Physics Model Reference

The generation pipeline assembles physically accurate stellar populations for any galaxy morphology — dwarf spheroidals, lenticulars, spiral arms with active nuclei, irregular starbursts. Every star is drawn from a stellar initial mass function (IMF) validated against observed galactic demographics, spanning the full mass range from brown dwarf candidates through O-class supergiants.

Each star is assigned an age, metallicity, and evolutionary stage consistent with its host galaxy's star formation history. Chemical enrichment accumulates across generations — older stellar populations carry lower metallicity, younger ones reflect the iron and alpha-element abundance built up by preceding supernova cycles. Galaxy-scale kinematics follow observed rotation curves, including the flat outer-disk profile that implicates dark matter halos.

At full scale the pipeline produces and individually tracks objects across galaxies of trillions of stars, using distributed compute on EMR Serverless with Iceberg tables on S3 for columnar access at any resolution.

Every star in a generated system evolves through its full life cycle within the simulation. Main sequence evolution tracks how luminosity, radius, and surface temperature change over gigayear timescales as hydrogen is consumed in the stellar core.

Post-main-sequence phases — the subgiant branch, red giant branch, and asymptotic giant branch — are each modeled as distinct evolutionary stages with physics-appropriate luminosity amplification and mass loss. Stellar winds carry mass away continuously throughout the giant phases, following the empirical mass-loss scaling established by Reimers for evolved stars. The final evolutionary outcome — white dwarf, neutron star, or stellar-mass black hole — is determined by the star's initial mass.

Every star is evolved forward, once, from its formation age to a chosen simulation epoch — so a generated galaxy presents a realistic distribution of evolutionary stages rather than a frozen snapshot of zero-age main sequence objects. That evolution stops at the requested epoch until the system is aged again.

High-mass stars end their lives in core-collapse supernovae — explosive events that release approximately 10⁵¹ erg of energy into the surrounding interstellar medium. The simulation tracks the expanding blast wave, calculates the affected volume, and assigns damage severity to neighboring systems within the blast radius.

Environmental impact cascades: atmospheric stripping removes significant fractions of planetary envelopes in close-proximity systems, and orbital perturbations from the gravitational kick can destabilize planetary architectures. The severity model scales with distance from the progenitor and the progenitor's mass.

Intermediate-mass stars eject their outer envelopes as planetary nebulae, producing UV-ionized shells described by Strömgren sphere geometry. These ionized regions can disrupt protoplanetary disks and alter habitability conditions in surrounding systems, coupling stellar death to planetary environment physics across light-year scales.

In a generated galaxy, these outcomes are assigned as part of the generation pass — a star already past its death is generated in its post-event state. Authoring a death live, on a star that hasn't reached it yet, is a separate capability under active development; see Stellar Evolution Studio.

Over half of all stars in the Milky Way exist in gravitationally bound pairs or higher-order systems. The simulation models the key physical processes that govern how these systems evolve across cosmic timescales.

Tidal forces progressively circularize initially eccentric orbits — the timescale for this process is a steeply nonlinear function of orbital separation, so compact binaries circularize on million-year timescales while wide binaries may retain significant eccentricity through their entire lifetimes. In close binaries, one star can overflow its gravitational sphere of influence — the Roche lobe — transferring mass to its companion at rates that fundamentally alter both stars' evolutionary trajectories.

For the tightest binaries, energy loss through gravitational wave emission steadily shrinks the orbit on gigayear timescales, with merger the eventual outcome for some systems. Planetary orbits in multi-star environments are classified as dynamically stable or unstable using the Holman & Wiegert analytical stability limits, distinguishing S-type (around one star) and P-type (around both) configurations.

Stellar birth is accompanied by a rotating disk of gas and dust — the protoplanetary disk — from which planets form. The simulation tracks disk evolution through two competing processes: viscous accretion, which drains mass inward toward the star over millions of years following the Shakura-Sunyaev alpha-disk model; and photoevaporation driven by stellar ultraviolet and X-ray flux, which strips mass from the disk surface at a rate that accelerates as the disk thins.

Planet formation depletes the available solid and gas reservoir within the first few million years of a system's life. By the time a typical disk has dispersed — generally within ten million years — the planetary inventory is set. Habitability zone boundaries are computed from stellar luminosity and shift as the star ages and brightens.

Planets are not static — their orbits, atmospheres, and surfaces evolve continuously under gravitational, radiative, and thermal forcing. Hot Jupiters in close orbits lose orbital energy to tidal dissipation, drifting gradually inward over hundreds of millions of years. Low-mass planets near luminous stars lose atmospheric gas through XUV-driven photoevaporation, with small worlds potentially shedding entire envelopes on gigayear timescales.

Natural satellites experience outward tidal recession from their parent planets — the Moon's observed recession from Earth (currently ~3.8 cm/yr) is the calibration case for the model. Surface temperatures couple to stellar luminosity evolution, tracking the shift in habitable conditions as a star ages off the main sequence and the habitable zone migrates outward.

Complex gravitational environments — multi-star systems, dense clusters, planet-moon-ring hierarchies, and spacecraft trajectories — require direct numerical integration of the equations of motion for each interacting body. The N-body engine computes gravitational forces between all relevant bodies and integrates orbits forward while conserving energy over long time spans, enabling multi-body simulations of realistically complex systems at scale.

For solar system bodies and spacecraft, SPICE ephemeris data provides verified positional ground truth at epoch — anchoring numerical trajectories to observed positions published by JPL Horizons and the NAIF toolkit.

Black holes formed by stellar collapse grow through accretion of surrounding matter. The maximum rate at which a black hole can accrete is limited by radiation pressure — the Eddington limit — which sets the characteristic timescale for black hole mass doubling and governs how quickly supermassive black holes can reach their observed masses.

Accreting black holes with strong magnetic fields can launch relativistic jets perpendicular to the accretion disk via the Blandford-Znajek mechanism, producing some of the most energetic continuous emission in the universe. Jet power scales with black hole spin and mass, enabling physically grounded modeling of active galactic nuclei at galactic center positions.

For primordial black holes lighter than roughly 10¹² kg, Hawking radiation — thermal emission arising from quantum field effects near the event horizon — is the dominant mass-loss mechanism, with evaporation timescales that can be shorter than the age of the universe for the lightest objects.