convex hull

Among many things which were—believe it or not—more boring than this, I spent part of today getting a geometric picture of the convex hull for stellar colors. An unresolved binary star system is composed of two stars; a galaxy is composed of many stars. Stars do not fill all of color space, but live on some restricted set of loci in the multi-dimensional space of astronomically observed colors. Combinations of stars, such as binaries and galaxies, must therefore also live in a subspace of the whole space. This is a no-brainer, but it is not trivial to visualize the situation, because color space is not linear in any sense. Some of the results were a bit surprising to me, which is embarrassing.