(Every day until Christmas, I’ll be posting a science-related image.)
Day 15
Galaxies, despite their appearance, are mostly empty space. They may contain hundreds of billions of stars and vast amounts of gas and dust, but those are rarefied, spread across a hundred thousand light-years or more. (The nearest star to the Solar System is 4 light-years away, so we could imagine a third star passing between without ill effect.) As a result, when galaxies “collide”, their stars mostly don’t come close to each other. However, the motion of stars, gas, and dust are determined by the overall gravity of the galaxy, collective behavior from collective causes. The Cartwheel Galaxy’s shape was altered radically by the collision with another galaxy, which on our time scales would have been a ponderously slow event; in time it will settle into a new form.
We know that galaxies collide, and sometimes merge to form larger galaxies. Eventually (in about 5 billion years, give or take), the Milky Way will likely collide and meld together with our largest neighbor M31, the Andromeda Galaxy. During that process, some distant alien observer may look at our galactic home as it flexes and stretches under the influence of gravity, and remark upon its beauty just as we admire the glory of the Cartwheel Galaxy.