 

 cisely what the spectroscope has demonstrated. Not only these stars, 

 but many other stars in the constellation of Orion, are shown by the 

 spectroscope to contain the same gases which constitute the nebula. 

 I'or this and other reasons they are considered to represent one of the 

 earliest stages of stellar growth. 



It may be many years before the exact nature of the process by 

 which a star is formed from a nebulous mass is clearly understood. 

 Shortly before his death the late Professor Keeler made a most im- 

 portant discovery in the course of his photographic work with the 

 Crossley reflector of the Lick Observatory. Spiral nebulse have long 

 been known, but it was not supposed that they were sufficiently numer- 

 ous to be regarded as type objects. The great spiral nebula illustrated 

 in Fig. 7 from one of Mr. Ritchey's recent reflector photographs has 

 long been regarded as one of the most remarkable objects in the 

 heavens, and the possible significance of its form had by no means been 

 overlooked. But few astronomers were prepared for Professor Keeler's 

 announcement that the majority of nebulse are of the spiral form and 

 that many thousands of these objects are within the reach of such an 

 instrument as the Crossley reflector. It does not seem improbable that 

 this spiral form may prove to represent the original condensing mass 

 more truly than the lenticular form from which Laplace imagined the 

 solar system to be evolved. 



Enough has already been said to indicate how large a part the 

 methods of spectroscopy must play in a study of the life history of 

 stars. In spite of the common opinion that the spectroscope is an intri- 

 cate instrument and that the principles of spectroscopy are obscure and 

 difficult of comprehension, it is a fact that the processes used in thife 

 field of investigation can be easily understood by any one who will 



