168 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
dense into small planetesimals moving around the sun and that 
these planetesimals would later stick together to form the planets 
by accretion. 
In 1917 the English astronomers Jeans and Jeffreys made more 
exact calculations and concluded that the eruptions would not have 
taken place; rather, the intruding star would have to sidewipe the 
sun, peeling off a long filament of solar material which would then 
condense into the planets. They pointed out that this filament would 
be thicker in the middle than at the ends, thereby accounting for the 
progression of planetary sizes. 
The Jeans-Jeffreys hypothesis seemed satisfactory until 1930, when 
Nolke in Germany and Russell at Princeton pointed out another clue: 
the angular momentum of the planets. Just as a spinning top would 
keep on spinning forever if there were no friction, so the planets must 
have maintained constant angular momentum in their orbits around 
the sun, since nothing analogous to friction is known in the solar 
system. If the planets were formed from material pulled out of the 
sun, this law of conservation of angular momentum requires that the 
original planetary material must have started moving around the 
sun with the same angular momentum the planets have today. 
Russell showed mathematically that a grazing collision with another 
star could not start the filament of planetary material off with any- 
where near enough angular momentum. 
In an effort to patch things up, one of Russell’s students, Lyttleton, 
analyzed mathematically the case of a collision between three stars, 
and found that it was just possible to produce a filament of material 
moving with sufficient angular momentum about one of them. An 
English astronomer, Hoyle, showed it was also possible if one of two 
close stars blew up, as a somewhat asymmetrical nova, propelling 
itself away and leaving some planetary material moving around the 
other star. 
But these mathematical exercises and the whole sequence of specula- 
tions based on the two-star hypothesis were brought sharply to a close 
in 1939 when Spitzer, another of Russell’s students, calculated that 
the material pulled out of the sun, or any other star, could not con- 
dense into planets or planetesimals anyway—it would expand with 
explosive violence to form a tenuous gaseous nebula! 
BACK TO THE NEBULAR HYPOTHESIS 
Long before Spitzer had showed that the two-star hypothesis would 
lead to a nebula, other scientists had been working away on the 
nebular hypothesis, trying to find some means by which material 
near the sun would form a group of planets all moving in the same 
direction in nearly circular orbits and in nearly the same plane. In 
