‘Ghost stars’ have aligned themselves in a strange pattern in our galaxy – and scientists might now know why

“Ghost” stars are aligned in beautiful structures near the centre of our galaxy, scientists say – and they might finally be getting to know why.

Researchers discovered the unusual alignment of these planetary nebulae ten years ago, when Manchester doctoral student Bryan Rees spotted them. But it has remained a mystery how they came to be that way.

Now scientists have been able to confirm that unusual alignment. But they have also made a breakthrough in finding out why they are there, after they found that a particular group of stars known as binary stars is responsible.

Planetary nebulae are gas clouds that are thrown out from stars when they come to the end of their life. Our own star, the Sun, will do the same in about five billion years.

Those ejected clouds are like ghosts of their dying stars, and assemble themselves in beautiful shapes, researchers say, such as an hourglass or butterfly.

Researchers studied a range of planetary nebulae that are near the centre of our Milky Way. Though they are not related and come from different stars and different times, many of their shapes are similar, lining up in the same way and on the same plane.

In the new study, scientists found that the alignment happens when those ghosts have a close companion star. The companion orbits around the main star, at the centre of the planetary nebulae, at a very close orbit.

Without such a companion star, the nebulae do not line up in the mysterious pattern. That suggests that the alignment is linked to the splitting of the binary components when the star is born.

“This finding pushes us closer to understanding the cause for this mysterious alignment,” said Albert Zijlstra, co-author and professor in astrophysics at The University of Manchester.

“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 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.”

Researchers looked at 136 confirmed planetary nebulae in the galactic bulge, or the thickest section of our Milky Way. They used the European Southern Observatory’s Very Large Telescope, and then looked at 40 more of them using images from the Hubble Space Telescope.

The findings are reported in a new paper, ‘When the Stars Align: A 5 σ Concordance of Planetary Nebulae Major Axes in the Centre of our Galaxy’, in the Astrophysical Journal Letters.