Janssen was engaged for many years in trying to discover evidence of
the existence of oxygen in the sun, and he constructed his observatory
on the summit of Mount Blanc specially to pursue that research. He
believed that oxygen must surely exist in the sun since we find so
many other familiar elements included in the constitution of the solar
globe, and as he was unable to discover satisfactory evidence of its
presence he assumed that it existed in a form unknown on the earth. If
it were normally in the sun's chromosphere, or coronal atmosphere, he
said, it would combine with the hydrogen which we know is there and
form an obscuring envelope of water vapor. It exists, then, in a
special state, uncombined with hydrogen; but let the temperature of
the sun sink to a critical point and the oxygen will assume its normal
properties and combine with the hydrogen, producing a mighty outburst
of light and heat. This, Janssen thought, might explain the phenomena
of the temporary stars. It would also, he suggested, account for their
brief career, because the combination of the elements would be quickly
accomplished, and then the resulting water vapor would form an
atmosphere cutting off the radiation from the star within.

This theory may be said to have a livelier human interest than some of
the others, since, according to it, the sun may carry in its very
constitution a menace to mankind; one does not like to think of it
being suddenly transformed into a gigantic laboratory for the
explosive combination of oxygen and hydrogen! But while Janssen's
theory might do for some temporary stars, it is inadequate to explain
all the phenomena of Nova Persei, and particularly the appearance of
the great spiral nebula that seemed to exhale from the heart of the
star. Upon the whole, the theory of an encounter between a star and a
dark nebula seems best to fit the observations. By that hypothesis the
expanding billow of light surrounding the core of the conflagration is