THE FORMATION OF STARS! 
By Lyman Spirzer, Jr. 
Princeton University Observatory 
[With 2 plates} 
If research in astronomy had stopped in 1913, our knowledge of 
stellar evolution today would be in a satisfactory state. At that time 
astronomers had a plausible theory of a star’s life cycle. Einstein’s 
theory of relativity, advanced only a few years before, showed that 
mass and energy were interchangeable. It was therefore natural for 
astronomers to assume that stars were formed as large massive bodies 
which through successive century after century continued to radiate 
away matter. Ultimately most of the matter in a star, according to 
this picture, would be radiated away as light and heat. In this way 
all the stars, despite their large differences in mass, formed part of 
the same evolutionary sequence. 
Unfortunately, this simple, sweeping, and satisfying picture became 
discredited by additional information, both astronomical and physi- 
cal. On the astronomical side, evidence began to accumulate that 
the universe has not lasted long enough for most stars to radiate 
away much of their matter. The expansion of the universe, the 
presence of uranium on the earth, the existence of certain relatively 
transitory clusters of stars, all indicate that something happened 
about 3 billion years ago. If the universe was not created then, it 
was certainly very extensively reorganized; some sort of cosmic ex- 
plosion apparently took place at that time. Since the sun, a fairly 
typical star, would require many hundreds of billions of years to 
radiate an appreciable fraction of its mass, its total mass has ob- 
viously not changed appreciably within this last few billion years. 
On the physical side, nuclear physicists have learned a great deal 
about the specific processes by which matter can be converted into 
energy. The only known process of importance which can liberate 
energy inside a star is the combination of four hydrogen atoms to 
form a helium atom. Calculations carried out by the nuclear phys- 
icist, Prof. Hans Bethe, show that in the stars this process occurs 
through the catalytic action of carbon and nitrogen nuclei. Since 
1 Reprinted by permission from Physics Today, vol. 1, No. 5, September 1948. 
153 
