Andromeda is headed our way at about seventy five miles per second.
In about three or four hundred billion years the Andromeda and Milky Way galaxies will collide. The result, if anybody is around to see it, will be beautiful. The two galaxies will pass through each other first. There is so much empty space, and the distance between stars so great, that it’s extremely unlikely that any stars will actually collide. But dust and gas clouds will, and that means that the rate of star creation will be greatly boosted.
Additionally, supernovae (which currently happen within either galaxy every 50 years or so) will likely start happening about once a year with some of them being close enough to outshine all the rest of the stars as seen from Earth.
Having passed through on a grazing course, Andromeda will retreat for a while, and then come back for a head-on collision. This time, the super-massive black holes at the centers of the galaxies will pass close enough together to pull each other in and merge. Gravitational waves will fling many stars out of the galaxies into intergalactic space, and will pull many others closer to the core. There’s a good chance that our sun (which will be a bit bigger, but still exist) along with its planets (Earth will still be there, but it will be parched and lifeless) will get pulled towards the center of the combined galaxies and then flung outward. (There’s a smaller chance that the flinging could send us right into one of the black holes.)
Our night sky right now is beautiful. But compared to what will be visible during these goings-on, it’s downright dull. The sky will be filled with so many stars that the night will no longer be dark. Gas clouds will glow brightly and in many colors. If we get close enough to the center, we’ll even be able to see massive gas jets shooting from the black holes in the core.
Shortly thereafter, the sun will finally expand to engulf the Earth bringing our planet to its final demise. But what an exit!!
(That article is about eight years old, but still accurate. The most substantive change to our knowledge since then is the confirmation in 2012 via Hubble data that the collision is, in fact, inevitable.)