Why do planetary nebulae have hourglass shapes?
The details are not fully understood at this time. Magnetic fields are one possibility. Another possibility involves the ejection of fast-moving material within a pre-existing outflow of gas from the same star or a companion. The above image, of the “Southern Crab Nebula” (He2-104) taken with the Hubble Space Telescope is described as follows by the investigators: The possible creators of these shapes cannot be seen at all in this Wide Field and Planetary Camera 2 image. It’s a pair of aging stars buried in the glow of the tiny, central nebula. One of them is a red giant, a bloated star that is exhausting its nuclear fuel and is shedding its outer layers in a powerful stellar wind. Its companion is a hot, white dwarf, a stellar zombie of a burned-out star. This odd duo of a red giant and a white dwarf is called a symbiotic system. The red giant is also a Mira Variable, a pulsating red giant, that is far away from its partner. It could take as much as 100 years for the two to orbit arou
