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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