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A region of space 190 parsecs (620 light-years) across, followed for 30 million years. At normal speed a second of playback is 0.4 million years. The colours are the Hubble narrowband palette used for images like the Pillars of Creation: sulphur [S II] in red, hydrogen Hα in green and oxygen [O III] in blue, so hot ionised gas near young stars looks blue-green and cooler gas at the edges looks red. Everything is computed on your device; nothing is pre-recorded, and each run of the same seed can differ slightly.
Real star-forming clouds are networks of filaments meeting at hubs. The seed draws three to six curved filaments of cold molecular gas from one to three hubs, each filament a few parsecs wide and denser toward its hub, broken into clumps along its length, inside a thin envelope of diffuse gas. The whole cloud holds about 100,000 solar masses, with gas densities of roughly 100 to 200 molecules per cubic centimetre in the filaments, as in a real giant molecular cloud. No stars exist at the start: the cloud is lit only by a distant cluster off to one side, so it begins dark, with its faces toward that cluster faintly lit.
The gas is a few hundred thousand to a million moving parcels, each with a position and a velocity, advanced on the graphics card in steps of a few thousand years. Each step the parcels are spread onto a three-dimensional grid of density, and every parcel feels the pressure of the gas around it, gravity, drag and the push of any nearby supernova. Cold neutral gas has a sound speed of about 0.7 kilometres per second; gas that starlight has ionised is heated to 10,000 degrees and its sound speed jumps to about 10 kilometres per second. That jump in pressure at an ionisation front is what drives H II regions outward, compresses the clumps they run into and boils gas off their lit faces.
The grid cannot follow collapse below about four cells (6 parsecs), so a pressure floor stops the gas piling into unresolved points; a clump that would collapse becomes a star nursery instead (below). Parcels that move far from the local flow relax back toward it, which stands in for the viscosity and shocks the grid cannot resolve, and gas driven out into the surrounding interstellar medium is slowed by its ram pressure.
At the start every parcel is given a velocity from a sum of 48 seeded waves with wavelengths from 5 to 120 parsecs and an energy spectrum like Larson's law for real clouds, scaled to a dispersion of about 1.8 kilometres per second (the Turbulence slider). This is what stirs the filaments, makes the clumps that later collapse and keeps the cloud from falling straight into its hub.
Every quarter of a million years an observer reads the density grid back and solves Poisson's equation for the gas's own gravity with a fast Fourier transform on a 32-cubed grid (6 parsec cells), and the gas accelerates in that field. Collapsing clumps add their own unresolved pull below the grid scale until their stars ignite. Stars, once formed, move in the same gravity field.
Nothing after the start is scripted. At each observation the densest spots are tested against the Jeans mass: a block of gas 18 parsecs across becomes unstable when its mass exceeds what its thermal pressure can hold up at the density the gas actually sits at (measured on the fine grid, so a thin dense filament is not averaged away), and half that when starlight is already compressing it. A clump that stays unstable for a million years collapses into a nursery. Each nursery forms five to sixteen stars drawn from a Salpeter initial mass function between 1.5 and 60 solar masses, so most are small and long-lived and about one in six is massive. They glow as faint red protostars while they accrete and ignite about 3.5 million years after the collapse, taking the clump's motion plus a small random velocity. Recent nurseries and young clusters block new nurseries within 10 to 14 parsecs, as the gas there has been used or blown away.
The light of the brightest stars above 5 solar masses is traced through the dusty gas on the graphics card onto a grid of 1.5 to 4 parsec cells, with shadows where dense gas absorbs it and blue light lost before red; this is what lights the cloud's surfaces, casts the shadows behind clumps and reddens what you see through dust. A coarser copy drives the gas physics. Ionisation follows the Strömgren balance: ultraviolet photons from stars above 8 solar masses are counted outward along rays and used up by recombinations, which rise with the square of the density, so thin gas ionises far out while dense clumps stay neutral and cast shadows. Only ionised gas is hot and emits the hydrogen, oxygen and sulphur lines, with the oxygen line strongest nearest the hottest stars and the sulphur line at the fronts and edges. Emission scales with the square of the density, which is why rims and compressed fronts glow brightest and the thin interior of an H II region is faint.
Stars above 8 solar masses live 3 to 38 million years depending on mass and end as core-collapse supernovae. A supernova is modelled by its terminal momentum, about 300,000 solar masses times kilometres per second, delivered along rays to the gas inside the radius where the remnant's shell forms (which depends on the density it runs into), so the blast runs far out along thin gas and stalls against dense gas. The flash itself lasts months, far below the time step, so no flash is drawn; the remnant shows as shocked, glowing gas. Smaller stars never explode and stay as the cluster's lasting members.
Stars are particles. A cluster stays bound while the gas of its birth clump is there and loosens as the gas is ionised and blown away, so old clusters drift apart over tens of millions of years, as real open clusters do.
Magnetic fields, stellar winds, radiation pressure, cooling physics and chemistry are left out. The finest structure is set by the grid, 1.5 to 2 parsecs: the pillars, fingers and globules of the Hubble close-ups are smaller than one cell here, so at this scale they are only suggested by the rendering. The Refine button and capture sizes render a chosen view at higher sampling but cannot add physics below the grid; a separate pillar-scale simulation for prints is planned for desktop computers.