156 STELLAR EVOLUTION. 



in presentino- themselves. In the heart of the Orion nebula are four 

 samll stars which constitute the well-known Trapezium, Situated as 

 they are in the midst of this far-reachino- luass of g-as, 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 

 precisel}" what the spectroscope has demonstrated. Mot only these 

 stars, but man}" other stars in the constellation of Orion are shown by 

 the spectroscope to contain the same gases which constitute the nebula. 

 For this and other reasons they are considered to represent one of the 

 earliest stages of stellar growth. 



It may ha many 3'ears before the exact nature of the process by 

 'A'hich a star is formed from a nebulous mass is clearly understood. 

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

 tant discovery in the course of his photographic work with the Crossley 

 reflector of the Lick Observatory. Spiral nebula? have long been 

 known, but it was not supposed that they were sufficiently numerous 

 to be j-egarded as type ol)jects. The great spiral nebula illustrated 

 b}^ one of Mr. Ritchey's recent reflector photographs (PI. VII) has long- 

 been regarded as one of the most remarkable ol)jects in the heavens, and 

 the possible significance of its form had b}" no means been overlooked. 

 But few astronomers were prepared for Professor Keeler's announce- 

 ment that the majority of nebuUe are of the spiral form and that many 

 thousands of these objects are within the reach of such an instrument 

 as the Crossle}^ 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 spectroscopj^ must play in the stud}' of the life history of 

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

 intricate instrument and that the principles of spectroscopy are obscure 

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

 this fleld of investigation can be easily understood by anyone who will 

 devote a very small amount of time to the subject. As you dou])tless 

 know, the essential feature of a star spectroscope is the prism or train 

 of prisms by which the star light is divided into its constituent parts. 

 After passing through the pi'isnis the light of the star is spread out 

 into a long band, which shows all the colors of the rainbow, beginning 

 with red at one end and passing through orange, yellow, green, and 

 blue to violet at the other. This band is crossed by lines, and the 

 problem of the spectroscoi)ist is to interpr<'t the lueaning of these lines. 



