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Bizarre Planetary Nebulae of Butterfly Shape

Bipolar planetary nebula PN Hb 12An image showing a typical example of a bipolar planetary nebula known as PN Hb 12–also known as Hubble 12—in the constellation of Cassiopeia.


Using the NASA/ESA Hubble Space Telescope with ESO’s New Technology Telescope, astronomers have explored more than 100 planetary nebulae in the central bulge of the galaxy. It is found that some butterfly-shaped members in this cosmic family tend to be aligned mysteriously, which is a surprising result given their varied histories and properties.

For a star such as our sun, the final stages of life result in the star puffing its out layers out into surrounding space and form objects know as planetary nebulae with beautiful and striking shapes. Among these nebulae, one type called bipolar planetary nebulae creates butterfly shapes or ghostly hourglass around their parent stars.

All these nebulae formed in different places and have different characteristics. Neither the individual nebulae, nor the stars that formed them, interact with other planetary nebulae. However, a new study by astronomers from the University of Manchester, UK, now shows surprising similarities between some of these nebulae: many of them line up in the sky in the same way.

These nebulae are formed in various places and have different characteristics. There is no interaction between an individual nebulae or the star that formed it with other planetary nebulae. However, a new study conducted by two astronomers from the University of Manchester, UK, have found that these planetary nebulae have surprising similarities that many of them line up in the sky in the same fashion. This new finding has been published on Monthly Notices of the Royal Astronomical Society on September 4th, 2013.

“This is really a surprising finding and if it is true, it will be a very important one,” explains Bryan Rees, one of the two authors of the paper, the University of Manchester. “Many of the ghostly butterflies seem to align their long axes along the plane of the galaxy. Using images from Hubble and NTT, we are able to get good view of these objects, so we can study them in great details.”

A selection of Hubble’s planetary nebulae
A group of images of bipolar planetary nebulae by Hubble Space Telescope. Image source: NASA/ESA.


The two astronomers studied at 130 planetary nebulae in the Milky Way’s central bulge and they divided them into three types, and closely peered at their appearance and characteristics. “Although two of the populations were totally random in their alignment in the sky as we expected, the third type—the bipolar nebulae—showed an amazing preference for a particular alignment,” noted the second author of the paper, Albert Zijlstra, also from the University of Manchester. “While any preference of alignment is surprising, to have it in the crowded central of our galaxy is even more unexpected.”

People generally believe that planetary nebulae are sculpted by the rotation of the star system from which they form. The particular shapes are dependent on the properties of the system, for instance, whether it is a binary, or has a fixed number of planets orbiting around it, both of which could greatly affect the form of the blown bubble. The shapes of bipolar nebulae are the most extreme and thought to be formed by jets blowing mass outwards from star systems perpendicular to their orbits.

“The alignment of the bipolar nebulae indicates some bizarre things about the start systems within the central bulge,” said Rees. “To form the lineup in the manner we see, the star systems that formed the nebulae have to be rotating perpendicular to the interstellar clouds from which they formed and this is very strange.”
Although the properties of their progenitor stars do conceive the shapes of these nebulae, the new discovery hints at a more mysterious factor. In addition to these complicated stellar characteristics, our Milky Way also gets involved; the whole central bulge rotates around the entire galaxy. The bulge might exert a greater influence than it has been ever though over the entire galaxy via its magnetic fields. Astronomers suggest that such orderly aligned behavior of the planetary nebulae might come from the presence of strong magnetic fields as the bulge formed.

Because the nebulae closer to us don’t line up in same orderly fashion, these magnetic fields would have been several times stronger than they are in the present of the sun’s neighborhood.
“We can learn a lot from researching on these objects,” concludes Zijlstra. “If they do behave in such unexpected way, it will not only influence the past of individual stars, but also the past for our whole galaxy.”