What is "scale radius" I've tried posting this on the Cosmolgy and astronomy forum but got no reponses yet.* Of course it only been a few days but I'm impatient... *I've been researching dark matter lately.* Evidence for it in spiral galaxies is the fact that the stars revolve about the center of a galaxy in a strange way: they all travel at the same speed.* When one graphs "rotation curves", orbit speed vs. distance from the center, the velocity curve rises (in the central bulge) then flattens in the disk.* When one plots curves based on the matter in the bulge and disk and sums them, the curve isn't flat.* Therefore, there must be invisible matter in the form of a sphere, or halo, enveloping the galaxy. My problem is that I can't reproduce those curves.* When I look at curves from actual galaxies my curves for the bulge and the central black hole are similar, but the curves for the disk and dark matter halo aren't.* I've downloaded explanations of the mass distrubution in spiral galaxies, which should solve my problem, but they always talk about "scale radius" and use that value in their formulae --- but I don't know what that means. It seems to mean a number compared to some arbitrary value, what what is that value?* One article mentions a "scale radius" for the disk of the Milky way of 3.5.* Can't be megaparsecs, since the MW disk is about 16 Mpc in radius. *So, I'm stuck.* Can anybody help me understand this stuff?