48 Recent Advances in Stellar Astronomy
their distances great, averaging about 100 parsecs. Two
of them are of the first magnitude, and much excel in
luminosity any of the stars which we have previously men-
tioned, Beta Centauri possessing a luminosity 2,500 times
the Sun’s while Antares (Alpha Scorpii) exceeds the Sun’s
light a little more than three thousand fold. There are
many other stars in this cluster which are several hundred
times brighter than the Sun.
Another similar star group comprises most of the con-
spicuous stars in Orion, and many in Canis Major. This
group is even farther off, but it is difficult to estimate its
distance with precision, for it is moving almost straight
away from us, so that the stars seem to stand almost still
in the sky. The majority of the stars appear to be at
distances of between ISO and 200 parsecs. Among these
are the three stars of Orion’s belt, which average about
4,500 times as bright as the Sun, and the still brighter
Rigel, in the southern part of the constellation, which ap-
pears to give out some 13,000 times the Sun’s light and
be the brightest star at present known. Two other stars
of the first magnitude, Canopus and Alpha Cygni, are also
known to be exceedingly remote, but we cannot yet measure
their distances. One or both may be as luminous as Rigel,
or even brighter.
We have, therefore, among the stars a range in real
luminosity of at least a hundred million fold. The physical
interpretation of such great differences must obviously be
of prime importance. But before we can discuss it intel-
ligently we must analyze the light of the stars more
thoroughly. We can do this best by photographing their
spectra. For those visible to the naked eye this can be
done in considerable detail; and the brighter telescopic
stars, down to the ninth or tenth magnitude, are accessible