It is very likely that, after the chemists have taught us how to
interpret all the varieties of spectrum, it will be possible to
ascribe the different spectrum-classes to different stages in the
life-history of every star. Already there are plenty of people ready
to lay down arbitrary assumptions about the lessons to be drawn from
stellar spectra. Some say that they know with certainty that each star
begins by being a nebula, and is condensed and heated by condensation
until it begins to shine as a star; that it attains a climax of
temperature, then cools down, and eventually becomes extinct. They go
so far as to declare that they know what class of spectrum belongs to
each stage of a star's life, and how to distinguish between one that
is increasing and another that is decreasing in temperature.

The more cautious astronomers believe that chemistry is not
sufficiently advanced to justify all of these deductions; that, until
chemists have settled the lately raised question of the transmutation
of elements, no theory can be sure. It is also held that until they
have explained, without room for doubt, the reasons for the presence
of some lines, and the absence of others, of any element in a stellar
spectrum; why the arc-spectrum of each element differs from its spark
spectrum; what are all the various changes produced in the spectrum of
a gas by all possible concomitant variations of pressure and
temperature; also the meanings of all the flutings in the spectra of
metalloids and compounds; and other equally pertinent matters--until
that time arrives the part to be played by the astronomer is one of
observation. By all means, they say, make use of "working hypotheses"
to add an interest to years of laborious research, and to serve as a
guide to the direction of further labours; but be sure not to fall
into the error of calling any mere hypothesis a theory.