EMBARGOED UNTIL:  1:00 P.M. (EST) DECEMBER 17, 1997

PHOTO NO.:  STScI-PRC97-38a

HUBBLE SEES SUPERSONIC EXHAUST FROM NEBULA

M2-9 is a striking example of a "butterfly" or a bipolar 
planetary nebula. Another more revealing name might be the 
"Twin Jet Nebula."  If the nebula is sliced across the star, 
each side of it appears much like a pair of exhausts from 
jet engines. Indeed, because of the nebula's shape and the 
measured velocity of the gas, in excess of 200 miles per second, 
astronomers believe that the description as a super-super-sonic 
jet exhaust is quite apt. Ground-based studies have shown 
that the nebula's size increases with time, suggesting that the 
stellar outburst that formed the lobes occurred just 1,200 
years ago.

The central star in M2-9 is known to be one of a very close pair 
which orbit one another at perilously close distances. It is 
even possible that one star is being engulfed by the other.  
Astronomers suspect the gravity of one star pulls weakly bound 
gas from the surface of the other and flings it into a thin, dense disk 
which surrounds both stars and extends well into space.

The disk can actually be seen in shorter exposure images 
obtained with the Hubble telescope. It measures 
approximately 10 times the diameter of Pluto's orbit. Models 
of the type that are used to design jet engines 
("hydrodynamics") show that such a disk can successfully 
account for the jet-exhaust-like appearance of M2-9. The 
high-speed wind from one of the stars rams into the 
surrounding disk, which serves as a nozzle. The wind is
deflected in a perpendicular direction and forms the pair of 
jets that we see in the nebula's image. This is much the 
same process that takes place in a jet engine: The burning 
and expanding gases are deflected by the engine walls 
through a nozzle to form long, collimated jets of hot air at 
high speeds.

M2-9 is 2,100 light-years away in the constellation 
Ophiucus. The observation was taken Aug. 2, 1997 by the 
Hubble telescope's Wide Field and Planetary Camera 2. In this 
image, neutral oxygen is shown in red, once-ionized nitrogen 
in green, and twice-ionized oxygen in blue.

Credits:  Bruce Balick (University of Washington),
Vincent Icke (Leiden University, The Netherlands),
Garrelt Mellema (Stockholm University), and NASA