By a comparison of the spectra of stars Sir William Huggins has
developed the idea that these bodies, like human beings, have a
life history. They are nebulae in infancy, while the progress to
old age is marked by a constant increase in the density of their
substance. Their temperature also changes in a way analogous to
the vigor of the human being. During a certain time the star
continually grows hotter and hotter. But an end to this must come,
and it cools off in old age. What the age of a star may be is hard
even to guess. It is many millions of years, perhaps hundreds,
possibly even thousands, of millions.
Some attempt at giving the magnitude is included in every
considerable list of stars. The work of determining the magnitudes
with the greatest precision is so laborious that it must go on
rather slowly. It is being pursued on a large scale at the Harvard
Observatory, as well as in that of Potsdam, Germany.
We come now to the question of changes in the appearance of bright
stars. It seems pretty certain that more than one per cent of
these bodies fluctuate to a greater or less extent in their light.
Observations of these fluctuations, in the case of at least the
brighter stars, may be carried on without any instrument more
expensive than a good opera-glass--in fact, in the case of stars
visible to the naked eye, with no instrument at all.
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