FORMATION OF STARS—SPITZER 155 
If it is assumed that these stars have in fact been formed within 
the last hundred million years, the mechanism for this formation is a 
problem which astronomers may hope to investigate with some hope 
of success. Within this interval, conditions in the universe have 
apparently not changed very much and an examination of the universe 
about us may actually indicate how supergiant stars have formed in 
the past, and may even be forming at the present time. 
CLOUDS—THE CLUE 
The clouds of matter which float about between the stars are an 
obvious source of material for star formation. Recent investigations 
show that these clouds are in fact so closely associated with supergiant 
stars that a physical connection between them seems very likely. 
In brief, the observations indicate that supergiant stars are found 
only in those aggregations of stars where interstellar clouds of matter 
are also present. More specifically, observations of stellar galaxies, 
each one a million or so light-years away and each, like our own 
galaxy, containing many billions of stars, show that supergiant stars 
are found only in spiral galaxies. These spiral systems, like the 
huge galaxy in which our sun is located, are flattened, disk-shaped 
systems some hundred thousand light-years in diameter, each one 
rotating about an axis perpendicular to the plane of its disk. A 
typical spiral galaxy is shown in plate 1, figure 2. The characteristic 
feature of these systems, after which they are named, is the presence 
of a pair of arms which apparently come out of the central nucleus and 
wind around the system. 
In the elliptical galaxies—which are not rotating so rapidly, are 
not so flattened, and show no spiral structure—no supergiant stars 
are found. In fact, long-exposure plates at the Mount Wilson Ob- 
servatory have shown that the stars in these systems have a sharp 
upper limit on their brightness; no star greater than the critical bright- 
ness can be found, while below this critical brightness myriads of 
stars appear on the photographic plate. This result is in marked 
contrast to the observed brightness of the stars in spiral galaxies, 
where there are always one or two brightest supergiant stars, a number 
of less bright supergiants, and a gradually increasing number of 
fainter and fainter stars. This sharp upper limit on the brightness 
of stars in elliptical galaxies is just what one would expect if no new 
stars had been formed since the beginning of the universe, and if 
the brightest ones had burned up all their fuel and gone out. 
Detailed examination of galaxies also indicates that clouds of matter 
between the stars are found only in spiral systems. In elliptical 
galaxies the vast stretches between the stars are very nearly empty, 
but in flattened spiral galaxies like our own there is about as much 
