In the vastness of the Galaxy, stars die and leave behind ethereal remnants known as planetary nebulae, or, as they are more poetically called, “ghost stars.” These magnificent gas clouds have relatively short lifetimes – tens of thousands of years compared to the billions of years that most stars live. However, an unusual pattern was discovered among these ghost stars a decade ago, sparking curiosity and intrigue in the scientific community. A recent study has suggested that close binary companions play a crucial role in this phenomenon, although the underlying cause is still unclear.
Contrary to their name, planetary nebulae have nothing to do with planets. The term was coined before their true nature was understood. Instead, nebulae form when stars too small for a supernova explosion end their life cycle and eject their outer layers into space. Planetary nebulae visible from Earth are a diverse collection of stars that have recently died but were born at different times. Some of these stars are ancient and resemble the size of our Sun, while others are much larger and have much shorter lifetimes.
Given the wide range of stellar ages and sizes, one would not expect to see a pattern of alignment among planetary nebulae. However, ten years ago, Dr. Brian Rees, then a PhD student at the University of Manchester, made a surprising discovery. He noticed that planetary nebulae near the Galactic Arc had major axes disproportionately oriented parallel to the plane of the Milky Way. The impression was that some powerful galactic-scale force was forcing these nebulae into alignment.
Planetary nebulae have different shapes, but elliptical and bipolar nebulae possess a major axis. The new study not only confirmed Rees’ observations on a larger sample, but also uncovered an additional clue that had initially escaped his attention. The alignment was found exclusively among planetary nebulae whose progenitor stars were part of a close binary system.
Within each planetary nebula, there is a white dwarf at its core, and the companion star continues to orbit around it. In all cases, the companion star’s orbit is smaller than Mercury’s orbit around the Sun. This discovery further complicates the already confusing process of star formation in the galactic bulge.
“Star formation in the bulge of our galaxy is a complex process involving various factors such as gravity, turbulence and magnetic fields. Until now, we had no evidence as to which of these mechanisms can trigger this process and lead to this alignment,” explained study author Professor Albert Zejlstra from the University of Manchester. “The significance of this study is that we now know that alignment is observed in this very specific subset of planetary nebulae.”
Interestingly, the extent of these nebulae often exceeds the orbits of their companion stars. This suggests that the companions are in some sense orbiting within their primary stars, or at least within the remnants of primary stars. The authors suggest that the rapid orbital motion of the less massive star shapes the nebula, contributing to its alignment.
To reach these conclusions, Reese initially analyzed 130 planetary nebulae using images taken by the Hubble Space Telescope. In the latest study, forty of these nebulae were re-examined, confirming the results of Rees’ analysis, and the sample size was expanded to include an additional 136 nebulae obtained with the European Southern Observatory’s Very Large Telescope. With this extensive data set, it became clear that the observed alignment of planetary nebulae is not a simple coincidence.
By continuing to investigate this mysterious phenomenon, scientists hope to discover the force behind the alignment of ghost stars. Whether it is the influence of gravity, turbulence, magnetic fields, or a combination of these factors, the alignment of planetary nebulae is a fascinating mystery that promises to reveal more information about the complex structure of our Galaxy.
