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We don't have very accurate distance determination to most planetary nebulae. This is due to most of our standard candle methods of distance determination not working on this class of objects.
@evilscientistca How come? Why do planetary nebulae lack standard candles?
@DialMforMara Their mass and size are too variable and difficult to measure to compensate for the variability. This prevents an accurate estimate of their luminosity and hence a determination of their distance based on how bright we see them as and how bright they really are.