This 2D simulator renders a star-forming nebula as layered, drifting noise fields shaded to mimic real interstellar gas: a nearby massive star ionizes the hydrogen around it, turning cool neutral gas violet-blue and pushing an expanding front of pink recombination glow and hot blue-white ionized gas outward as its brightness rises. Raise the density to thicken the cloud, add turbulence to see the gas churn and fray into filaments, and dial up the ionizing star's brightness to watch the HII region grow and the live temperature-zone readout climb from cool molecular gas toward a hot ionized bubble.