Stars are formed in loose groups called open clusters. For the first few hundred millions of years, they pull and tug on each other in chaotic orbits till, one by one, they drift away in unpredictable directions and the cluster evaporates. Although, on a grand scale, these stars all follow similar orbits around the core of the galaxy, locally their motions and orientations are in all directions. So when astronomers from the university of Manchester conducted a study looking at over 100 different bipolar planetary nebulae near the galactic core, they were surprised to find that they tended to line up with the axis of the Milky Way galaxy.According to a paper published in The Monthly Notices of the Royal Astronomical Society, the astronomers looked at over 130 planetary nebulae and found that, of the three different types, those of the bipolar variety (which have hourglass or butterfly shapes) near the centre of the galaxy were all neatly aligned. These nebulae all formed in different places, at different times, so there is no way for them to have influenced each other. “This really is a surprising find and, if it holds true, a very important one,” explains Bryan Rees of the University of Manchester, one of the paper’s two authors. “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 different types of planetary nebula, and what causes a dying star to form one or the other, is an ongoing field of study. It is known that a star’s planets affect the ejected gas envelope which forms the nebula, as does the presence of a binary companion star. None of this gives any clue about why scientists are seeing this alignment, though. But one intriguing possiblility has been suggested by the paper’s authors: The central bulge of the Milky Way, where these nebulae were found, rotates en masse, and may have a magnetic field far stronger than previiously believed. This magnetic field could have a strong influence on the formation of bipolar planetary nebulae, possibly enough to force them to align with the galactic pole.
“While two of these populations were completely randomly aligned in the sky, as expected, we found that the third — the bipolar nebulae — showed a surprising preference for a particular alignment,” says the paper’s second author Albert Zijlstra, also of the University of Manchester. “While any alignment at all is a surprise, to have it in the crowded central region of the galaxy is even more unexpected. We can learn a lot from studying these objects,” he concluded. “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.”