The turbulent past and future of the Milky Way

The Milky Way galaxy has had a rough history of collisions with other galactic bodies, and as we get closer to our crash with Andromeda galaxy, we’re learning more about the mergers of the past.

Our galaxy is built on collisions – over the last 12 billion years, at least a dozen other galaxies have merged into our own, with each one changing the shape and size of our galactic home into how we know it today.

In a study published in the October 2020 issue of Monthly Notices of the Royal Astronomical Society, researches have found evidence of five large collisions with our galaxy. Four of these were already known to us, but one of them is new.

All five of these collisions date back more than 10 billion years, and this newfound crash with the aptly named “Kracken Galaxy” fills the gap in the Milky Way’s ‘family tree’.

“The collision with Kraken must have been the most significant merger the Milky Way ever experienced,” Diederik Kruijssen, lead author on the study and an astronomer at the University of Heidelberg, said in a statement. “The merger with Kraken took place 11 billion years ago, when the Milky Way was four times less massive [than today].”

The Kracken merger didn’t contribute as much solar mass than the ingloriously titled ‘Gaia sausage merger’ but it did change the structure of our galaxy a lot more due to its comparative size being much smaller at the time.

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The five main contributors to our galaxy. Image credit: D. Kruijssen / Heidelberg University

These five mergers, however, are nothing compared to the eventual Andromeda-Milky Way collision.

The Andromeda-Milky Way Collision

In roughly 4 billion years, we will crash into our neighbour galaxy, Andromeda. From the light it produces, we have calculated that it is moving towards us at a rate of 110 kilometres per second. That’s right – in the time that it takes to make your morning coffee, Andromeda has come almost 15,000 km closer to ripping through the Milky Way.

Andromeda and the Milky Way are comparable in size, though Andromeda is denser, containing approximately 1 trillion stars, while our galaxy contains anywhere between 200 million and 400 million.

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An impression of Andromeda’s halo. Image credit: NASA, ESA, J. DePasquale and E. Wheatley [STScI] and Z. Levay

The result of the collision will be the galaxies dancing for a long time before merging completely and becoming what has been named ‘Milkdromeda’ or ‘Milkomeda’. It’s unlikely that any stars will directly collide with each other, and individual solar systems will be largely untouched.

Andromeda’s massive halo is actually already touching the Milky Way, reaching across the 2.5-million-light-year gap between us. If we could see the halo with the naked eye, it would appear to be over 100 times the diameter of the full moon from Earth. Observing the halo via the Hubble Telescope is giving us an insight into Andromeda’s past.

What about Earth?

There is roughly a 50% chance that the Solar System will be swept out three times further from the core of the merged galaxy, and a 12% chance that it’ll be ejected entirely. The direct effect on the Sun and our planets themselves in this situation are uncertain, but it will likely not have any adverse effects.

Due to the increased luminosity of the Sun by this time, however, life on Earth that relies on water to survive will likely have been wiped out, so we’d better have started packing our bags for another solar system by the time this all comes around.

Theoretically, though, what would this all look like from Earth?

As the galaxies approach one another, Andromeda will continue to grow larger in the sky. It isn’t currently very visible to the naked eye, but here’s what it would look like if it were brighter.

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Andromeda’s current size from Earth (just brighter). Composite photo by Tom Buckley-Houston. Original photo by Stephen Rahn.

As it gets closer, it will become more and more visible, with millions of stars lighting up the sky and creating new constellations. The stars we know will become jumbled, and as we pass Andromeda it will fade, before it makes a U turn towards us and we’ll have another mess of stars join the fray.

It will be a beautiful sight, but the merge will occur over hundreds of millions of years, meaning it’ll be kind of like watching grass grow, except several million times slower.

Although we may not live to see Milkomeda come to fruition, it will be forever be infinitely fascinating to both speculate about the future and learn about the past. Maybe one day, if we find other planets suitable for human colonisation, our collision with Andromeda will bring us closer to finding life outside our own world.

Featured image: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger

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