On July 24th, 2018 we learned about

The Andromeda galaxy’s past and future growth is fueled by collisions with its cosmic kin

The Earth is currently flying around the Sun at around 70,000 miles-per-hour. The Sun is dragging our solar system through the Milky Way galaxy at around 450,000 miles-per-hour. The Milky Way, though composed of 150 to 250 billion different stars, is moving through a cluster of galaxies known as the Local Group at around 250,000 miles-per-hour. While there’s enough open space in space to allow our solar system room to maneuver, it seems like massive galaxies flying around would likely lead to some kind of collision. As it turns out, intuition is actually correct in this case, as researchers have calculated that nearby galaxies have recently crashed into each other in the not too distant past, and more collisions are expected in the future.

Andromeda’s acquisitions

The largest galaxy in the Local Group is Andromeda, which is estimated to be home to around a trillion stars. It wasn’t always that big though, as astronomers believe it has a long history of colliding with, and consuming, smaller galaxies in its path. Two billion years ago, it likely gobbled up what would have been the third-largest galaxy in the Local Group, the blandly-named M32 galaxy. Based on the age of the stars M32 stars still clustered together in Andromeda, researchers believe this collision was substantial. The jump in Andromeda’s celestial population was probably 20 times greater than any acquisitions in the past.

The simulations that lead to those figures match up well with other research teams’ observations, who also determined that Andromeda experienced a major collision between 1.8 and 3 billion years ago. The one major surprise in all this is how these collisions haven’t reshaped Andromeda more dramatically. Researchers expected that sucking up so many new stars would force Andromeda to adopt a smoother oval shape, but somehow the galaxy has retained a spiral, suggesting we don’t fully understand how these shapes are formed.

Milky Way Merger

If any humans are still around in four billion years, we may be able to find out more about that process from front row seats here on Earth. Andromeda’s next acquisition has been calculated to be our very own Milky Way galaxy. The Milky Way is the second-largest galaxy in the local group, and also a spiral galaxy, so the merger of these two massive galaxies should be quite impressive. As far as we know, it would be the largest restructuring the Milky Way has ever experienced, and will take around two billion years to complete.

If you have a distant relative still on this planet somehow, they’ll have plenty to watch, but not directly experience. The night sky will start to fill with stars from Andromeda’s core, and new stars will start popping up as various forms of mass merge together. Our solar system probably won’t be subject to any violent collisions though, instead being pushed towards the outer edge of the resulting “Milkomeda” galaxy. Granted, our own Sun will likely prevent this from being much of a concern as it should be expanding into an Earth-consuming red giant in the middle of all this, but at least the view from our solar system’s gas giants will be impressive.

Source: The Milky Way Had a Big Sibling Long Ago — And Andromeda Ate It by Mike Wall, Space.com