Asked by: TOM REES, BY EMAILA typical galaxy has about 100 billion stars, which sounds like a lot, but they are also very big. A spiral galaxy, like ours, is 100,000 light-years across and 3000 light-years thick. That gives an average density of just one star for every 225,000 cubic light-years. So two galaxies colliding is like two swarms of bees colliding. Except that to get the scale right, there would be 6000 kilometers between each bee. If galaxies were just made of stars, they would slide through each other with hardly any interaction at all. But the space between the stars contains significant amounts of dust and gas. Gravitational effects and frictional heating distorts the shape of both galaxies as they pass and causes new stars to form. But the collision takes more than a million years.