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.

As a general rule, the light of these stars goes through its
changes in a regular period, which is sometimes as short as a few
hours, but generally several days, frequently a large fraction of
a year or even eighteen months. Observations of these stars are
made to determine the length of the period and the law of
variation of the brightness. Any person with a good eye and skill
in making estimates can make the observations if he will devote
sufficient pains to training himself; but they require a degree of
care and assiduity which is not to be expected of any one but an
enthusiast on the subject. One of the most successful observers of