ORIGIN OF EARTH 



135 



of more or less circular spots which are exceptionally im- 

 permeable to light. These were called ' globules '. The 

 smallest known globule has a diameter of o-oo6 parsecs and 

 its mass is 1/500 that of the Sun. Other globules have 

 considerably greater masses, in some cases several times that 



BOI B2 BS Ba B9 AO AZ A3 As Fn F2 FS Fa Co GS KO KZ K5 M 

 ■S 



.. SPECTRAL CLASS 



.j5- ■ -iaooo 



■ 3 



■ 2 

 1 

 0- 



.1 



-J 



■ S- 10 



.6 y 



.7 S 

 10 

 ■0 



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iuPOC'e'trs 



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Fig. 10. Hertzsprung-Russell diagram. 



of the Sun: i.e. they would be large enough to form one or 

 several stars. In connection with such a possibility one must 

 bear in mind the extremely high density of the globules. 

 This is thousands of times greater than the density of the 

 interstellar medium which surrounds them. 



A theory enjoying considerable popularity among contem- 

 porary cosmogonists is that one such globule was the ' proto- 

 star ' from which our planetary system was formed. At some 

 stage in the development of this globule there arose a central 



