When the Dying Breaths of Ancient Stars Align

Contributed by
Sep 5, 2013
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Astronomers just announced something that I have to admit has me scratching my head: Near the galactic core, planetary nebulae—the winds from dying stars—tend to have their axes aligned along the plane of our galaxy. I know, that sounds a bit esoteric, but that’s only because it is. Still, it actually is really weird. And it may have some interesting implications for how stars form in our galaxy.

This gets pretty technical, so I’ll boil it down somewhat for you, but it’ll still be a little technical. So let me show you some of the guts of this, and then if you want you can skip to the bottom to get the overview.

The Stellar Guts

OK, first off, what’s a planetary nebula? I’ve written about them eleventy billion times because I love them (I studied them for both my Masters and PhD, actually). When a star like the Sun gets old and starts to die, it blows off a slow dense wind of gas, like a solar wind. After a while this starts to deplete the outer layers of the star, exposing the hotter interior. The wind speeds up, and new gas blown from the stars slams into the older, slower gas. The collision compresses the gas, forming cool and weird shapes.

If the dying star is in a binary system, that is, it orbits another star, the early, slower wind tends to blow primarily along the equator of that orbit. Think of yourself going around a fast carousel, and you throw a ball: it’ll tend to go out, away from you, parallel to the ground. Same thing with these stars. You wind up with a dense ring of material around the two stars. But then the faster wind kicks in. It sees more material around the equator, and less along the poles, so it’s easier to blow up and down, along the axis of the binary orbit. In the carousel analogy, it’s like you’ve thrown so many balls early on that they’ve piled up around you, so later ones have an easier time going up and down, rather than trying to plow through the ones on the ground.

In the end, you can get fantastic shapes to these nebulae. Many have elongated structures, double lobes, extending away. Some are butterfly shaped, too, with wings of material reaching out. Many planetary nebulae, though, are not in binaries, and look more circular.

Imagine drawing a line along the lobes of an elongated (also called a bipolar) nebula. Now look at hundreds of them. You’d expect they’d point every which way, because their orientations only depend on physics internal to the stars themselves; that is, every one is unique. They don’t interact with each other, and different binary star systems should have their orbits at all different angles. It’s like throwing a bunch of pencils in the air and taking a photo of them; they’d be pointing in all different directions. Planetary nebulae should be the same way.

When Stars Align

But that’s not what the astronomers found. Our galaxy, the Milky Way, is a flat disk with a spherical bulge of stars in the middle. The astronomers looked at 130 planetary nebulae located very close to the center of the galaxy, in the middle of the central bulge. What they found is that the elongated nebulae were far more likely to have their long axis parallel to the galactic disk than perpendicular to it, like pencils floating in water. The odds of this happening by random chance, they determined, was very small.

In other words, some bizarre force was reaching out from the center of the galaxy and lining up all those nebulae. That’s the weird part.

But we may know what it is! That force, it turns out, may be the galaxy’s magnetic field. And it doesn’t affect the planetary nebulae, really, but the stars themselves as they formed!

We know that stars form from gas clouds, and we also know that some of those gas clouds have magnetic fields, like giant quadrillion-kilometer-long bar magnets (though with way more complicated fields). The galaxy itself has a magnetic field, and the clouds near the galactic center do tend to have their fields lined up with the galaxy’s. So it’s not too far-fetched to think that when those clouds form stars, the orbits of the stars will be affected too.

We already think magnetic fields affect star formation, though just how is unclear (the physics is incredibly complicated). Still, overall, it makes some sense that as the cloud collapses to form stars, the magnetic field gently shapes that collapse. The stars that form would then become aligned with the magnetic field, and in the end you get binary stars which have orbits aligned with the galaxy’s plane—in this case, aligned along the disk of the galaxy, again like pencils floating in water (as opposed to buoys that stick out of the water, or at some angle in between). They may point in different directions, but they all wind up with axes aligned along the plane.

Interestingly, when the astronomers looked at non-elongated nebulae, they did not see this alignment. It turns out that the magnetic fields of clouds would have a greater effect on wide binary stars, the kind that form elongated nebulae, than they would on single stars. The pattern makes some sort of sense.

The Core of the Problem

Amazingly, what all this means is that the magnetic field of the galaxy itself—or at least, very near the center of the galaxy—weak as it is can profoundly shape the way stars form there. The way the stars spin and orbit each other line up, a bit like iron filings sprinkled over a sheet of paper with a bar magnet underneath it. But this only happens when the stars form; the magnetic field probably has little or no effect on them today.

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 not clear, though, what other effects this might have. Our Sun and planets are located pretty far out form the core, halfway or so to the galaxy’s edge. Planetary nebulae out here don’t appear to be particularly aligned. Maybe the magnetic field strength isn’t as powerful this far out, and maybe things have changed since the stars in our neighborhood were young.

But it’s fascinating to think that magnetism could have an affect that strong even near the hub of the Milky Way. It’s the first time this has been so clearly seen, so maybe we’ll find other signs of this influence as time goes on. And this may yet show some effect on the greater galaxy at large, perhaps when it was younger and more active. The one thing that’s for sure is this shows us that our local island Universe, as galaxies were once called, has a few surprises up its sleeve left for us to discover.