The Ghosts of Dead Stars Form a Mysterious Alignment, But Why?

The Ghosts of Dead Stars Form a Mysterious Alignment, But Why?

Planetary nebulae (from left) NGC 6302, NGC 6881, and NGC 5189 as imaged by Hubble. (ESA/Hubble & NASA)

There's something weird about the ghostly remnants of dead stars in the middle of the Milky Way. Planetary nebulae of a particular kind all seem to be aligned the same way, close to parallel to the galactic plane. 

This peculiarity was first spotted more than a decade ago by eagle-eyed astronomer Bryan Rees of The University of Manchester in the UK. Now, Rees and his team, led by astronomer Shuyu Tan of the University of Hong Kong, think they're closer to figuring out the reason: the magnetic environment in the region of the galaxy where this alignment is observed. 

"This finding," says astrophysicist Albert Zijlstra of The University of Manchester, "pushes us closer to understanding the cause for this mysterious alignment." 

A collage of 22 famous planetary nebulae. (ESA/Hubble and NASA, ESO, NOAO/AURA/NSF, Ivan Bojičić, David Frew)

Planetary nebulae are a particularly ethereal, relatively short-lived phenomenon.

They're the cast-off remains of stars like the Sun that reach the end of their lives and eject their outer material before the core collapses down into a stellar remnant known as a white dwarf. 

Because they're shed without the blast of a supernova, the nebulae often remain relatively intact, floating like ghostly glowing bubbles in space. This is how they got their name – because some of them are round, like planets. 

If the white dwarf shares its system with another star, as many do, the planetary nebula will look quite different. Scientists think the orbital motion between the two stars in binary systems carves the nebula into lobes, like an hourglass. 

In 2011, Rees noticed that these butterflied planetary nebulae are often curiously aligned in such a way that their long axis lies parallel, or nearly parallel, to the galactic plane. 

Tan and her colleagues investigated 136 planetary nebulae in the Milky Way's galactic bulge; a tightly-packed region of stars found in the centers of most spiral galaxies. 

They found the nebulae that exhibit the peculiar alignment are those in which the binary is on a very small, tight orbit. 

The Butterfly Nebula, a particularly spectacular lobed planetary nebula, imaged by Hubble. (NASA, ESA and the Hubble SM4 ERO Team)

This, they say, is a clue that can help unravel the history of the galaxy's formation. 

"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," Zijlstra explains. 

"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 only plausible explanation for the curiously neat alignment of this specific subset of planetary nebulae is magnetic fields, according to the researchers. While the magnetic fields in the galactic bulge aren't currently strong enough to account for the uniform arrangement, they could have been in the past, locking the binaries into alignment long prior to the formation of the planetary nebulae. 

This would imply that younger binaries, and those with wider orbital separation, would not necessarily be prone to the same effect – which is consistent with the team's observations. However, more observations and analysis will be needed to confirm that this is what is tilting planetary nebulae in the galactic center. 

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

The research has been published in The Astrophysical Journal Letters. 

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