Free Astronomy Magazine September-October 2025

52 SEPTEMBER-OCTOBER 2025 ASTRO PUBLISHING A ndromeda fills the field of view. The Milky Way begins to show distortion due to tidal pull from An- dromeda. This is just one of the sce- narios predicted within the next 10 billion years. The Milky Way and M31 may not collide at all. [Science Illustration: NASA, ESA, Z. Levay and R. van der Marel (STScI), T. Hallas, and A. Mellinger] called Monte Carlo simulations stretching to 10 billion years into the future. “Because there are so many vari- ables that each have their errors, that accumulates to rather large un- certainty about the outcome, lead- ing to the conclusion that the chance of a direct collision is only 50% within the next 10 billion years,” said Sawala. “The Milky Way and Andromeda alone would remain in the same plane as they orbit each other, but this doesn’t mean they need to crash. They could still go past each other,” Sawala added. Researchers also considered the ef- fects of the orbits of Andromeda’s large satellite galaxy, M33, and a satellite galaxy of the Milky Way called the Large Magellanic Cloud (LMC). “The extra mass of Andromeda’s satellite galaxy M33 pulls the Milky Way a little bit more towards it. However, we also show that the LMC pulls the Milky Way off the or- bital plane and away from Androm- eda. It doesn’t mean that the LMC will save us from that merger, but it makes it a bit less likely,” said Sawala. In about half of the simulations, the two main galaxies fly past each oth- er separated by around half a mil- lion light-years or less (five times the Milky Way’s diameter). They move outward but then come back and eventually merge in the far future. The gradual decay of the orbit is caused by a process called dynamical friction between the vast dark-mat- ter halos that surround each galaxy at the beginning. In most of the other cases, the galax- ies don’t even come close enough for dynamical friction to work effec- tively. In this case, the two galaxies can continue their orbital waltz for a very long time. The new result also still leaves a small chance of around 2% for a head-on collision between the gal- axies in only 4 to 5 billion years. Considering that the warming Sun makes Earth uninhabitable in roughly 1 billion years, and the Sun itself will likely burn out in 5 billion years, a collision with Andromeda is the least of our cosmic worries. !