170 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
The closest star—our sun—is radiating energy at such a stupendous 
rate that no ordinary energy generator could keep it going for the 3 
billion years we know it has been shining on the earth. However, it 
is now known that atomic energy provides the sun’s light and heat 
by a process in which four atoms of hydrogen are converted into one 
atom of helium and the excess mass changed into radiant energy. The 
details of this process, which can only proceed at the high temperature 
and pressure of a star’s interior, were established by Hans Bethe at 
Cornell in 1938. But there are many hot stars thousands of times 
brighter than the sun (if viewed from the same distance), and a 
simple calculation shows that they would use up all their atomic 
energy in a mere 10 million years. Where did these hot bright stars 
come from if they can last only one three-hundredth as long as the 
earth has been in existence? 
A possible answer was provided only last year (1947) by Lyman 
Spitzer at Yale, and Bart Bok at Harvard. Spitzer showed theoreti- 
cally that diffuse gas and dust which is observed between the stars 
could, under some circumstances, be compressed by the pressure of 
radiation from all the other stars, to condense into a new star. Bok 
observed in the Milky Way certain small dark knots of such inter- 
stellar material, which may well be stars in the process of formation. 
Here is the process of growth by accretion on a much larger scale. 
This theory is well enough established that Whipple at Harvard has 
recently proposed that the planets coagulated in the manner postu- 
lated by Kant and by Weizsicker during the formation of the sun 
itself. 
GALAXIES 
As we are pushed farther and farther in explaining the origin of our 
planet, new sources of evidence come into the problem. The next 
evidence comes from a study of the large groups of stars called galaxies. 
Passing from the solar system to the stars is no larger a jump—and 
no smaller—than from the earth to the solar system. Our galaxy 
includes all the visible stars and is a correspondingly large system, 
outside of which the telescope shows many other galaxies. These are 
believed to be very like our own galaxy—a disk-shaped conglomera- 
tion with a mass, determined from its rotation, of about 200 billion 
star masses. There are about 100 billion stars in a galaxy, the rest 
of the material being spread between the stars in the form of gas and 
dust. 
The outside galaxies, often called “spiral nebulae,” are being 
studied by Hubble at the Mount Wilson Observatory in California, 
and by other astronomers with large telescopes. As Hubble looks 
farther and farther out into space (by taking longer photographic 
exposures with larger and larger telescopes), he finds more and more 
