Sky Reporter 
Debut of an Interstellar Bubble 
A new, fourth type of emission nebula is found 
by Stephen P. Maran 
The Rosette nebula is a beautiful 
glowing gas cloud that surrounds a 
cluster of young hot stars in the constel- 
lation Monoceros, about 5,200 light- 
years from the earth. It has long 
puzzled astronomers, but a consensus 
has now been reached: the Rosette is a 
large interstellar bubble. 
The term interstellar bubble was it- 
self invented only six years ago, a dec- 
ade after the first suggestion that such 
objects might exist. It refers to a type of 
emission nebula characterized by a 
low-density cavity formed by winds 
from one or more stars that push away 
the surrounding interstellar gas. The 
displaced gas forms a thick shell 
around the cavity, like the piles of snow 
at the side of a freshly plowed road. 
Emission nebulae are clouds of cos- 
mic gas that shine by their own light, 
rather than by reflecting the light of 
stars. Until now, they were classified 
into three types: H II regions, planetary 
nebulae, and remnants. H II regions 
consist of interstellar gas that is heated 
and made to glow by the ultraviolet 
rays from adjacent large hot stars. The 
Orion nebula ( Natural History, May 
1980) is a good example. Planetary neb- 
ulae, such as the Ring nebula in Lyra, 
are formed from the outer layers of red 
giant stars. After the layers are ejected, 
only the small blue cores of the once 
grossly extended giants remain. Rem- 
nants consist of rapidly expanding ma- 
terial from exploding stars (novae and 
supernovae). The Crab nebula is the 
best-known supernova remnant. 
Interstellar bubbles, now recognized 
as a fourth type of emission nebula, are 
like H II regions and planetary nebulae 
in that they shine because the stars 
within them stimulate them with ultra- 
violet light. However, this type of emis- 
sion nebula differs from the other types 
in being shaped by the winds from 
those stars. 
The recognition of interstellar bub- 
bles as a distinct type of nebula can be 
traced back to an investigation of the 
Rosette nebula by the radio astronomer 
T. K. Menon at Green Bank, West Vir- 
ginia, in 1961. He mapped the Rosette 
with an 85-foot radio telescope of the 
National Radio Astronomy Observa- 
tory at Green Bank. Although Menon 
did not himself develop the theory of 
interstellar bubbles, his map was the 
first of several clues that eventually led 
astrophysicists to identify this new type 
of nebula. 
At the time of Menon’s study, the 
Rosette nebula was classified as an H II 
region. In fact, a photograph of the neb- 
ula was used to illustrate the concept of 
an H II region in a college astronomy 
textbook published in the same year 
that Menon mapped the Rosette. The 
caption for the photograph did not 
mention the striking difference between 
the Rosette and other H II regions such 
as the Orion nebula, although the dif- 
ference was readily apparent. The Ro- 
sette nebula seems to have a relatively 
darker central region, surrounded by a 
brighter zone, whereas the Orion neb- 
ula has a bright center. 
Menon wished to explore the nature 
of the dark central region in the Rosette 
nebula. Telescopic photographs record 
nebular light that is produced by gas 
with temperatures of about 4,000° to 
10,000°C. The “hole” seen in photo- 
graphs of the Rosette might be due, 
Menon reasoned, either to a substantial 
deficiency of gas or to the presence of 
gas just as dense as that found in the 
bright rim of the nebula but at a much 
higher temperature. Nebular gas of a 
few hundred thousand degrees or hot- 
ter would not glow noticeably in visible 
light but would emit strong radio 
waves. Thus, if the radio map of the Ro- 
sette were to show a bright central re- 
gion, the presence of much superhot 
gas would be indicated, but if the map 
showed a dim hole, like that in the tele- 
scopic photographs, a deficiency of gas 
would be confirmed. A third possibili- 
ty, that the apparent hole might be due 
to a cloud of dark interstellar dust, is 
ruled out because the star cluster at the 
center of the Rosette is plainly visible 
on the photographs. 
After scanning the Rosette nebula 
with the 85-foot Green Bank radio tele- 
scope, Menon presented the results in 
the form of a contour map of radio- 
brightness levels that resembles a 
geographer’s contour map of terrain al- 
titudes. At the center of the series of 
closed solid curves on this map, there 
appears a small, dashed loop. The 
dashes indicate a dip in the middle of 
the nebula, rather than a peak in radio 
brightness. Thus there is indeed a defi- 
ciency of gas in the center of the Ro- 
sette. The observations were incapable 
of determining just how hot and thin 
the residual gas in the aforementioned 
hole seen in the photographs might be. 
Subsequent observations made with 
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