The Milky Way is mind-boggingly big. This massive galaxy has 300 billion stars of which the Sun is just one of them.
All these stars and a bunch of hydrogen gas that will be used to make more stars in the future are contained in a disk with a diameter of 100,000 light years. Although astronomers still do not know exactly how it got there, at the center of it all is a 30 million solar mass supermassive black hole.
As rare as the Milky Way might sound, just “next door” to us is a near-twin called the Andromeda galaxy. We can see Andromeda with the unaided eye in the winter night sky (see previous post). Twin heavyweights that they are, Andromeda and the Milky Way are gravitationally attracted to each other, and as a result are on a direct collision course.
If this impact story is true, then why haven’t you heard of this before? It turns out that the collision will not happen for a while. The impact will take place only in about another 1 billion years, it is not going to make the agenda of the typical presidential candidate.
Importantly for us, or for any life on Earth that will hopefully still be around in 1 billion years, the impact will not affect the Earth, the Sun or even the orbit of the Earth around the Sun at all.
What will happen? Well some bits of the two galaxies will collide rather dramatically. In particular the hydrogen gas component in both galaxies will hit with a gigantic SMACK. The gas will heat up to high temperatures, produce shock waves, and this impact will instigate a bunch of new star formation.
Meanwhile the individual stars of each galaxy like the Sun will not collide at all, but will fly past one other. In the end there will be only one galaxy bigger in size and twice the weight of the Milky Way, and that galaxy shape will be elliptical instead of spiral. The good news is our descendants should be just fine.