Colliding galaxies electronically
Computer simulations reveal how normal galaxies get transformed as they fall under each other's gravitational spell.
Anytime astronomers witness a collision between two large galaxies, they see only a snapshot. Galaxies are so big, and gravity is such a weak force, that collisions take hundreds of millions — if not billions — of years from first brush to final outcome. Observers can see different galaxy pairs at different stages of interaction, but this gives only a vague sense of what goes on in individual cases.