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Collision of Two Spiral Galaxies - a Simulation
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Computer simulation by Chris Mihos, Case Western Reserve University & Lars Hernquist, Harvard University. 262,144 particle calculation with gravity and smoothed particle hydrodynamics, TREESPH code.

Visualization by Frank Summers, Space Telescope Science Institute. Color shows the different galaxy components: gas is blue, old stars are yellow, young stars are white.

Examples of actual galaxies in collision: Mice Galaxies, Tadpole Galaxy, Antennae Galaxies, Bird's-Head Galaxy