We are sadly in need of information concerning the constitution

of the spiral nebulae
We are sadly in need of information concerning the constitution
of the spiral nebulae. Their spectra appear to be prevailingly
of the solar type, except that a very small proportion contain
some bright lines in addition to the continuous spectrum. So
far as their spectra are concerned, they may be great clusters
of stars, or they may consist each of a central star sending
its light out upon surrounding dark materials and thus
rendering these materials visible to us. The first alternative
is unsatisfactory, for all parts of spirals have hazy borders,
as if the structure is nebulous or consists of irregular groups
of small masses; and the second alternative is unsatisfactory,
for in many spirals the most outlying masses seem to be as
bright as masses of the same areas situated only one half as
far from the center, whereas in general the inner area should
be at least four times as bright as the outer area. All
astronomers are ready to confess that we do not know much about
the conditions existing in spiral nebulae.


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