Today let’s see when what happens when two galaxies collide. The pair featured here is located about 230 million light years away in the constellation of Hercules. When they were first discovered by William Herschel in 1784 they looked like a single object in his telescope and received a single catalog number, NGC 6052. But the Hubble Space Telescope clearly reveals two galaxies in mid-collision, each pulling on the other, distorting its shape as they merge into a single, stable galaxy.
Star collisions are rare when galaxies collide because stars are tiny compared to the space between them. A typical star might be a million miles across but the distance to the next star is measured in trillions of miles, making stars more like scattered fireflies in a great, dark forest. Galaxies can pass through one another as if they’re made of nearly empty space, which despite appearances, they are!
That doesn’t mean collisions don’t have consequences. Galaxies also harbor giant molecular clouds composed of hydrogen molecules (and other gases and dust) that can span hundreds of light years. When they slam into each during a merger, the denser regions collapse through gravity to give birth to thousands of new stars that sparkle like blue firecrackers. In NGC 6052 dozens of them spangle either galaxy’s distorted spiral arms.
Galaxies often gather into clusters. The Milky Way belongs to one such cluster of some 50-plus galaxies called the Local Group. Distance between galaxies in groups is several order of magnitude less than the distances between stars, so close approaches and mergers are common. Galaxies don’t have to collide to wreak havoc on one another. A close flyby is enough for their gravities to twist and distort each other into bizarre shapes with long tidal tails of stars pulled from each other like taffy.
Milky Way and Andromeda collision simulation
Our own galaxy, the Milky Way, will undergo a similar collision in the future with our nearest, largest galactic neighbor, the Andromeda Galaxy some 4 billion years from now. In detailed computer simulations, it won’t be a simple close flyby and merger but rather a dance where each will pivot about the other twice before falling into a single galaxy more than 6 billion years from now.
During that time of mayhem and reorganization, there’s a modest chance the sun will get kicked out of the galaxy’s disk altogether, either ending up in a tidal tail or captured by the Andromeda Galaxy. While it won’t affect the steady cycling of the planets around the sun, any humans still remaining in the solar system may get the opportunity to see the Milky Way from outside, as an external galaxy under a sky aglow with fresh stars. What I wouldn’t give to be see that!
Since you and I won’t be around to witness the event, at least we can enjoy the video.