The Milky Way had a sibling
The Milky Way is our local galaxy and one of the celestial bodies that we know the most about. What we are just discovering however is that the Milky Way at one time had a larger sibling that was torn apart many years ago by the Andromeda galaxy.
The Andromeda galaxy and the Milky Way are in the local group and they are the two largest members of this group which is a collection of 50 galaxies all packed within 10 million light years of space. Around 2 billion years ago Andromeda completely devoured the third-largest member of the local group family.
Of course this is not an unusual event. Andromeda is a very large spiral galaxy and just the way that it is shaped it is thought to have shredded up hundreds of smaller galaxies throughout its lifespan. The mergers of these galaxies together have made it difficult to know exactly what Andromeda could have consumed throughout its existence, until now.
With the help of a team using computer simulations the Andromeda galaxy could be examined much more closely. The stars that make up the Andromeda Halo surrounding the galaxy disk actually came from a smashup in their simulation from a larger galaxy. The outer stellar halo suggests that Andromeda actually shredded larger galaxies and with the computer model that they created, likely a sibling to the Milky Way.
The sibling to our own galaxy was estimated to be 20 times larger than any galaxy that the Milky Way had merged with in the past. Evidence of this sibling can be found all along the edge of the elliptical galaxy. Older stars are close by to young stars and this is exactly the type of shakeup that can occur when galaxies merge.
With future use of computer models for galaxy collision simulation, we can find out more about how our local group has changed over time.