| 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.
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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. In this
image, neutral oxygen is shown in red, once-ionized nitrogen in green,
and twice-ionized oxygen in blue.
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