surrealist

Stars are created when a cloud of particles/dust/whatever collapses under its own gravity. I've read this in several places, but none of them tells me exactly how dense a cloud of particles must be in order to collapse. What are the numbers here? If a cloud were close to the right density, could humans (in theory) jump-start the process?

A parabola is defined at the locus of points equally distant from a point (called a focus) and a line (called the directrix). I would like to generalize on this idea. My questions are: 1) In 3-space, what is the locus of points equally distant from a point and a plane? (My guess is a paraboloid.) 2) In 3-space, what is the locus of points equally distant from a line and a plane? 3) In 3-space, what is the locus of points equally distant from two lines? Can any of you give me help in either computing or visualizing these loci?