We can’t avoid the inevitable end of our Sun. In roughly five billion years, that friendly globe in the sky is going to expand out, eat at least Mercury, probably Venus, and maybe us, and even Mars is going to get a bit charbroiled in the process. While it’s bad enough that our Sun is going to bloat up and destroy things, in five billion years that might not be the most insane thing going on. At the same time our Sun is evolving to its next phase, our galaxy is going to be merging with its nearest large neighbor, the Andromeda Galaxy. 

Getting a precise measure of the size of our galaxy, from inside our galaxy, is a bit tricky, but we think that Andromeda is the bigger of the two systems, and as we all gravitationally tangle, both systems will lose their amazing spiral structure, and in the tangle, wild things will happen. Some gas and dust in both systems will collapse to form stars, while more of it falls into the system’s supermassive black holes, and the remainder will get strung out in trails of debris called tidal tails. This will make for an amazing sky, with twin jets potentially coming out of the cores of both systems and amazing nebulae in all directions.

We understand galaxy collisions from looking at other galaxies like ours in various stages of merging together. We see what they are doing and understand this as our fate too. This is like looking at all your grandmothers and realizing you are destined for grey hair and cataracts; some things just come with your genes. Galaxies don’t have genes, but the building blocks of their structure dictate the future of their evolution just the same.

According to models, the resulting Milkdromeda Galaxy will be an elliptical galaxy, and while it will start with two supermassive black holes, newly published results show that roughly seventeen million years after the merger completes, which will be about ten billion years from now, the supermassive black holes will also merge. This supermassive black hole merger will release, in the form of gravitational waves, ten quintillion Suns of power; that’s a billion billions worth of solar power! Any civilization with a system like LIGO that is within our local group of galaxies should be able to detect this future event!

Space is big. It’s also mostly empty and the amount of time it takes things to travel can be mind-boggling. Andromeda is heading toward us at a breakneck 116 km/s. At this rate, Andromeda could travel from New York to London in 48 seconds. Nevertheless, it will be on the order of five billion years before the slow-motion impact begins. And unfortunately, none of us will be here to witness it.

More Information

Science News article

“Future merger of the Milky Way with the Andromeda galaxy and the fate of their supermassive black holes,” Riccardo Schiavi et al., 2020 October 1, Astronomy & Astrophysics (preprint on arxiv.org)