STAR-CLUSTERS AND GROUPS 
149 
scope. A curious one may be noticed in Orion just 
below the belt. It seems to be luminous pale-green 
matter. The spectroscope shows that it is burning 
hydrogen. It covers an area many times larger than 
our own solar system. 
Other nebuliB are shown by spectrum analysis to 
be more condensed. One in the Great Bear has two 
central nuclei. 
These nebulae, together with others, could be ar¬ 
ranged in the form of a series, from the simple gase¬ 
ous matter apparently just beginning condensation, 
through various stages, until we reach the central 
nucleus, a star surrounded by more or less detached 
bands of luminous matter. Is it possible that inter¬ 
stellar space is filled with invisible star dust coming 
out of the abyss of nothing and gradually gravitating 
N N 
Fig. 72. — C Herculis, a double Fig. 73.— ( Herculis, as it 
star, as it appeared in 1865. appeared in 1871. 
here and there into nebulous clouds, just as the invisi¬ 
ble vapor of our atmosphere piles up into clouds 
of various shapes and densities ^ 
11 
