and Father Secchi,[24] of Rome. The former began by devoting years of
work principally to the most accurate study of a few stars. The
latter devoted the years from 1863 to 1867 to a general survey of the
whole heavens, including 4,000 stars. He divided these into four
principal classes, which have been of the greatest service. Half of
his stars belonged to the first class, including Sirius, Vega,
Regulus, Altair. The characteristic feature of their spectra is the
strength and breadth of the hydrogen lines and the extreme faintness
of the metallic lines. This class of star is white to the eye, and
rich in ultra violet light.

The second class includes about three-eighths of his stars, including
Capella, Pollux, and Arcturus. These stars give a spectrum like that
of our sun, and appear yellowish to the eye.

The third class includes alpha Herculis, alpha Orionis (Betelgeux), Mira
Ceti, and about 500 red and variable stars. The spectrum has fluted
bands shaded from blue to red, and sharply defined at the more
refrangible edge.

The fourth class is a small one, containing no stars over fifth
magnitude, of which 152 Schjellerup, in Canes Venatici, is a good
example. This spectrum also has bands, but these are shaded on the
violet side and sharp on the red side. They are due to carbon in some
form. These stars are ruby red in the telescope.

It would appear, then, that all stars are suns with continuous
spectra, and the classes are differentiated by the character of the
absorbent vapours of their atmospheres.