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EXPLODING stars leave behind glowing clouds of gas with myriad shapes, much
to the puzzlement of astrophysicists. Now an Australian astronomer has suggested
that barrel-shaped supernova remnants may arise due to interstellar magnetic
fields and 鈥渂ubbles鈥�. The work might give astronomers a tool to map the
invisible interstellar medium.

Brian Gaensler, an astronomer at the University of Sydney, analysed the
orientation of supernova remnants which are elongated and barrel-shaped rather
than spherical. He discovered that of 17 of these remnants, more than half were
aligned along the plane of the Galaxy鈥攆ar more than would be expected by
chance alone.

Gaensler suggests in the current issue of The Astrophysical Journal (vol 493,
p 781) that the shapes arise because of the Galaxy鈥檚 magnetic field, which is
roughly aligned with the Galactic plane. He says that during a star鈥檚 lifetime,
material streaming out from its atmosphere would expand most easily along the
field lines, creating an elongated 鈥渂ubble鈥� in interstellar hydrogen clouds. If
the star eventually exploded, the debris would expand more easily into the
bubble than into the denser interstellar medium, explaining the symmetrical
barrel shape.

鈥淚magine a pile of leaves which is mostly flat, and a firecracker explodes in
the centre,鈥� explains John Dickey, an astronomer at the University of Minnesota
in Minneapolis. 鈥淭he leaves would flatten out the explosion, but sideways it
would shoot out without much resistance.鈥�

鈥淚t seems that the surroundings influence the expansion of supernova remnants
more than the ejection itself,鈥� says John Dickel of the University of Illinois
in Urbana-Champaign. 鈥淚t鈥檚 something that Brian has helped bring to people鈥檚
attention.鈥� Dickey agrees, noting that it might be possible to use supernovae to
figure out the shapes of invisible regions of hydrogen gas. 鈥淚t鈥檚 a significant
step forward.鈥�