Part one
The physicsPhysics · Origin
Two missing ingredients
Watching the flock cluster under cohesion-vs-temperature looks like star formation — but the mechanics differ in exactly two ways. Flock cohesion is a short-range, overdamped nudge on bodies that all move at one fixed speed. Gravity is long-range (every pair attracts, however far) and acts on bodies with momentum — they fall, overshoot, and orbit. Astro mode adds precisely those two ingredients and removes the society: no breeding, no disease, no predators. Bodies only ever merge or escape, so the count only falls.
Physics · Stability
A knife-edge called virial balance
Long-lived orbiting structure exists only in a narrow band: kinetic energy must sit near half the gravitational binding energy. Slower → collapse; faster → the cloud unbinds. That is real astrophysics, and it is why the sliders feel delicate. Two protections widen the sweet spot: moving gravity live rescales every velocity by √(G′/G) so the balance rides along (structures tighten or relax instead of shattering), and the integrator subdivides its steps whenever anything would move further than the softening length can resolve — without that, close encounters inject phantom energy and the cluster boils into noise.
Physics · Disk formation
Birth of a pancake
The cloud seeds as a 3D ball spinning about an axis tilted 65° from your line of sight — because a disk whose spin axis points at the viewer forms face-on, and flattening along the line of sight is invisible. Tilted, you watch the collapse edge-on, like an inclined galaxy. But rotation alone is not enough: a collisionless system conserves energy, so bodies oscillate through the plane forever and the cloud stays a puffy spheroid — real stellar systems never flatten. Disks need dissipation. The settle (gas) slider damps only the motion along the spin axis, the way colliding gas radiates energy away while angular momentum protects the rotation: the cloud thins into a rotating band before your eyes.
Physics · Growth & decay
Accretion and evaporation
Not every collision sticks. A contact merges — conserving mass and momentum — only when the bodies meet slower than their mutual escape speed (× the stickiness slider); harder hits bounce inelastically. So the hot young disk resists clumping, every bounce sheds energy, and accretion accelerates as the disk cools — the way real disks grow. Size grows as ∛mass and heavy bodies gain a glow. Growth is hierarchical and runaway — grains sweep up grains, planetesimals sweep up planetesimals, until a few massive survivors orbit each other. The counter-current is evaporation: close encounters slingshot bodies outward while the core tightens (gravitating systems get hotter as they lose energy — the famous negative heat capacity of star clusters). Escapers past 12× the half-mass radius are removed; they were never coming back.
Part two
Watching itWatching · The camera
A camera that frames the mass
There are no walls in astro, so a camera has to choose what matters. It tracks the centre of mass and zooms to frame the half-mass radius — the standard astrophysical measure of a cluster's core. Mass-weighted, so a runaway minority of escapers can't drag the view out, and once accretion builds heavy bodies the camera stays on the mass, not the leftover dust. Positions use the true zoom; sizes use √zoom — the cartographer's compromise (astronomy software draws stars by brightness, not true angular size): bodies stay visible when the camera pulls far out and don't balloon when it dives in. Floors guarantee nothing ever renders below ~6 px or a hairline trail.
Watching · Recipes
Five experiments
Enter ✦ Astro and the defaults already give the pancake. From there, one knob at a time: