Scientists from the Universities of Manchester and Hong Kong have discovered the origin of a mysterious arrangement of stars near the center of the Milky Way. Manchester PhD student Bryan Rees first discovered the arrangement of planetary nebulae a decade ago, but it still cannot be explained. New data from the European Southern Observatory's Very Large Telescope and the Hubble Space Telescope in Chile (published in the Astrophysical Journal Letters) confirm this alignment, but also identify the special group of stars responsible for the phenomenon, known as close binaries.
A now-iconic collage showing 22 of the famous PNe, artistically arranged in a spiral pattern in order of their approximate physical dimensions. Image credit: ESA/Hubble and NASA, ESO, NOAO/AURA/NSF, from an idea of the corresponding author and Ivan Bojičić, and rendered by Ivan Bojičić, with input from David Frew and the author.
Planetary nebulae are clouds of gas expelled by stars at the end of their lives—the same gas clouds our sun will form about five billion years from now. The ejected clouds are the "ghosts" of dying stars, and they form beautiful structures like hourglass or butterfly shapes.
The team studied a group of so-called planetary nebulae found in the galactic bulge near the center of the Milky Way. Each of these nebulae are unrelated, coming from different stars, being born at different times and spending their lives in completely different places. However, the study found that many of their shapes line up the same way across the sky, and are aligned almost parallel to the galactic plane (our Milky Way). This is the same direction Bryan Rees discovered a decade ago.
The new study, led by University of Hong Kong student Shu-yu Tam, found that this arrangement only occurs in planetary nebulae with a close companion star. The companion star orbits the primary star at the center of the planetary nebula in an orbit closer than Mercury is to the Sun. Planetary nebulae without close companions do not show this arrangement, suggesting that this arrangement may be related to the initial separation when the binary stars are born.
Co-author Albert Zijlstra, Professor of Astrophysics at the University of Manchester, said: "This discovery brings us closer to understanding the cause of this mysterious arrangement. Planetary nebulae provide us with a window into the center of the Milky Way. This insight deepens our understanding of the dynamics and evolution of the Milky Way's bulge region. Solution. Star formation in the Milky Way's bulge is a complex process involving many factors, including gravity, turbulence and magnetic fields. The significance of this study is that we now know that this arrangement is observed in this very special subset of planetary nebulae."
Researchers used the European Southern Observatory's Very Large Telescope, whose primary mirror is 8 meters in diameter, to survey 136 confirmed planetary nebulae in the bulge (the thickest part of the Milky Way, made up of stars, gas and dust).
They also re-examined and measured 40 from the original study using high-resolution Hubble Space Telescope images. Corresponding author Professor Quentin Parker of the University of Hong Kong believes that the nebula may have been shaped by the rapid orbital motion of the companion star, which may even end up orbiting within the host star. The arrangement of the nebulae may mean that tight binary star systems formed preferentially, with their orbits lying in the same plane.
Although further research is needed to fully understand the mechanisms behind the alignment, these findings provide important evidence for the existence of an ongoing and controlled process that influences star formation over billions of years and vast distances.