How Planetary Nebulae Get Their Shapes – Sky & Telescope

Planetary nebulae are like a field of sweets: You by no means know what you’re going to get.

This assortment exhibits 99 planetary nebulae (and “Gomez’s Hamburger,” a protoplanetary disk) imaged by the Hubble Space Telescope.Hubble / ESA / NASA; processed by Judy Schmidt CC BY 2.0The huge number of shapes and colours these celestial gems show has fascinated observers and theorists alike. These incredible fuel-and-mud eruptions from low-mass stars kind shapes equivalent to spirals, roses, butterflies, and even cosmic eyes. Now, a staff of observers has appeared one step earlier in stellar evolution to clarify how these shapes come to be.

One Step Back, Two Steps Forward

Astronomers have lengthy identified that planetary nebulae come about within the dying days of much less-large stars — together with, someday, the Sun. Such stars shed their outer layers, making the fantastic shows we see from Earth, even because the stellar cores collapse into white dwarfs.

But what astronomers have struggled to grasp is how the presumably symmetrical winds coming off getting older stars make such lovely uneven shapes. Some concepts have included the consequences of stellar companions, although newer analysis has proven that even large planets can have an effect on the winds expelled by stars just like the Sun.

To perceive the evolution of planetary nebulae, Leen Decin (KU Leuven University, Belgium) and colleagues went a step backward in stellar evolution, to asymptotic large department (AGB) stars. These cool, purple giants can broaden to the scale of Earth’s orbit, and so they shine hundreds of occasions brighter than the Sun. Yet they’re operating out of gasoline, lowered to fusing carbon of their cores. (Later of their lives, AGB stars can seem as ruby-purple carbon stars.)

AGB stars lose as much as 0.01% occasions the Sun’s mass yearly, starting the method of stripping away the celebs’ outer layers. But the excessive charges of mass loss have additionally obscured what’s taking place nearer to the star.

The AGB section is brief-lived, and that has additionally difficult our understanding of the top levels of low-mass stellar evolution. Depending on their particular person plenty, AGB stars are solely 100,000 to 20 million years away from turning into planetary nebulae. The course of of constructing the planetary nebula takes a couple of thousand years, a blink of the astronomical eye, and the nebula itself solely lasts some 20,000 years extra earlier than it disperses into interstellar area.


Winds and Companions

Decin’s staff studied a pattern of 12 AGB stars utilizing the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. By observing such lengthy wavelengths, ALMA penetrates the fuel and dirt round these stars to disclose particulars of their winds — and overturning earlier assumptions about their shapes.

This picture gallery of stellar winds round cool getting older stars exhibits quite a lot of morphologies, together with disks, cones, and spirals, as seen at submillimeter wavelengths (slightly than the seen-wavelength photos captured by Hubble above). Gases are coloured by their movement, with blue materials shifting towards us and purple stuff shifting away.L. Decin / ESO / ALMANone of the 12 AGB stars have symmetric winds, the researchers report within the September 18th Science. All of them have some type of arcs or shells, and a few of them even exhibit the sorts 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 (additionally 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 enormous populations of stars present that AGB stars, on common, have no less than one companion with greater than 5 occasions Jupiter’s mass. Whether these companions are stars or large planets, they’re seemingly affecting the evolving shapes of the stellar winds.

The wind’s pace can be key, as a result of because the star loses extra mass, the companions’ orbits will widen. Based on the ALMA observations, the researchers develop an equation that predicts whether or not a star will blow its winds primarily out alongside its poles — a construction discovered for stars with weak winds and closeby companions — or whether or not the wind will take a spiral form alongside the equator, discovered extra usually for sturdy 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 concerned within the analysis. “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 imply for the Sun? One day some 7 billion years from now, it too will change into a white dwarf surrounded by a lovely planetary nebula. Based on their findings, Decin speculates that Jupiter and even Saturn are large sufficient to play a job in deciding that nebula’s form.

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

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