Gravity, not choreography.

A physical sandbox for one possible Milky Way–Andromeda encounter.

Current preset

What is actually simulated

One or two moving analytic galaxy potentials, each combining a Hernquist-style dark-matter component with a softened stellar component. Thousands of massless stellar tracers respond to the modeled gravity using a fixed-step leapfrog integrator. In two-galaxy views, the centers follow mutual gravity plus an adjustable Chandrasekhar-style dynamical-friction approximation. Tidal bridges and tails arise from those forces rather than a keyframed animation.

What the galaxy classes change

Spiral and lenticular presets begin with exponential stellar disks, ellipticals use a pressure-supported three-dimensional distribution, rings place young stars outside a central gap, and irregulars use lopsided stellar clumps. Those morphologies change how tracers are initialized; the live gravitational potential remains an intentionally simplified spherical approximation.

What this browser model leaves out

Sol marks an illustrative Milky Way arm location, attached to a model stellar tracer. It is a navigation landmark, not an accurately placed star or a prediction of the Sun’s future orbit. First entry from Earth or the Solar System starts at 1.5 million years per second; later visits retain your chosen Galaxy speed. Returning restores Earth’s last valid date and speed.

Real catalog inputs do not make every model parameter a measurement. Catalog-based galaxies use assumed halos and appearance; estimated black-hole masses are not detections. The optional real-star sky is a fixed J2000 Earth-view reference with stylized brightness and an illustrative Milky Way band, not the evolving sky seen from an external galaxy. Data credits, adaptations and licenses are listed below.

The timeline keeps growing past its initial range. Later epochs continue the same simplified gravity model, not a forecast of the universe: stars do not age or form, and space does not expand here. Space facts describe real astronomy; “What if?” cards are invitations to imagine or experiment, not predictions. Old trajectory snapshots are discarded to bound memory; revisiting an older epoch recalculates it.

No live self-gravitating dark-matter halos, stellar self-gravity, gas hydrodynamics, star formation, black-hole evolution, cosmological expansion, dust radiative transfer or uncertainty ensemble. Dynamical friction is approximate. Late merger remnants and custom galaxies are illustrative, not predictions of detailed morphology.

Particle colors and point sizes are enhanced for visibility. The wide-separation display option can enlarge stellar disks without changing forces, center separation or time. If the 3D renderer or WebGL is unavailable, a lower-detail Canvas renderer shows the same precomputed three-dimensional trajectories.

Black holes inside the galaxy simulation

Each included central black hole contributes a softened gravitational field to the stellar trajectories. Its mass is allocated from the galaxy’s stellar budget, preserving the total mass. Black holes follow their host centers until galaxy core coalescence, then remain distinct moving masses in the remnant; core coalescence is not a black-hole merger. Their symbols are enlarged location markers, not rendered event horizons. Open “Black holes” beside the scale legend for masses, model limits and sources.

A separate Black Hole Lab

Open Black Hole Lab in Explore galaxies to launch point tracers around an imagined central black hole. It uses its own time and distance scales and pauses the galaxy view. Newtonian orbits stop at a chosen model limit of eight event-horizon radii; reaching that line does not establish capture. The lab never changes this simulation’s black holes or replays its stellar trajectories. Its model notes and NASA sources are available inside the lab.