How planetary nebulae get their shapes
About 7.5 billion years from now, our sun will have converted most of its hydrogen fuel into helium through fusion, and then burned most of that helium into carbon and oxygen. It will have swollen to a size large enough to fill the solar system nearly to the current orbit of Mars, and lost almost half of its mass in winds. At this stage the very hot remnant star will ionize the ejected material, lighting it up and causing it to glow as a planetary nebula (so-called not because it is a planet but because it surrounds its star). All low-to-intermediate mass stars (stars with between about 0.8 to 8 solar masses) will eventually mature into stars hosting planetary nebulae. This simple description suggests that planetary nebulae should all be spherically symmetric shells, but in fact they come in a wide range of shapes from butterfly or bipolar to eye-like or spiral- shapes. Astronomers think that the stellar wind is somehow responsible for these asymmetries, or perhaps the rapid spinning of the host star plays a role, but so far most of the proposed processes are not efficient enough.
phys.org
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