A visual guide to what the cosmic event will look like from space -- and from right here on Earth.
The Andromeda galaxy is about 2.5 million light years away -- for now.
NASA scientists announced yesterday that Andromeda (also called M31) and our own galaxy, the Milky Way, are on a collision course that will result in a dramatic rearranging of the two galaxies that will begin in about four billion years and last for another two billion. In the end, the two galaxies will merge completely into a single elliptical galaxy. A third galaxy, M33, may join in the collision, and, possibly, even reach the Milky Way first.
It had long been known that Andromeda is rushing towards Earth at about 250,000 miles per hour -- or about the distance from Earth to the moon -- but scientists were unsure whether the two galaxies would full-on collide or just barely brush. The new research is the result of careful observations of Andromeda by the Hubble Space Telescope, which show that the gravitational pull of each galaxy is pulling them closer and closer together, and "remove any doubt that [M31] is destined to collide and merge with the Milky Way," NASA said. The sun could end up in a completely different region of the galaxy, one much farther from the galactic center, but that the Earth and solar system would survive.
These sorts of collisions were more common in the early universe, when the universe was smaller, but they still happen because of the gravitational pull of dark matter.
NASA has provided artistic renderings of what the whole event -- unfolding over billions of years -- will look like from here on Earth:
In addition, a video shows the trajectory of the two galaxies and how the collision will appear from space:
To learn more about how scientists measure distances of light years in space, we have an explainer video up here.