The San Francisco call., 1906-06-24

parent size Of the stars. but they do in
crease their brilliancy, and, more i'm
portant still, they serve TO reveal many
stars which are invisible to the naked
eye. 1t is estimated that there are
about 5000 stars in the entire heavens
that can be seen without the aid of a
telescope. and that fewer than half that
number are in sight at any one time
because half of the heavens is con-
cealed below the horizon and many of
the fainter stars are lost to view when
they approach the horizon, owing to
the greater thickness of air through
which we must view them. But with
a small telescope of only an inch aper
ture the number of visible stars will be
that Of these fully 10000 are variable
stars. This offers vast field for study
to any one who possesses a good chart
of the heavens and can identify the
stars he sees with those on his chart
1t was the custom of the ancients
to classify the stars according to their
brilliancy. the fifteen or twenty bright
est ones being graded as first magni-
tude stars and those barely visible to
the naked eye as sixth magnitude stars,
the other stars being assigned accord
ing TO their light to the four interme-
diate grades. Modern astronomy has
adopted this same system of classifi-
cation but has extended it to include
the stars which can be seen only with
the aid of the telescope. These require
the addition of ten or eleven grades of
ma gnitude.
Each magnitude is taken as about
two and one-half times as bright as the
next lower magnitude, or as exactly
a hundred times brighter than the mag
nitude FIVE times lower in the scale.
Thus a first ma gnitude star is a hum
dred times as bright as a sixth magni-
tude star ten thousand times as bright
as a eleventh magnitude star and mill
lion times as bright as a sixteenth mag
nitude star, which is about the faintest
point of light visible through our larg-
est TELESCOPES.
The magnitude of a star is deter
mined by comparing it with another
star and cutting off the light of one or
the other with shade glasses until they
appear equally bright Another and
more modern method is to photograph
a group of stars and then compare the
impressions on the Photographic plate.
1t is found that the brighter the star
the larger will be the spot ii makes In
the photograph. This method is, of
course, not open to the layman, because
it requires a specially mounted tele-
scope, arranged to be moved by clock
work, so that it will accurately follow
the stars as they move across the sky.
But remarkably accurate observations
of stars can be made by simply looking
ar a stor for a moment until its image
is well measured by the eye and then
comparing this image with an adjacent
star Of about the same magnitude. OBJ
servations OF this sort taken two or
three times each night a star which
is suspected to be a variable, may re-
veal remarkable fluctuations in it's
brightness.
There are many different kinds of
variable stars. Some, like Mars, give a
constant light except at certain irreg-
ular periods, when they brighten meas
urably- others are normally bright, but
at short periods fade for few hours.
Others seem 10 obey no law or system.
changing most srregularly, and still
others flash forth for few days, then
fade rapidly and soon disappear entire
~, even from the view of the highest
powered telescope
In the case o. stars that vary at reg