Six hundred billion stars, nebulae and black holes are headed straight toward us. When they hit, some stars will collide with others. Gas clouds will slam into each other and even more stars, causing a furious burst of new star formation. Calculations show that our own solar system may be thrown out of its nice, neat orbit around the center of the Milky Way to drift erratically through space.
This isn't science fiction: It is a picture of the future. The Andromeda Galaxy, our galactic big sister, is on a head-on collision course with our own Milky Way. Since so much of space is empty, actual collisions between stars will be rare. But there will be massive gravitational upheavals, with many stars ripped from both galaxies to wander intergalactic space as orphans.
Nebulae are large, extended gas clouds, the location of stellar nurseries. Some from Andromeda will collide with some from our galaxy. These impacts will compress the gas clouds, triggering an era of rapid star birth. Many of these will be very massive stars that live fast and furious and die quite young, as stars go, in fiery supernova explosions that will sear any nearby planets with deadly gamma and cosmic rays.
Astronomers have known about this for some time. But don't worry, this collision won't happen for another 4 billion years.
And, really, our planet has much more to fret about, anyway. Long before Andromeda arrives, our own sun will begin to swell as a red giant star, eventually growing so large that it will swallow Mercury and Venus and, probably, Earth, before eventually running completely out of fuel and turning into a stellar ember, emitting virtually no heat. Before all of that, the sun's increased heat will turn Earth's surface molten and completely evaporate our oceans.