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Strömgren Sphere: HII Region Growth

Interactive Strömgren-sphere simulator: watch a hot star's UV photons carve out and grow a spherical ionization front through surrounding hydrogen gas until photoionization balances recombination, with live physically accurate radius and timescale readouts.

Space & Astronomy3DModerate60 FPS📱 Mobile-adapted⇄ 2D version

When a hot young star switches on, its ultraviolet photons race outward and strip electrons from every hydrogen atom they meet, carving a growing bubble of ionized gas out of the surrounding cloud. This simulation models that process from first principles: a real balance between photoionization and case-B recombination sets the equilibrium Strömgren radius, and the time-dependent solution shows the ionization front sweeping outward through several thousand instanced gas particles until it saturates. Tune the star's ionizing photon output and the ambient gas density to see how both the final radius and the time it takes to get there respond, with live readouts for the Strömgren radius, the instantaneous front radius, the recombination timescale and the ionized volume fraction.

⚙ Under the hood

Watch a hot star's ultraviolet photons carve out a growing sphere of ionized hydrogen gas, balancing photoionization against recombination until the front reaches its equilibrium Strömgren radius.

nebulaHII regionionizationastrophysicsstar formationphotoionization

3D · Three.js / WebGL renderer · 60 FPS target · runs fully client-side, no install