Space bubbles knock exploded stars into shape

By Charles Seife 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 “bubbles”. 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—far 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’s magnetic field, which is roughly aligned with the Galactic plane. He says that during a star’s lifetime, material streaming out from its atmosphere would expand most easily along the field lines, creating an elongated “bubble” 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. “Imagine a pile of leaves which is mostly flat, and a firecracker explodes in the centre,” explains John Dickey, an astronomer at the University of Minnesota in Minneapolis. “The leaves would flatten out the explosion, but sideways it would shoot out without much resistance.” “It seems that the surroundings influence the expansion of supernova remnants more than the ejection itself,” says John Dickel of the University of Illinois in Urbana-Champaign. “It’s something that Brian has helped bring to people’s attention.” Dickey agrees,
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