Row 1 (from upper left): NGC 6302, NGC 6881, NGC 5189
Row 2 (from lower left) : M2-9, Hen 3-1475, Hubble 5
Credit: NASA, ESA; Acknowledgement: Josh Barrington

Planetary nebulae are some of the most beautiful objects in the cosmos. These short-lived objects, only having a lifespan of several thousand years, are the remnants of a dying star. Apparently, they also have a tendency not to line up with Earth either, even though thousands of light-years separate them.


We know from our observations that star systems are oriented in different ways. The orientation of a star system depends mostly on the internal dynamics of the star and the formation of the solar system. Logically, you would assume planetary nebula would also be oriented in a more random fashion because the a planetary nebula is unique to the star system that created it. Contrary to what we think we should see, astronomers discovered the nebulae are found near the core of the Milky Way. Here, due to some peculiar forces, the nebulae axis of the nebulae aligns with the plane of the Milky Way. Studying the forces involved could have profound implications on the formation of stars and the structure of the Milky Way.


Out of a survey of 130 or so elongated planetary nebulae, astronomers discovered the axis was far more likely to be parallel to the galactic plane than perpendicular to it. Given random chance, the possibility that this would occur seems very small, so something else must be at work. Astronomers determined the most likely explanation lies with the Milky Way’s magnetic field. This process of alignment would start long before the formation of the planetary nebula, dating back as far as the formation of the actual star.


Image Credit: Bill Saxton, NRAO/AUI/NSF

Physicists have long suspected that the galaxy’s magnetic field plays a role in star formation, having a particularly strong affect on stars forming near the galactic center, though they’ve had a hard time explaining how or why this influence matters. Astronomers have observed that binary star systems closer to the galactic nucleus have a higher than normal chance of aligning their orbits with the galactic plane. The conclusion is, as the binary system is forming, while it’s just a condensing cloud of dust, the galaxy’s magnetic field can influence the cloud and help it form in such a way to allow it to line up with the galactic plane. Binary systems are more likely to produce these elongated nebulae. For those of you wondering, non-elongated nebulae, which are commonly created by a single star, do not show the same alignment anomalies.


Schematic of the Milky Way, seen face-on (left) and edge-on (right).
Photo via University of Oregon Physics Dept. modified by Phil Plait

Planetary nebulae further away from the galactic nucleus don’t show the same alignment properties either. The thinking is the strength of the magnetic out in this part of the neighborhood is too weak to have the same types of effects. It’s also important to note that astronomers are looking tens of thousands of years into the past when observing the planetary nebulae closer to the galactic center. As Phil Plait, an astronomer and creator of Bad Astronomy, said, “Mind you, the stars the astronomers observed are billions of years old; they take a long time to age and eventually die. So really, what we’re seeing are the ghostly fingers of the Milky Way’s ancient magnetism, reaching across the eons and leaving its fingerprints on stars as they die today.” It’s possible that conditions in the galaxy have changed since the time the stars responsible for producing these planetary nebula formed.


All of this allows astrophysicists to gain a deeper understanding of the way stars form, the structure of galaxies, and it allows us to peer into the intricate dance between stars and their host galaxy.


Sources and Further Reading:

Alignment of the Angular Momentum Vectors of Planetary Nebulae in the Galactic Bulge

The Spatial Orientation of Planetary Nebulae Within the Milky Way (paper)

When the Dying Breaths of Ancient Stars Align

Mystery Alignment of Dying Stars Puzzles Scientists

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