The name is absent

The Light of the Stars 61
Harvard, had noticed that the lines were exceptionally
narrow and sharp, were much superior in absolute mag-
nitude to the general run of stars.

The greatest advance, however, was made eight or
ten years later by Adams and Kohlschiitter. By this time
reliable information was at hand regarding the absolute
magnitudes of a large number of stars. Photographing
the spectra of a selected list of these with the same in-
strument, and comparing the plates, the Mount Wilson
investigators were able to detect many small differences
which had escaped earlier notice. Certain lines were
stronger in the giant stars, and others in the dwarfs.
What is more, the relative intensities of certain pairs of
such lines (one of each kind) was found to vary in a
regular manner with the absolute magnitude. When the
law of this variation had once been determined, with the
aid of a dozen or so stars of a given spectral class for
which the parallaxes and absolute magnitudes were known,
it became possible to take the spectrum of any other star
of this class, examine the lines in question, and find from
their relative intensity what is the star’s absolute magni-
tude—in other words, how bright it really is. Knowing
this, and the star’s apparent brightness, we can immedi-
ately calculate its distance and parallax.

In this way the way opened for the determination of
“spectroscopic parallaxes.” The method is surprisingly
accurate—the average error of a determination of paral-
lax being about twenty per cent.—and very rapid in prac-
tice, since it takes much less time to get the two or three
spectrograms which are sufficient for the purpose than to
take and measure the dozen or fifteen plates which are
required for a determination by the direct or “trigono-
metric” method. Moreover, since the probable error is

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