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a more advanced radio telescope con- 
firmed Menon’s findings, and we now 
know that the Rosette nebula consists 
of a central cavity almost seventy light- 
years in diameter surrounded by a 
bright, roughly spherical shell of gas, 
about twenty-three light-years thick 
and containing enough material to 
make over 1 1,000 suns. The nebula has 
attracted the attention of many as- 
tronomers since Menon’s study, and a 
variety of ideas have been proposed to 
explain its unusual form. 
The first of these ideas was that as the 
young stars at the heart of the nebula 
were formed, their ultraviolet radiation 
began traveling out through the sur- 
rounding gas, heating and ionizing it 
and also producing a shock wave that 
pushed the gas outward. Mathematical 
analysis, however, soon showed that 
any such shock wave would be too 
weak to produce a cavity of the ob- 
served proportions. 
Two other theories, which appeared 
to be based on sounder principles, were 
proposed in 1966 by William G. Math- 
ews, a young postdoctoral fellow at the 
Mount Wilson and Palomar Observa- 
tories in Pasadena, California. One the- 
ory he advanced was the idea that the 
hole in the Rosette nebula had resulted 
from the formation of the star cluster in 
its center. In other words, according to 
this “cavitation” theory, the missing 
gas had condensed to form the observed 
stars. Mathews’s second theory, now 
recognized as the first proposal for the 
existence of interstellar bubbles, was 
that the hottest stars in the Rosette 
cluster have powerful, high-tempera- 
ture stellar winds, which sweep away 
the surrounding gas, thus producing 
the hole. 
One point that favors the cavitation 
theory is that the amount of missing gas 
(which can be estimated by assuming 
that the hole in the Rosette nebula was 
once filled by gas as dense as that now 
found in its bright rim) is about equal to 
the total mass of the stars in the cluster 
at the nebula’s center. Since nature ab- 
hors a vacuum, the cavitation theory 
predicts that the gas in the rim is now 
pouring into the hole. Specifically, an 
inflow of gas at a velocity higher than 
the speed of sound (about six miles per 
second in the nebula as compared with 
one-fifth of a mile per second on Earth) 
is predicted. 
Mathews’s second theory, which as- 
cribes the hole in the Rosette nebula to 
the action of hot stellar winds, has been 
updated by several other astrophysi- 
cists. In its revised form, the wind the- 
ory predicts that the bright gas on the 
rim of the hole is flowing outward at a 
speed above the speed of sound. Ac- 
cording to this theory, since the hole is 
filled by outward-blowing winds, it is 
also predicted that the thin gas in the 
hole is at a temperature of about 
500,000°C. 
At the time his first two theories 
were proposed, Mathews recognized 
that both of them invoked processes not 
actually known to occur. There were no 
other known cases of holes formed in 
nebulae as a result of the condensation 
of stars nor were stars of the type found 
in the center of the Rosette nebula 
known to possess powerful stellar 
winds. In 1967, these uncertainties led 
Mathews to propose a third theory. 
According to this theory, the central 
region of the Rosette nebula was origi- 
nally filled by a mixture of gas and mi- 
croscopic particles of interstellar dust. 
Such dust is found, for example, mixed 
in with the shining gases of the Orion 
nebula. Mathews proposed that the 
dust was electrified by ultraviolet light 
from the centrally located stars and 
that it was pushed outward from the 
center of the nebula by the pressure of 
light from the newly formed stars in the 
cluster. This process would be analo- 
gous to the formation of the dust tail of 
a comet, which occurs when sunlight 
pushes tiny dust particles away from 
the head of the comet. The outward- 
moving dust of the Rosette would then 
in effect drag the adjacent gas outward 
with it, accounting for the formation of 
the central hole. This theory predicts 
that the gas on the rim of the hole is 
flowing outward, as in the wind theory, 
but at less than the speed of sound. 
In summary, two of Mathews’s theo- 
ries predict outward flow although they 
differ from each other in whether the 
flow is subsonic or supersonic. Math- 
ews’s other theory predicts supersonic 
inward flow. Thus no two theories 
agree on both the direction and the 
speed of the gas flow, and so observa- 
tional tests should be possible. This cir- 
cumstance inspired numerous observa- 
tions including those of astronomers at 
the Cerro Tololo Inter-American Ob- 
servatory in La Serena, Chile; the Mar- 
shall Space Flight Center in Huntsville, 
Alabama; the University of Manches- 
ter in England; and the Algonquin Ra- 
dio Observatory in Ottawa, Canada. 
These many studies of the Rosette 
nebula came to a common conclusion. 
Speeds well above the six miles per sec- 
ond velocity of sound were observed in 
the nebula. Hence the one theory that 
30 
