Collision of the Milky Way and Andromeda

 

It fascinates me that the world is so fascinated by the impending "collision" (more like gradual merger with a lot of gas cloud collisions) of our Milky Way with Andromeda at least four million years in the future. Sheesh, that's the same order of time as the Solar System has existed.

Exactly. The phrase “collision of the Milky Way and Andromeda” makes it sound rather more dramatic than the physics actually is.

Four billion years is almost exactly the age of the Solar System—about 4.6 billion years. So when astronomers say Andromeda will begin interacting strongly with the Milky Way in roughly that timeframe, they're talking about a span comparable to the entire existence of Earth and the Solar System so far.

And even “collision” is misleading. The stars themselves are extraordinarily unlikely to hit one another. The galaxies are mostly empty space. What actually happens is that their enormous gravitational fields distort each other, producing tidal streams and warped disks, while the interstellar gas clouds have a much better chance of interacting and triggering bursts of star formation.

Eventually the two galaxies should settle into a single much larger galaxy—historically nicknamed Milkomeda. And the really amusing part is that, on those timescales, the Solar System may still be around, although the Sun will be approaching the end of its normal main-sequence life and will be a very different star.

So from the perspective of a human observer, “Andromeda is going to collide with us!” sounds like an impending cosmic disaster. From the perspective of the universe, it's more like two enormous clouds of stars slowly noticing that they are gravitationally entangled and spending a few billion years sorting it out.

And four billion years isn't particularly “soon” in astronomical terms. It's almost comically close to the entire history of our planetary system, while still being so far away that practically everything we regard as the present universe will have become ancient history.

Still, I would like to be around to see the sky then.

Oh, absolutely. And that would be a spectacularly good seat in the universe.

The night sky would change enormously long before the galaxies actually merge. Andromeda would grow larger and brighter over hundreds of millions of years, eventually becoming an unmistakable huge structure in the sky rather than that faint little smudge we see today. As the two galaxies interact, their disks would become distorted, with enormous streams of stars stretching across the heavens.

The strange thing is that it wouldn't necessarily look like a disaster. There would be no crashing sound, no stars smashing into one another, probably no sudden fireworks—just a slowly transforming sky on timescales so long that generations of civilizations could watch the process.

Of course, by then the Sun will have its own ideas about hospitality. 😄

But I understand the sentiment. Given the choice between being immortal and seeing that sky, I suspect the latter would be the more interesting perk.