Astronomers have long known that the Andromeda Galaxy, a city of a trillion stars, is heading our way. To a distant observer the collision would look like this.
The two galaxies are expected to pass through each other in about four billion years, scattering stars as they fall apart and eventually merge to form a new galaxy.
Recently they used the Hubble Space Telescope to measure how stars in Andromeda are moving relative to the stars in our own galaxy: The Milky Way.
They found, in fact, that Andromeda is on course for a head-on collision with us. Galaxies are mostly empty space so M31 and the Milky Way are expected to glide right through each other without any individual stars colliding.
And what about us? Our solar system might be thrown from its path around the Milky Way and orbit Andromeda for a while before being absorbed into the larger, merged galaxy.
The earth might survive, but we will be long gone. Our planet would have been sterilized by the swelling sun long before that.
In only two billion years it will be here and the sky will look like this for anyone and anything that might still be around to see it.
Gravity will rearrange the stars into streams of light and scramble both galaxies as they pass through each other.
Like fighters or dancers circling each other, the two galaxies will part and then slowly come together again.
The death throes of the Milky Way will spangle a successor galaxy with new stars. The clouds of gas and dust are shaken and compressed by gravitational interactions.
After several billion years, they will merge into a single giant galaxy. A big egg-shaped ball of stars. .
The ashes of dead stars will become the seeds for new ones. New worlds and perhaps a chance for new life as the cosmic cycle of destruction and creation goes on.