Planetary NebulaMike Swanson
Planetary - similar to a planet. Nebula - gaseous, ethereal appearance.
The term planetary nebula came from some of the first astronomers with access to telescopes. The brighter planetary nebulae that they were able to detect were similar to planets in the size they presented in the eyepiece, yet they were obviously gaseous in nature. But they are anything but planets. Planetary nebulae are the remains of old stars similar in size to our Sun. In fact, in a few billion years, if everything goes as planned ;-), our Sun will become a planetary nebula.
The process from main sequence star to planetary nebulae is thought to go this way. Originally the star is mainly fueled by converting hydrogen to helium in the extremely high pressure of it core via the process of nuclear fusion. After most of the hydrogen in the core is expended, the fusion reaction starts combining helium atoms to form carbon and other heavier elements. The out-flowing radiation from this helium fusion reaction causes the star to balloon into a red giant phase, where the some helium and the remaining hydrogen expand outward.
But this helium to carbon reaction is not the most stable of nuclear furnaces and eventually the instability causes an explosive outflow of radiated energy, blowing the outer, lighter elements off into space. What is left is a shell, glowing from the energy emitted by the remaining white dwarf star - the typical anatomy of a planetary nebula.
They come in a variety of shapes. Twin cones expanding out from the central star are well known. So are spheres of glowing gas around the parent star. Others are more intricate shapes suggesting very complex circumstances at the time of the explosion or perhaps the star had a binary companion that helped create the convoluted shape due to its gravity.
Arguably the four most famous planetary nebulae are:
Ryukyu Astronomy Club
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