But
astronomers do not believe that the mere fact of distance explains all
changes. There is a star known as Mira, which for eleven months is
wholly invisible to the naked eye, then flames forth as a star of the
first magnitude, and is visible for a period of nearly three months,
fading at its close into darkness again. The star Algol, in the
constellation Perseus, is usually of the second magnitude, but every two
and a-half days it begins to decline in brilliancy, becomes very faint,
and remains thus for about three hours, and then waxes bright again.
Possibly this may be caused by the shadow of another star. In 1866 a
star of the eighth magnitude, in the Northern Crown, suddenly flamed up
into extraordinary brilliancy, remained thus for several months and
gradually subsided. This star was examined with the spectrum, and showed
lines of burning hydrogen. This led to the theory, now held, that the
increase in brilliancy of these stars is caused by the incandescence of
this gas. These fixed stars are all supposed to be suns of other
systems, and to be surrounded--like our sun--with envelopes of fiery
gases; from some cause not at all understood these gases may, at
regular periods, flame up with fiercer heat than usual, and produce this
appearance of greatly increased light.
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