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Galaxy Collision

Cosmic Ballet​

When galaxies collide, they create some of the most spectacular events in the universe. Gravitational tidal forces rip stars from their orbits, creating sweeping tidal tails and bridges that span hundreds of thousands of light-years.

This simulation uses N-body gravitational dynamics to model the merger of two disk galaxies, similar to the famous Antennae Galaxies or the predicted future collision of the Milky Way and Andromeda.

The Physics​

N-Body Gravitation​

Each star particle experiences gravitational attraction from all others. The acceleration includes a softening parameter ε that prevents:

  • Numerical instabilities from close encounters
  • Unrealistically high velocities
  • Effectively models the extended mass of real star systems

Initial Conditions​

Each galaxy is created with:

  1. Central bulge: Pressure-supported (random velocities)
  2. Disk component: Rotation-supported (circular orbits)
  3. Massive core particles: Simulate dark matter halo concentration
Galaxy Parameters:
- Stars per galaxy: 1500
- Disk radius: ~12 kpc
- Bulge fraction: 20%
- Initial separation: 70 kpc
- Impact parameter: 20 kpc

Simulation Stages​

1. Approach​

Two spiral galaxies approach each other on a hyperbolic orbit. Tidal forces begin to distort the outer regions.

2. First Passage​

The galaxies pass through each other (stars rarely collide directly). Strong tidal forces pull out long streamers of stars.

3. Tidal Tail Formation​

Material stripped from the outer disks forms spectacular tidal tails extending far from the merger site.

4. Second Passage & Merger​

Gravitational friction slows the galaxies. They fall back together and eventually merge into a single, larger galaxy.

Animation​

The animation above shows the gravitational N-body simulation in real-time.

Run It Yourself​

claude -p "Load the galaxy_collision distribution, run MD simulation with 6000 steps \
  at dt=0.018, render trajectory with per-particle colors (blue and red galaxies), \
  and save to /tmp/galaxy_collision.gif" --allowedTools "mcp__molecular-mcp__*"

Real Galaxy Collisions​

The Antennae Galaxies (NGC 4038/4039)​

The Antennae are the closest example of a galaxy merger:

  • Distance: 45 million light-years
  • Status: Mid-merger
  • Features: Spectacular tidal tails, intense star formation
  • Future: Will become a single elliptical galaxy

Our simulation captures the essential physics of this interaction.

Milky Way - Andromeda Collision​

In about 4.5 billion years, our Milky Way will collide with the Andromeda Galaxy (M31):

  • Current separation: 2.5 million light-years
  • Approach velocity: ~110 km/s
  • Result: A new elliptical galaxy ("Milkomeda")

The Science​

Why Stars Don't Collide​

Even though galaxies "collide," individual stars almost never hit each other:

  • Stars are tiny compared to the space between them
  • If the Sun were a grain of sand, the nearest star would be 4 miles away
  • Galaxies are mostly empty space

Tidal Forces​

The gravitational gradient across a galaxy creates differential forces: ΔF ~ GM Δr / r³

This stretches the galaxy along the line connecting the two centers and compresses it perpendicular to this line.

Dynamical Friction​

Galaxies lose orbital energy through:

  1. Gravitational wake: Each galaxy creates a wake of particles behind it
  2. Momentum transfer: Energy goes from bulk motion to random stellar motion
  3. Result: Galaxies spiral inward and eventually merge

Collision Scenarios​

TypeImpact ParameterFeatures
Head-on~0Violent disruption, rapid merger
Moderate0.3 × RTidal tails, bridges, eventual merger
Glancing> RLong tidal streamers, may not merge

Technical Notes​

Integration Method​

Velocity Verlet algorithm:

  1. Half-step velocities
  2. Full-step positions
  3. Compute new accelerations
  4. Complete velocity step

Computational Complexity​

Direct N-body is O(N²) per timestep. For larger simulations, use:

  • Barnes-Hut tree code: O(N log N)
  • Fast Multipole Method: O(N)
  • Particle-Mesh: O(N log N) with FFT

Further Reading​

  • Toomre & Toomre (1972): Classic paper on tidal tails
  • Barnes & Hernquist (1992): Merger dynamics and remnant properties
  • Hopkins et al. (2006): Star formation in galaxy mergers