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| Star Birth and the Main Sequence |
1. It depends on the average particle mass because the more massive the particle the slower it moves (for a given energy) and the less effective it is in generating pressure to counteract gravity. In the Jeans formula, the cube of the particle mass appears in the denominator, so the critical density is lowered (favoring collapse) if the average particle mass is increased. This is as we would expect from our interpretation.
3. Because of the large disparity of stellar lifetimes as a function of mass. The hottest stars have lifetimes of millions of years, which is essentially instantaneous on the 15 billion year or so timescale for the age of the Universe, but the coolest stars have main sequence lifetimes far longer than the present age of the Universe.