Why Do Butterfly-shaped Planetary Nebulae

Have Bizarre Alignments?

You’ve heard of planetary nebulae before, right? Those are the celestial objects that stars like the Sun become as they age and die. As these stars get old, they lose their outer layers to space, creating rings and shells of material that get heated by what’s left of their star. They’re beautiful objects and have fascinated astronomers ever since the first one was discovered (the Dumbbell Nebula, by Charles Messier in 1764). The size and shape of a given planetary nebula is dictated by the characteristics of the star that created it, plus the rotation of the star (that is, the spin on its axis), plus other environmental factors. Sometimes a planetary nebula has two stars and the presence of the second star also helps sculpt the cloud. It’s also possible that planets in orbit around the progenitor star (the one that created the planetary nebula) could affect how the nebula looks.

Some dying sun-like stars create very interesting shapes called bipolar planetary nebulae. They often look like hourglasses or butterflies. One or more stars in the system has a pair of jets flowing away as a sort of release mechanism to let pent-up heat and plasma flow away to space.  Those jets sculpt the material blown away previously, and heat it up in the bargain. One look at the planetary nebulae shown here and you can see that some of these dying stars are busy!

The shapes of bipolar nebulae are some of the most extreme ever seen, and are probably caused by their jets that blow mass from the binary system, perpendicular to the orbit— i.e., from the poles of the star(s).

Astronomers using the New Technology Telescope (European Southern Observatory) and the NASA/ESA Hubble Space Telescope have explored more than 130 planetary nebulae that exist in the central bulge of our galaxy. The bulge area has more stars than the rest of the galaxy’s disk, and there are a great many planetary nebulae there, too. That makes sense: if you have more stars, you’re likely to have more remnants of those stars dying, including planetary nebulae.

Most of the nebulae they looked at were randomly aligned on the sky. This means that their axes of rotation didn’t point in any one given direction. But, the bipolar nebulae they looked at in the sampling lined up in the same way. According to one of the scientists, Bryan Rees of the University of Manchester in England, the alignments were intriguing. “Many of these ghostly butterflies appear to have their long axes aligned along the plane of our galaxy,” he said. “By using images from both Hubble and the NTT we could get a really good view of these objects, so we could study them in great detail.”

So, why would a population of bipolar planetary nebulae in the galactic bulge all point their axes in the same direction?  It’s a good question and one that astronomers are working to answer. Rees speculated about the cause of this interesting preferential alignment. “The alignment we’re seeing for these bipolar nebulae indicates something bizarre about star systems within the central bulge,” he said. “For them to line up in the way we see, the star systems that formed these nebulae would have to be rotating perpendicular to the interstellar clouds from which they formed, which is very strange.”

There’s one possible galactic explanation for the phenomenon. Along with the characteristics of the progenitor stars (the stars that become planetary nebulae), the Milky Way itself may play a role in aligning these nebulae. First, you have to understand that the whole central bulge rotates around the galactic center. The bulge region may have more influence over the entire galaxy than previously thought. The mechanism that some astronomers are suggesting are the bulge magnetic fields. They work to shape and constrain objects and events that exist within them, and it’s possible that this planetary nebula lineup could have been caused by the presence of strong magnetic fields as the bulge formed.

It’s true that planetary nebulae outside the bulge region don’t show any alignment preference, so they probably weren’t subjected to a strong field from the bulge by the time they formed. It also implies that the magnetic fields of the bulge region were much stronger in the past. So, the alignment of planetary nebulae in the bulge could give astronomers another way of looking at and understanding the history of our galaxy and its evolution. Stay tuned!