What examples are there of fuzzy concepts in astronomy? In particular, how fuzzy are the boundaries between different types of stars?
As an example of a fuzzy concept I'm thinking of the planet/brown dwarf distinction:
The IAU decided to define the term "planet" (at least for the solar system) by giving specific conditions, but for exoplanets the distinction between brown dwarfs of 13 Jupiter masses (MJ) is observationally difficult, and scientifically dubious anyway for various reasons: the initial mass function of star clusters forming does not stop at 13 MJ but could go as low as 1 MJ; objects larger than 13 MJ could form in the same way as planets; and in any case the 13 MJ limit of deuterium burning is for objects of solar metallicity and otherwise doesn't apply - especially if the object has a rocky core, and "deuterium burning" is itself ambiguous - how much deuterium must burn before it qualifies?1 It seems that many objects would be of interest to planetologists and brown dwarf studiers. The Extrasolar Planets Encyclopaedia goes up to 25 MJ.