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Mystery of the 'butterfly' nebulae alignment

Compass needles? Astronomers have discovered something weird in the Milky Way's galactic bulge - a population of planetary nebula are all mysteriously pointing in the same direction.

They noticed the mysterious alignment in the long axes of bipolar planetary nebulae.

Planetary nebulae are caused by the death of red giant stars. During their final years, long after the hydrogen fuel has run out in their cores, these puffed up stars begin to shed their outer layers, blasting huge quantities of material into space.

At the end of its life our Sun will also enter into a red giant phase, swallowing up the inner solar system planets (possibly even Earth), eventually creating its own planetary nebula.

The resulting nebulous clouds can take on many beautiful shapes, but bipolar planetary nebulae can be the most striking, generating two lobes of material expanding in opposite directions. These nebulae often resemble butterfly wings.

Although the surveyed nebulae are completely separate, non-interacting and are of various ages, the researchers noticed a large number of the nebulae long axes are aligned.

"This really is a surprising find and, if it holds true, a very important one," says Bryan Rees of the University of Manchester, co-author of the paper to appear in the journal Monthly Notices of the Royal Astronomical Society.

"Many of these ghostly butterflies appear to have their long axes aligned along the plane of our galaxy. By using images from both Hubble and the NTT we could get a really good view of these objects, so we could study them in great detail."

The other two populations of planetary nebulae appear to be randomly oriented in relation to the galactic disk.

"While any alignment at all is a surprise, to have it in the crowded central region of the galaxy is even more unexpected," says the paper's second author Albert Zijlstra, also of the University of Manchester.

Why aligned the same way?

So what could be causing this strange alignment inside the galactic bulge?

The shapes of planetary nebulae are thought to be caused by factors such as the orientation of its system before the star turned into a red giant, or whether the star was part of a binary pair. But as for a common alignment across an apparently independent selection of nebulae, some external factor appears to be having a strong influence.

"The alignment we're seeing for these bipolar nebulae indicates something bizarre about star systems within the central bulge," says Rees. "For them to line up in the way we see, the star systems that formed these nebulae would have to be rotating perpendicular to the interstellar clouds from which they formed, which is very strange."

Interestingly, bipolar planetary nebulae do not appear to have a preferential orientation in our galactic neighbourhood many thousands of light-years from the galactic core. The alignment effect only seems to act near the centre of the Milky Way.

The researchers suspect that powerful magnetic fields emanating from the galactic core as it formed during the evolution of our galaxy may be influencing the direction of the nebulae - akin to a magnetic field forcing the direction of compass needles.

"We can learn a lot from studying these objects," says Zijlstra. "If they really behave in this unexpected way, it has consequences for not just the past of individual stars, but for the past of our whole galaxy."