How Planetary Nebulae Get Their Shapes

Planetary nebulae are like a box of chocolates: You never know what you’re going to get.

The wide variety of shapes and colors these celestial gems display has fascinated observers and theorists alike. These fantastic gas-and-dust eruptions from low-mass stars form shapes such as spirals, roses, butterflies, and even cosmic eyes. Now, a team of observers has looked one step earlier in stellar evolution to explain how these shapes come to be.

One Step Back, Two Steps Forward

Astronomers have long known that planetary nebulae come about in the dying days of less-massive stars — including, one day, the Sun. Such stars shed their outer layers, making the glorious displays we see from Earth, even as the stellar cores collapse into white dwarfs.

But what astronomers have struggled to understand is how the presumably symmetrical winds coming off aging stars make such beautiful asymmetric shapes. Some ideas have included the effects of stellar companions, though more recent research has shown that even giant planets can affect the winds expelled by stars like the Sun.

To understand the evolution of planetary nebulae, Leen Decin (KU Leuven University, Belgium) and colleagues went a step backward in stellar evolution, to asymptotic giant branch (AGB) stars. These cool, red giants can expand to the size of Earth’s orbit, and they shine thousands of times brighter than the Sun. Yet they’re running out of fuel, reduced to fusing carbon in their cores. (Later in their lives, AGB stars can appear as ruby-red carbon stars.)

AGB stars lose up to 0.01% times the Sun’s mass every year, beginning the process of stripping away the stars’ outer layers. But the high rates of mass loss have also obscured what’s happening closer to the star.

The AGB phase is short-lived, and that has also complicated our understanding of the end stages of low-mass stellar evolution. Depending on their individual masses, AGB stars are only 100,000 to 20 million years away from becoming planetary nebulae. The process of making the planetary nebula takes a few thousand years, a blink of the astronomical eye, and the nebula itself only lasts some 20,000 years more before it disperses into interstellar space.

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Winds and Companions

Decin’s team studied a sample of 12 AGB stars using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. By observing such long wavelengths, ALMA penetrates the gas and dust around these stars to reveal details in their winds — and overturning previous assumptions about their shapes.

None of the 12 AGB stars have symmetric winds, the researchers report in the September 18th Science. All of them have some form of arcs or shells, and some of them even exhibit the kinds of spiral, bipolar, or eye-like shapes that planetary nebulae have.

“Each star, which was only a number before, became an individual by itself,” says coauthor Miguel Montargès (also at KU Leuven). “Now, to us, they have their own identity. This is the magic of having high-precision observations: stars are no longer just points anymore.”

Studies of large populations of stars show that AGB stars, on average, have at least one companion with more than five times Jupiter’s mass. Whether these companions are stars or giant planets, they are likely affecting the evolving shapes of the stellar winds.

The wind’s speed is also key, because as the star loses more mass, the companions’ orbits will widen. Based on the ALMA observations, the researchers develop an equation that predicts whether a star will blow its winds primarily out along its poles — a structure found for stars with weak winds and closeby companions — or whether the wind will take a spiral shape along the equator, found more often for strong winds and farther-out companions.

“This is a wonderful, groundbreaking result in that it makes solid a connection that has been long speculated,” says Morgan MacLeod (Center for Astrophysics, Harvard & Smithsonian), who was not involved in the research. “This result provides a revolutionary degree of evidence for the idea that the multiplicity of stars is key in how evolved stars lose their envelopes.”

The Sun’s Future

What does all this mean for the Sun? One day some 7 billion years from now, it too will become a white dwarf surrounded by a beautiful planetary nebula. Based on their findings, Decin speculates that Jupiter or even Saturn are massive enough to play a role in deciding that nebula’s shape.

“Our calculations now indicate that a weak spiral will form in the stellar wind of the old dying Sun,” she says.