Galaxies are composed of hundreds of billions of stars. But galaxies are of immense size ... many are over 100,000 light years long ...that's 1,000,000,000,000,000,000 kilometres! Individual stars are tiny in comparison, and the spaces between them are many trillions of kilometres wide. So when two galaxies collide, they can pass right through each other without the individual stars coming into contact! This isn't to say that the galaxies remain unaffected by a collision, however; galaxies are very massive, and their huge gravitational forces can pull other galaxies apart.
Here are some photographs of actual galaxies in collision, which can be observed through large telescopes. Click on each image to see a larger version. We also have a galactic collision simulator, which will let you experiment with colliding galaxies over hundreds of millions of years.
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Galaxy NGC 6872 is longer than normal ... over 700,000 light years across from top to bottom, making it one of the largest barred spiral galaxies known. Its elongated shape may be a result of its continuing collision with the smaller galaxy IC 4970, just above center. NGC 6872's spiral arm on the upper left exhibits an unusually high amount of blue star forming regions.
The light we see today in our telescopes left these colliding giants before the days of the dinosaurs, about 300 million years ago.
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When two galaxies collide, the stars that compose them usually do not. This is because galaxies are mostly empty space and, however bright, stars only take up only a small amount of that space. But during the slow, hundred million year collision, one galaxy can rip the other apart gravitationally. Also, dust and gas common to both galaxies does collide. Clouds of gas and dust compressed by the collision cause the rapid birth of millions of stars.
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*** Try our galactic collision simulator. ***
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