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SYFY WIRE Bad Astronomy

A dying star’s magnetic racing stripes

By Phil Plait
A deep color image of the planetary nebula NGC 6804 shows it to be a relatively standard example of its kind, but there are glowing red clouds near it whose origin is unclear. Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona

I have an enduring affection for planetary nebulae, despite their rather gloomy origin: They are clouds of gas blown out when stars like the Sun die.

The details are a bit complicated — aren’t they always — but when a star runs out of fuel in its core, the core contracts and heats up. The outer layers of the star absorb that extra energy and expand, eventually blowing off the star entirely. As they fly away, the now exposed and very hot core (at this point called a white dwarf) energizes the gas, causing it to glow.

The gas (what we call the planetary nebula) can take on some pretty unusual shapes, depending on the way the layers get blown off; at first the winds are slow and dense, then speed up as the star peels away more and more material. The two winds interact, and that can make the overall shape be round, elongated, extremely elongated with a pinched middle, and more. The various gases in them (hydrogen, oxygen, sulfur, and more) all glow at different wavelengths, different colors, making planetary nebulae gasp-worthy when seen in deep, high-resolution images.

NGC 6894, located about 5,000 light years from Earth, is no exception. It’s quite beautiful, but also very odd. Take a look:

A deep color image of the planetary nebula NGC 6804 shows it to be a relatively standard example of its kind, but there are glowing red clouds near it whose origin is unclear. Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona

That image is by my friend and astronomer Adam Block. The inner part of NGC 6894 is round, with an elliptical rim and some extended glow around it. The rim is likely actually a ring, a circle, but we see it at an angle so it looks oval. Its structure is a bit complex, but not more so than other nebulae seen before.

What’s weird are those faint red clouds to the upper right. What are those?

After Adam posted that image I poked around the scientific journal paper archive to see if I could find an explanation. To my delight I found a paper from 1997 co-authored by Noam Soker, who was my advisor for my Master’s degree! Noam has studied planetaries (as we in the know call them) for a long time, and together he and I analyzed the giant outer halo of one called NGC 6826, which I had observed using the University of Virginia’s Fan Mountain observatory. In fact, his 1997 paper references our work! 

His ’97 paper has a possible explanation for those clouds to the upper right of NGC 6894. They’re elongated, more like stripes than diffuse clouds, which can be seen in observations made for his paper:

NGC 6894 (circular feature) seen in the light of glowing hydrogen gas (the right side is a closer view), with mysterious stripes of gas clouds to the upper right. Credit: Soker and Zucker

They do appear to be very flat. The red glow is characteristic of warm hydrogen, and is seen ubiquitously in nebulae of all kinds. NGC 6894 is located almost exactly in the plane of the disk of our Milky Way galaxy, where gas is everywhere, so maybe this isn’t surprising. But… there’s little to no gas anywhere else around the nebula (a lovely wide-field shot of the nebula makes that more clear).

Noam’s idea (together with his co-author) is that the gas in those stripes came from the nebula itself. The outer halo of the nebula is gas that was the very first to be expelled by the star, and has moved off a long way. Usually that’s a spherical shell around the star (which is what we found in NGC 6826), but that’s only if it’s allowed to expand freely.

That’s where this gets weird. Noam’s speculation is that this gas has interacted with the galaxy’s magnetic field, which has influenced the gas to spread out into those long stripes! That’s… odd. But it also makes sense. The galactic field is pretty weak (the Earth’s magnetic field is many tens of thousands times stronger) but it acts over a huge volume, and the gas is very tenuous. The galaxy’s magnetic field is relatively flat in its disk, like parallel lines, and the gas stripes are parallel to the galactic magnetic field lines in that part of space. The gas then is just flowing along the path of least resistance, along the magnetic field lines.

So why only on one side? Studies show that the nebula itself is moving through space to the lower left in that image. If the gas were stripped off, you’d expect to see more of it to the upper right. Interesting.

I’ll note that since the paper was published, another set of stripes near a different object has been seen, though in that case it’s not clear what the source is, though the shape is almost certainly due to the galaxy’s magnetic field as well.

What’s causing them to glow? Probably the star in the center of NGC 6894. It’s blasting out energy at a rate 300 times what the Sun does, and could make that gas glow even though it’s several light years away from the star.

One way to support this idea is to use a much bigger ‘scope to look more carefully at the stripes and the planetary nebula itself. Spectra will tell us their velocities, and if they are moving at about the same speed, then that could mean all that gas is associated physically. If not, it means the nebula just happens to be passing by some random gas shaped like stripes, which would still be a little odd.

This may be a minor mystery in the cosmic scheme of things, but it’s fun to think about. And it may be telling us about the eventual fate of gas blown out by dying stars. One day our Sun will do this — not for 7 billion years or so, so breathe easy — and I think it’s of some interest to know what will happen in that distant future. If Noam’s right, our Sun’s guts will be thrown out into the galaxy, diffused by the galactic magnetic field, and may someday form similar structures. Perhaps that gas will mix with a larger and denser cloud, one that will form new stars…?

That’s hard to say, but it’s a rather nice thought now, isn’t it?