EVOLUTION OF STARS—-ABBOT 177 
5. The telescope with its photographic plate shows us that the 
vast spaces of the universe hold cloudhke matter called nebulae. 
Some of these clouds are bright, some dark. ‘The star, Theta Orionis, 
near the base of Orion’s sword is really nebulous. It is involved in 
the great nebula of Orion. The Pleiades stars are also surrounded 
by nebulosity. In fact, nearly every one of the irregular nebulae 
is associated with a bright star. Hubble has suggested that it is 
these stars which cause the nebulae to glow. 
This view is the more probable because many of the nebulae do 
not glow. Thus in the Milky Way are many dark patches devoid of 
faint stars, apparently because a cloudy veil of nebulous matter lies 
between us and the starry background. <A very striking example of 
this sort of thing is in the horse-head shaped blackness in the con- 
stellation of Orion. Then there is the lacelike nebula in Cygnus, 
which we suppose extends farther than it glows, because the fainter 
stars are absent on one side of it. 
Thus we see that among the stars lies much cloudlike, unorganized 
substance which may seem to be suitable raw material to make up 
into stars. The spectroscope shows little complexity in the chemistry 
of such nebulae. Hydrogen is prominent, and some other gases 
whose spectra are unfamiliar. Perhaps these strange gases are 
really of some common variety whose unfamiliar spectra depend 
on an unknown means of exciting light. 
6. The stars themselves show much variety. [iven ordinary eye- 
sight can tell a difference between the blue star Rigel and the red 
Betelgeuse in Orion; or between the white Sirius and the yellow 
Procyon of the two Dogs. The spectrum gives this distinction more 
precisely. It shows a regular gradation of complexity from the 
blue Rigel, with its few lines revealing the presence of the gas 
helium; to the white Sirius, with hydrogen lines predominant; to 
the yellowish Procyon, which faintly shows evidences of many 
metals; to the yellow sun whose light is considerably dimmed 
by the very numerous dark absorption lines of its many chemical 
elements; to the reddish Aldebaran in whose spectrum, as in 
the light of sun-spots, evidences of compound molecules appear; and 
finally to the deep red Antares, from whose rays most of the visible 
strength is cut off by powerful bands of absorption, due to compounds 
of carbon, nitrogen, and various gases. 
These differences of spectrum are associated with temperature 
differences. Many years ago at the Harvard College Observatory, 
as the observers noted the spectrum peculiarities we have just spoken 
of, they named the various star types by an irregular sequence of 
letters that still prevails. The table shows the characteristic spectra 
and temperatures of 99 per cent of all the stars arranged after the 
Harvard type-letter system. 
