Stars Align: Ghost Star Answers

A team of scientists from The University of Manchester and the University of Hong Kong has made a significant breakthrough in uncovering the cause behind the enigmatic alignment of stars near the Galactic Centre.

The peculiar alignment of planetary nebulae, first discovered a decade ago by Bryan Rees, a Manchester Ph.D. student, has remained a puzzle until now.

By analyzing new data gathered from the European Southern Observatory Very Large Telescope in Chile and the Hubble Space Telescope, the team confirmed the alignment phenomenon. Moreover, they identified a specific group of stars responsible for this alignment—close binary stars.

Planetary nebulae are gaseous clouds expelled by dying stars at the end of their life cycles. Our own Sun is expected to produce one of these nebulae in about five billion years. These expelled clouds form stunning structures, such as hourglasses or butterfly shapes, known as “ghosts” of the deceased stars.

The researchers focused on a group of planetary nebulae located in the Galactic Bulge near the center of our Milky Way. Although each nebula is unrelated and originates from different stars born at different times and in different locations, the team found that many of their shapes align in the sky, almost parallel to the Galactic plane.

These findings are consistent with Bryan Rees’ discovery a decade earlier.

Shuyu Tan, a student at the University of Hong Kong, led the new research, which revealed that the alignment is only present in planetary nebulae accompanied by a close stellar companion. This companion star orbits the central star of the nebula at a distance closer than that between Mercury and our Sun.

Planetary nebulae without close companions do not exhibit this alignment, suggesting that it may be linked to the initial separation of the binary star components during the birth of the star.

Albert Zijlstra, co-author, and Professor in Astrophysics at The University of Manchester, expressed his excitement about this breakthrough: “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.”

He further explained, “The formation of stars in the bulge of our galaxy is a complex process that involves various factors such as gravity, turbulence, and magnetic fields. Until now, we have had a lack of evidence for which of these mechanisms could be causing this process to happen and generating this alignment.”

“The significance in this research lies in the fact that we now know that the alignment is observed in this very specific subset of planetary nebulae,” Zijlstra added.

To investigate further, the team examined 136 confirmed planetary nebulae within the galaxy’s bulge, the densest region comprising stars, gas, and dust, using the powerful eight-meter main mirror of the European Southern Observatory Very Large Telescope. They also reanalyzed and remeasured 40 of these nebulae from the original study, utilizing high-resolution images from the Hubble Space Telescope.

Corresponding author Prof Quentin Parker from the University of Hong Kong proposed that the shape of these nebulae might be influenced by the rapid orbital motion of the companion star, which could even result in its orbit residing inside the main star.

The alignment of these nebulae suggests that close binary systems tend to form with their orbits lying in the same plane.

While further investigations are necessary to fully comprehend the mechanisms driving this alignment, these findings provide crucial evidence for a consistent and regulated process that has shaped star formation across vast distances and billions of years.

Source: 10.3847/2041-8213/acdbcd

Image Credit: ESA/Hubble and NASA, ESO, NOAO/AURA/NSF from an idea by the corresponding author and Ivan Bojičić and rendered by Ivan Bojičić with input from David Frew and the author.