The name is absent

The Light of the Stars 63
a thousand times as bright as the Sun, or even brighter.
These all show the narrow sharp lines noticed by Miss
Maury, and are often called с-stars, following her nota-
tion. The average parallax of these stars can be ob-
tained with considerable accuracy from their proper mo-
tions, taking into account the apparent drift produced
by the Sun’s motion in space, so that the spectroscopic
parallaxes have here another firm point of support. Some
of these с-stars are variable in a highly distinctive fash-
ion, known as Cepheid variation, from the star Delta
Cephei. The changes in brightness are continuous, with
a regular period of a few days or weeks, and are ac-
companied by changes in color and spectrum—the star
being redder, and further advanced in spectral type, at
minimum. Whenever we find a star that varies in this
fashion we can be morally certain that it is several hun-
dred times brighter than the Sun.

Now a large number of faint stars which vary in
brightness in exactly this way occur in the Magellanic
Clouds—those remarkable patches, like outlying fragments
of the Milky Way, which lie near the south pole of the
heavens. They must be all at substantially the same dis-
tance from us. Miss Leavitt, at Harvard, found some
years ago that there is an extraordinarily exact connec-
tion between the average brightness of these stars and
their periods—those of longer period being the brighter.
It is extremely probable that this relation holds true also
among the nearer Cepheid variables which have already
been mentioned. If this is so we can determine the ab-
solute magnitude of such a variable, and hence its dis-
tance, from a mere knowledge of its period. Following
this line, Shapley has determined the distances of more
than a hundred isolated Cepheid variables, some of which

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