In the wikipedia entry it says it is likely a red dwarf between 50,000 AU and 100,000AU from the Sun and that it is probably a known star with an unknown distance.
I just don't see how that is possible. A red dwarf has enough intrinsic brightness to be visible by the naked eye on a clear night if it was as close as the entry says.
A red dwarf like Barnard's Star (~2pc away) has an apparent magnitude of 9.56 and therefore an absolute magnitude of about 13. If we assume this "red dwarf" that they say Nemesis is is 50 to 100 thousand AU away, and we assume it to be a similar spectral type to Barnard's Star (M4V), then its visual magnitude will be between 5.0 and 6.5 - visible to the naked eye. And it would have one hell of a parallax angle. It's parallax would be betwen two arcseconds and four arcseconds. Most near stars have a parallax along the order of 0.1-0.3 arcseconds. In other words, relatively speaking, in two different observations of the sky six months apart a 5th or 6th magnitude star would appear to shift positions quite noticably in the sky. Guys like Kepler and Galileo and Brahe would have noticed this.
Obviously this is not the case.
It is very obviously a brown dwarf if it is anything because brown dwarfs are not luminous because they are not fusing hydrogen into helium or helium into carbon or whatever.
And yes there is no "clean division", but the numbers I gave are astronomical guidelines.
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