laci141
Density Wave

Scientific basis

This simulation is inspired by density‑wave models of spiral structure. Early dynamical work by Bertil Lindblad helped establish this line of thinking, which was later developed into more complete density‑wave theories by C. C. Lin, Frank Shu and others.

How the model works

The simulation uses nested elliptical orbits with a common major axis. Each star follows a slightly eccentric orbit; the orientation of the orbits twists with radius, creating the spiral pattern. The bar is formed by aligning the inner orbits, while the outer disc retains the usual winding. The parameters (Hubble‑like morphology, bar strength, winding, inclination, etc.) are visual approximations, not a physical N‑body simulation.

The spiral pattern itself rotates slowly and rigidly, like a bar. Only at the corotation radius (dashed circle when orbits are shown) do stars keep pace with it: inside, stars overtake the arms; outside, the arms overtake the stars. Gas is compressed where it enters an arm, so the dark dust lanes sit on the upstream edge, pink H II regions just downstream, and short-lived blue stars a little further on — and this order flips across corotation.

Limitations

  • No gravitational interactions between stars are simulated.
  • The spiral pattern is purely kinematic – there is no self‑consistent density wave.
  • “Supernovae” are visual effects only (bright flares and expanding shells).
  • The “Milky Way‑inspired band” is a generated background, not a real star map.
  • Presets like “Andromeda‑like” are morphological matches, not precise physical models.
  • The galaxy encounter uses test particles in a fixed potential plus one companion mass, in the spirit of Toomre & Toomre (1972); the disc has no self-gravity.

Sources

  • Lindblad, B. (1925). On the Cause of Star-Streaming. Astrophysical Journal, 62, 191–197. NASA ADS / ApJ
  • Lin, C. C. & Shu, F. H. (1964). On the Spiral Structure of Disk Galaxies. Astrophysical Journal, 140, 646. DOI
  • Binney, J. & Tremaine, S. (2008). Galactic Dynamics: Second Edition. Princeton University Press. DOI / kiadói oldal
  • Shu, F. H. (1982). The Physical Universe: An Introduction to Astronomy. University Science Books. ISBN 978-0-935702-05-7. Kiadói oldal
  • Toomre, A. & Toomre, J. (1972). Galactic Bridges and Tails. Astrophysical Journal, 178, 623–666. DOI

Density Wave — Interactive Spiral Galaxy Simulation

A browser simulation of how the spiral arms of a disc galaxy can appear without any fixed structure holding them together. Every star follows its own elliptical orbit; the arms are simply where those orbits crowd.

What you are looking at

Each point is a star on its own slightly rotated elliptical orbit around a common centre, and stars near the core complete an orbit faster than stars further out. Because neighbouring orbits are turned by a small, steadily increasing angle, they bunch together along two or more sweeping curves. Individual stars drift into those crowded regions and out again: the pattern stays, the matter moves through it.

Density waves and the winding problem

A disc galaxy does not turn like a solid wheel — the inner parts orbit far faster than the outer parts. If spiral arms were fixed strings of stars and gas, this differential rotation would wind them into a tight coil within a few galactic rotations, and after billions of years no open spiral would be left. Yet the sky is full of galaxies with wide, open arms. This is the winding problem, recognised by Bertil Lindblad in the 1920s.

Density wave theory, developed by C. C. Lin and Frank Shu in the 1960s, answers it by treating the arms as a wave pattern that rotates at its own steady speed while stars and gas pass through it — closer to a traffic jam than to a wall. Gas compressed at the crest of the wave collapses into new stars, which is why real arms glow with young blue stars and pink hydrogen clouds. Switch on “Material arms” in the panel to watch the winding problem happen.

What this is not

This is a visual approximation, not an N-body or hydrodynamic simulation: the orbits are prescribed rather than computed from gravity. The supernova flashes and the Milky Way-inspired background band are decorative effects, not physical models, and the morphology slider approximates the look of the Sa–Sc sequence rather than classifying anything. The origin, lifetime and exact dynamics of spiral arms remain an active research question.

Controls

Scientific basis

Density wave theory is a serious, observationally supported model, but it is one explanation among several and the nature of spiral structure is not a settled question. These are the primary sources behind the simulation:

Lin, C. C. & Shu, F. H. (1964). On the Spiral Structure of Disk Galaxies. The Astrophysical Journal, 140, 646.
Shu, F. H. (2016). Six Decades of Spiral Density Wave Theory. Annual Review of Astronomy and Astrophysics, 54, 667–724.
Dobbs, C. & Baba, J. (2014). Dawes Review 4: Spiral Structures in Disc Galaxies. Publications of the Astronomical Society of Australia, 31, e035.

Open source, MIT licensed — built by laci141. Source and download on GitHub ↗ build 2026-09-26