(Every day until Christmas, I’ll be posting a science-related image.)
At one point in the past, the galaxy known euphoniously as ESO 350-40 was probably a spiral galaxy, not dissimilar to the Milky Way. However, a smaller galaxy—perhaps one of the two at the right of the image above—plunged through the disk of ESO 350-40. The result was astounding: much of the galaxy’s mass pushed to the center, while other gas and dust concentrated at the galaxy’s rim. Where the gas clumped up, new stars formed, creating the intense blue light seen in the image. The disturbance also gave rise to the spokes, which gave the object its name: the Cartwheel Galaxy.
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.