Mon. Not. R. Astron. Soc. http://doi.org/nsw (2013)

Planetary nebulae come in various shapes and sizes, the most striking of them resembling colourful butterflies or glowing cat's eyes. They are actually stellar remnants, each one the ghostly leftovers of the atmosphere ejected from low- to medium-mass stars as they died. Only a minority of these ionized gas clouds is spherically symmetric; most are elliptical or bipolar. All morphologies result from the particular properties of their progenitor stars, which are independent, so the orientations of the nebulae are expected to be randomly distributed. However, Bryan Rees and Albert Zijlstra have found that the bipolar ones — that is, the hourglass and butterfly nebulae — tend to have their long axis (the one bisecting the wings) aligned with the Galactic plane.

What makes the bipolar nebulae different? The authors argue that stellar magnetic fields would be too weak to shape the nebulae, and that angular momentum carried by binary stars must be the key. Working backwards, they propose that the progenitor binary stars formed within the Galactic bulge in the presence of a strong magnetic field aligned along the Galactic plane. Therefore, the characteristics of bipolar planetary nebulae should encode information from the early Universe that created the conditions under which our Galaxy was formed.