Well, it's already been established that these objects consist of more than one star and have not evolved to the planetary nebula stage. The "four of the others" that I mentioned earlier are, in fact, M27, M57, M76, and M97.
I'll drop a hint that dwelling on their physical characteristics is heading down the wrong path.
Dave Mitsky
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Chance favors the prepared mind.
De gustibus non est disputandum.
Never attribute to malice that which can be adequately explained by stupidity.
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