Star alignment traced to binary systems

29 July 2023

Scientists from the University of Hong Kong and University of Manchester have discovered more about a puzzling alignment of stars near the centre of our galaxy. Their results have been published in Astrophysical Journal Letters.

Planetary nebulae are gas clouds expelled by stars at the end of their lifecycle. In about five billion years, our Sun will also form one of these structures. Despite these nebulae originating from different stars, at different times, and existing in completely different locations, the shapes of the clouds are largely aligned with the galactic plane. This mysterious phenomenon has been known about for ten years but without explanation.

Now a study led by Shuyu Tan, a student at the University of Hong Kong, found that the alignments occur in planetary nebulae that have a close stellar companion. These companion stars orbit the main star at the centre of the nebulae in an orbit closer than Mercury is to our Sun. In contrast, planetary nebulae without close companions do not exhibit this alignment, suggesting that the alignment could be related to the initial separation of the binary components at the time of the star’s birth.

Measurement of the equatorial position angle (EPA). The orientation axis measured from the projected 2D image of a bulge PN was determined visually and the axis that best represents the long symmetry of each PN. The left panel shows a typical elliptical PN M 1–20 with a morphology of "Ems" and an EPA of 87° according to its major axis from HST observations. The right panel shows bipolar M 1–34 from our sample with a "Bps" morphology and an EPA of 31° measured from the VLT observation. Image: Parker et al, APL

Professor Albert Zijlstra of the University of Manchester, co-author, said, “This finding pushes us closer to understanding the cause for this mysterious alignment. Planetary nebulae offer us a window into the heart of our galaxy and this insight deepens our understanding of the dynamics and evolution of the Milky Way’s bulge region."

The researchers investigated 136 confirmed planetary nebulae in the galaxy bulge using the European Southern Observatory Very Large Telescope, and re-examined 40 of these using images from the high-resolution Hubble Space Telescope.

Professor Quentin Parker of the University of Hong Kong, suggests that the rapid orbital motion of the companion star could shape the nebulae. The companion star may even end up orbiting inside the main star.

The alignment of the nebulae could indicate that close binary systems preferentially form with their orbits in the same plane. While further research is needed to fully understand the mechanisms behind the alignment, these findings provide critical evidence for a constant and controlled process that has influenced star formation over billions of years and across vast distances.