Astronomers love to remind us that there’s no up or down in space. Look out into the depths of the universe and you’ll see galaxies floating edge-on, face-on and at every angle in between. Look at planets circling stars within the Milky Way, and their orbits might be oriented in any direction at all.
But for one class of celestial objects this seems to be untrue: planetary nebulae, the gorgeous clouds of gas puffed by stars in their last, gasping moments of life, seem to be mysteriously aligned with the plane of the Milky Way—something that Bryan Rees, an astronomer at the University of Manchester in England, and lead author of a paper in an upcoming issue of Monthly Notices of the Royal Astronomical Society calls “quite unexpected.”
That’s putting it mildly. Most planetary nebulae are roughly spherical; they’re not visibly “aligned” with anything. One especially spectacular subclass, however, is more hourglass-shaped, and when Rees and his colleague Albert Zijlstra examined this particular kind, the long dimensions of the clouds pointed more or less in the same direction. “They’re not exactly aligned,” he says, “but they’re not random.”
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The question is, why? To answer that, you’d have to know how the hourglass shape forms in the first place, and astronomers still aren’t certain about that. The gas clouds are created when a star in its death throes bloats out to many times its original size, then shrinks back to form a white dwarf, leaving its outer layers to expand. Usually, these layers maintain the star’s original, spherical shape as they grow, leading the observers who first spotted them in the 1700’s to describe them as planet-shaped.
One way to turn a sphere into an hourglass is to put a belt around it, and that’s a leading theory: a belt of dust leftover from the raw material that originally condensed into the star restricts expansion in that direction while leaving the star’s top and bottom halves free to move outward. Another is to surround the star with a magnetic field that shapes the expanding cloud—and such a field could be generated if the original star were part of a double-star system. The orbit of the second star, still intact after its partner expanded and contracted, could set up magnetic field lines that would sculpt the burgeoning cloud into its characteristic hourglass shape.
Neither of these explains why the hourglasses all point in the same direction, though. But Rees and Zijlstra have an idea. The interstellar cloud of gas and dust out of which stars form in the first place spreads out into a disk shape and then condenses, with the newly formed star toward the center of a platter of leftover dust swirling around it. That dust often forms planets. If the collapse happens in the presence of a strong magnetic field, the collapsing disk could be forced to align with that field.
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Since the nebulae Rees and Zijlstra looked at in this study are located toward the dense core of the Milky Way, there might well have been strong magnetic fields present when the original stars formed. Double stars and single stars with belts of dust might thus have been lined up with the plane of the Milky Way right from birth—an effect that wouldn’t happen further out from the core of the galaxy, where Earth is located.
This is by no means a definitive explanation yet, says Rees, but it’s at least a plausible one, which theorists will now chew over in detail. Whether it’s true or not doesn’t detract from the ghostly beauty of the nebulae, of course, and it also doesn’t detract from Rees’s own remarkable story. He did this work as part of his Ph.D. thesis, but unlike most Ph.D. students, he had a first career, as a telephone engineer. When he took early retirement from his job a few years ago, he says, “I didn’t want my brain to ossify so I thought I’d have a look at some astrophysics.” It seems to have turned out rather well.