t = 0.0 Myr
Dense core (collapsing)
Diffuse envelope
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Before a galaxy has stars, spiral arms or a rotation curve, it starts as diffuse gas — and every parcel of that gas faces the same fork in the road described by the Jeans instability: does self-gravity overcome thermal pressure, or does pressure win and the gas simply drifts apart? This simulator renders a 3D cloud of particles and evolves it under a physically grounded model — mean-field self-gravity toward the cloud's centre competing against thermal pressure support — while computing the real Jeans mass, Jeans length and free-fall time from your chosen mass, radius and temperature. Push the cloud's density above the Jeans threshold and watch it run away into a collapsing core; keep it below the threshold and watch pressure win instead. It's the same criterion astrophysicists use to explain why molecular clouds fragment into stars, and why the first structures condensed out of the gas that went on to build galaxies.