involves subsonic flow, Mathews’s 
third proposal in 1967 that the cavity 
was produced by the pressure of star- 
light, has been eliminated. 
You might think it an elementary 
matter to determine whether the super- 
sonic flow in the Rosette is directed in- 
ward or outward, but such is not the 
case. The velocity is determined by 
measuring the Doppler shifts of spec- 
tral lines. Due to the Doppler effect, gas 
moving away from the earth emits lines 
that are red-shifted, while gas ap- 
proaching the earth emits blue-shifted 
lines. The size of the Doppler shifts ob- 
served in the spectrum of the Rosette 
nebula tells us that the flow is superson- 
ic. If the nebular gas were flowing out- 
ward, then gas on the near side of the 
nebula would produce blue-shifted 
lines; if the gas were flowing inward, 
then blue-shifted lines would come 
from the gas on the far side. But since 
the nebula is transparent and light from 
both its near side and far side is record- 
ed simultaneously with a spectrograph, 
when we find a blue-shifted line, we do 
not know whether it comes from the 
front or the back of the Rosette. We 
know how/asr the gas is going, but not 
which way it is going. 
One team of astronomers found indi- 
cations that the flow 7 in the Rosette is 
inward, as predicted by the cavitation 
theory, but this result is not very per- 
suasive. A more conclusive finding 
comes from studies with the Interna- 
tional Ultraviolet Explorer (IUE) satel- 
lite. These observations show that the 
two brightest stars in the Rosette neb- 
ula actually do have strong stellar 
winds. 
According to Peter Conti, an expert 
on stellar winds and interstellar bubbles 
at the University of Colorado in Boul- 
der, the brightest star in the Rosette has 
more than fifty times the mass of the 
sun and is nearly a million times 
brighter. Its wind alone may have been 
sufficient to blow out the large cavity 
we now observe in the nebula. 
A final test, now being carried out 
with the IUE, is attempting to measure 
the temperature of the gas in the hole of 
the Rosette. If, as the wind theory pre- 
dicts, this thin gas has a temperature of 
about 500,000°C, then our recently 
formed conviction that the Rosette 
nebula is an interstellar bubble will be 
vindicated. 
Stephen P. Maran is a senior staff scientist 
in the Laboratory for Astronomy and So- 
lar Physics at NASA's Goddard Space 
Flight Center in Greenbelt, Maryland. 
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