A collaboration of scientists from The University of Manchester and the University of Hong Kong have discovered the source of the mysterious alignment of stars near the Galactic Centre.
The alignment of planetary nebulae was first identified a decade ago by Bryan Rees, a PhD student at Manchester, but has remained unexplained until now.
New data, published in Astrophysical Journal Letters, obtained from the European Southern Observatory Very Large Telescope in Chile and the Hubble Space Telescope, has not only confirmed the alignment but also identified a specific group of stars responsible – close binary stars.
Planetary nebulae are gas clouds expelled by dying stars, including our own Sun, which will also form one in about five billion years. These ejected clouds appear as ghost-like structures in various shapes, such as an hourglass or butterfly.
The team focused on planetary nebulae located in the Galactic Bulge near the centre of the Milky Way. Each of these nebulae originates from different stars born at different times and places. However, the study found that many of them align in the sky in the same manner, close to parallel with the Galactic plane.
This follows the same direction first discovered by Bryan Rees ten years ago.
The recent research, led by Shuyu Tan, a student at the University of Hong Kong, determined that the alignment is present only in planetary nebulae that have a close stellar companion. The companion star orbits the main star at the centre of the nebulae at a distance closer than Mercury is to our Sun.
Planetary nebulae without close companions do not exhibit the alignment, suggesting that the alignment may be linked to the initial separation of the binary components during the star’s birth.
Albert Zijlstra, a co-author and Professor in Astrophysics at The University of Manchester, said: “This finding brings us closer to understanding the cause behind this mysterious alignment.
“Planetary nebulae provide us with insight into the core of our galaxy, deepening our understanding of the dynamics and evolution of the Milky Way’s bulge region.
“The formation of stars in the bulge involves various factors such as gravity, turbulence, and magnetic fields. Until now, we lacked evidence for which mechanism might be responsible for this process and the resulting alignment.
“The significance of this research lies in our understanding that the alignment is observed only in this specific subset of planetary nebulae.”
The researchers examined 136 confirmed planetary nebulae in the galaxy bulge, the thickest section of the Milky Way composed of stars, gas, and dust, using the European Southern Observatory Very Large Telescope, which has an eight-meter main mirror diameter.
They also re-examined and re-measured 40 of these nebulae from the original study using high-resolution images from the Hubble Space Telescope.
Prof Quentin Parker, the corresponding author from the University of Hong Kong, suggests that the rapid orbital motion of the companion star may shape the nebulae, potentially even leading to the companion star orbiting inside the main star.
The alignment of the nebulae may indicate a preference for close binary systems to form with their orbits in the same plane.
While further research is needed to fully comprehend the mechanisms behind the alignment, these findings provide important evidence for a constant and controlled process that has influenced star formation over vast distances and billions of years.