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'Nebula' is a latin word, meaning 'mist' see http://en.wiktionary.org/wiki/nebulaYour pictures are very pretty but remember the colours are probably false - to make features stand out.If you drop ink into a bowl of water, the patterns will be fairly random too. There will be features which depend on the height from which you drop the ink (total energy available) but they never look just the same; it's a random process.Nebulae start off as huge gas and dust clouds with a range of sizes so you could expect quite a variety of shapes. Then there are the explosions which may be involved (they can be formed by supernovae) and the fact that we are seeing them at all different stages in their lifetime. . . .
"The name originated in the 18th century because of their similarity in appearance to giant planets when viewed through small optical telescopes, and is unrelated to the planets of the solar system.""Only about 20% of planetary nebulae are spherically symmetric (for example, see Abell 39.) A wide variety of shapes exist with some very complex forms seen. The reason for the huge variety of shapes is not fully understood, but may be caused by gravitational interactions with companion stars if the central stars are double stars. Another possibility is that planets disrupt the flow of material away from the star as the nebula forms. In January 2005, astronomers announced the first detection of magnetic fields around the central stars of two planetary nebulae, and hypothesised that the fields might be partly or wholly responsible for their remarkable shapes."http://en.wikipedia.org/wiki/Planetary_nebula
Different shaped nebulae can form as a result of inhomogeneities in the original gas cloud, or because clouds merge that had different angular alignment.I have also seen it speculated, although I can't find the reference at the moment, that heavier elements may play some part in it.
Dear Nebulizers,Do ewe like my collection of planetary nebulae ?