It is believed that the universe formed from an event known as the Big Bang. Some 15 billion years ago, a massive clump of matter had seemingly formed from nothing. Theological arguments aside, this ball of gas and dust expanded to the point of exploding. The pieces of this gigantic mass flew in all different directions, and are still traveling ever outward as the universe expands. Thus, everything that exists initially came from that Big Bang.
A significant result of the Big Bang was the formation of stars and galaxies. A galaxy is a massive collection of stars clumped into one specific shape. The galaxy in which Earthlings reside is the Milky Way, and it is described as a spiral-shaped disk. The Milky Way is a bit more than 100,000 light-years in diameter, with the sun that supports human life being located more or less in the middle.
The Andromeda galaxy is the nearest galactic neighbor to the Milky Way, and also happens to be spiral-shaped. This galaxy is about twice the size of the Milky Way and currently about 2.2 million light-years distant. However, as each hour passes, the Milky Way and Andromeda galaxies are getting some 600,000 miles closer to each other. So if the universe is expanding from the Big Bang, how is it possible that the Milky Way and Andromeda galaxies are getting closer together?
The best answer astronomers can give is as follows: The Milky Way and Andromeda are a bound pair of galaxies that orbit each other. Shortly after the Big Bang took place, they formed close together and initially drifted apart following the massive explosion, but since they are bound together by the force of gravity, they are now falling back together. Thus, around 2-3 billion years from now, they will collide with each other!
What will this mean for inhabitants of Earth? Assuming that the human race will still be around by then, there are different scenarios.
As the two galaxies collide, new stars will be born, but the process will take several millions of years before the Milky Way and Andromeda will have merged into one. Many would likely assume that such an intergalactic collision would result in the end of all life, but the only way that could happen would be if a star from the Andromeda galaxy hit the sun or Earth, and the odds of that occuring are quite remote. First of all, stars are spaced very far apart. To get an idea of just how far apart most stars are from each other, try to imagine two periods this size (.) spaced the entire length of a football field apart.
Instead, the Earth’s inhabitants could be treated to a spectacular show in the sky as the spiral-shaped Andromeda galaxy grows larger and larger over a period of millions of years. Finally, not only would new stars be born as others explode into supernovas, but the number of stars in the nighttime sky would more than double.
In another possible scenario, the sun could be thrown out into the depths of space, which would result in very dark nights, but keep in mind that the sun would still provide life and light to anyone anywhere in the solar system.
Galaxy collisions like the eventual one that will occur between the Milky Way and Andromeda are quite common when two happen to be close together. Seemingly, this directly contradicts the fact that individual stars are spaced far apart as noted above, but one must look at the big picture of the universe. Entire galaxies composed of billions of stars themselves make up a minuscule fraction of the entire universe. As a result, two galaxies that are just a few million light-years apart are considered “neighbors.”
Therefore, while the universe as a whole is still expanding from the Big Bang that took place 15 billion years ago, there are nevertheless many galaxies with “close” neighbors that are influenced by each others’ gravitational pull. As a result, they can eventually collide with one another and get clumped together in spite of the universe’s perpetual expansion.