extreme left, have not been matched in our laboratories and,
therefore, are of unknown origin. Most of the irregular nebulae
whose spectra have been observed, the ring nebulae, the
planetary and stellar nebulae, have very similar spectra,
though with many differences in the details.[1]

[1] My colleague, Wright, who has been making a study of the
nebular spectra, has determined the accurate positions of about
67 bright nebular lines.



The great spiral nebula in Andromeda has a continuous spectrum
crossed by a multitude of absorption lines. The spectrum is a
very close approach to the spectrum of our Sun. It is clear
that this spiral nebula is widely different from the
bright-line or gaseous nebulae in physical condition. The
spiral may be a great cluster of stars which are approximate
duplicates of our Sun, or there is a chance that it consists,
as Slipher has suggested, of a great central sun, or group of
suns, and of a multitude of small bodies or particles, such as
meteoric matter, revolving around the nucleus; this finely
divided matter being visible by reflected light which
originates in the center of the system.

There is an occasional star, like chi Carinae, whose spectrum
consists almost wholly of bright lines, in general bearing no
apparent relationship to the bright lines in the spectra of the
gaseous nebulae except that the hydrogen lines are there, as
they are almost everywhere. There is reason to believe that