One day, our own galaxy will collide with Andromeda — and when that happens, poor defenseless Earth will be smashed to bits. Right? Wrong. In this week's "Ask a Physicist," we'll find out why.


Like most io9 readers, I've been thinking a lot about the apocalypse since Annalee's new book hit the stands. In tribute, let me throw my own doomsday scenario into the mix: the galactic smackdown that will transpire when the Andromeda galaxy collides with our own. TWO GALAXIES ENTER, ONLY ONE WILL LEAVE. But what will happen to Earth when this transpires?


Top image: Hubble Space Telescope



The Future of the Local Group

Space is well named. Even traveling at the speed of light, it takes years to travel between stars, and millions of years to travel between galaxies. And as Edwin Hubble discovered back in 1929, space has a tendency to get even spacier as galaxies recede ever further from one another.

But despite popular misconceptions to the contrary, not everything in the universe is moving away from one another. The earth isn't expanding, for instance, and neither is the solar system. Whenever there's enough local gravity, big objects tend to coalesce rather than blow apart.


And as it turns out, there's a lot of local gravity in our neck of the universe. Our Milky Way Galaxy is the vice-president a collection of fifty-some galaxies known as "The Local Group," all which are more or less in orbit around one another other. Most of these galaxies are beneath our contempt – hundreds, or even thousands of times less massive than our own – but there's another major player in the Local Group, the Andromeda galaxy. Andromeda (M31 to aficionados) and the Milky Way are the 500 pound gorillas of the Local Group, and even though we're still about 2 1/2 million light-years away from one another, we're barreling toward one another at about roughly 270,000 mph. Though it's hard to get a feel for speeds that fast, it's roughly Mach 400.

It's harder than you think to say for certain that any two things are going to collide with one another in space. We're confined to the Earth, after all, which means that we only get a 2-D view of things. Andromeda might veer sideways just enough to miss us — at least on the first pass. As a result, we weren't entirely sure that we were really on track for a head-on collision until last year, when a team at the Space Telescope Science Institute used a combination of HST images and numerical simulations and found that a collision was inevitable – and due in only about 4 billion years. Or at least, that's how long it will take to get the party started. The ensuing merger will take a few billion years more.


Truth to be told, while the exact timing of the collision might have been in doubt, the ultimate fate of our two galaxies wasn't. As galaxies orbit around one another, they rip and tear at each other tidally, slowly losing energy and spiraling inward.


It's only a matter of time until the two galaxies meet in a sweet and eternal embrace... and ultimately settle down into a boring routine. This is, in fact, how we see merging galaxies in other corners of the universe. While both the Andromeda and Milky Way are both beautiful Spiral galaxies, the resulting Milkomeda will be a drab Elliptical. We may not be destroyed physically by the process, it's fair to say that the collision will destroy whatever sense of style our Galaxy ever had.

The Good News: Space is Empty

Will earthlings get to see the merger?

Four billion years is a long time, but not too long. The sun's still got a fair amount of life in her yet, and 4 billion years from now – and for another 1.4 billion years after that – it'll still be happily turning hydrogen into helium in the form of a perfectly ordinary "Main Sequence" star. After that, it'll turn into a red giant, reaching nearly to the radius of Earth, which will kill everything foolish enough to stick around that long.


Will the Earth get destroyed by the Andromeda-Milky Way collision, or will it last long enough to get eaten by the sun?

I've got some good news, and some bad news. We'll start with the good news, which, ironically, is based on just how lonely a place the universe can be.

Milky Way-Andromeda collision simulation, via Hubble.

Go out on a clear night and look at the sky. No matter how clear your location – even if you use a telescope – you're going to come to the conclusion that most typical points in the sky don't have stars in them. This is actually a good thing. If the heavens were literally filled with stars so densely that they overlapped, then every point in the sky would be as bright as the sun, and the earth would burn to a crisp in a matter of hours. That the sky isn't as bright as the sun (an idea known as Olber's Paradox), is actually a consequence of the fact that the universe had a beginning (and also is a big theme in my upcoming book).


Our nearest stellar neighbor, Proxima Centauri, is about 60 million solar radii from the solar system. Plug that in, and there's about a 1 in a hundred-billion probability that the sun would collide with any star from Andromeda.


Of course, that's not the only way the Earth could be destroyed. If a star passed close enough to earth to disrupt our orbit, we'd be in pretty rough shape, too. Luckily for us, even that sort of event is only about a 1 in a million shot. Post collision, the sun will keep on shining, and the earth will keep on circling. Hurray!

The Bad News: We're already dead

Just because a star isn't going to come along and smash us out of existence doesn't mean that we're going to just sit back and watch while the Milkomeda Galaxy forms. For one thing, while the solar system won't be ripped apart, it will be relocated, probably even further into the boondocks than we currently are.


This is probably good news, because the black holes at the centers of the Milky Way (about 4 million times the mass of the sun) and Andromeda (about a hundred million solar masses) are going to cannibalize one another, and throw out a lot of X-rays in the process. We probably don't want to be too close to that. This supermassive black hole will then proceed to chow down on the material in the middle of the galaxy, producing enough radiation to make it easy to spot even here on earth.


The newly merged galaxy is going to be pretty active for quite a while. As they collide, whatever gas remains in both galaxies are going to smack into one another like a shock wave and start producing stars. After only a couple of million years, some of the largest stars will go supernova, and it would really suck to be near one of those when it went off.

I'm saving the worst for last, because even though the Earth will still be here in 4 billion years, the truth of the matter is that we won't, at least not in anything even remotely resembling our current form. If you're planning for humanity's long-term survival, you need to keep in mind that the check-out time isn't when the sun runs out of fuel and becomes a red giant – we'd better be off the planet long before then.


The sun is a changing and evolving thing. In the 4 1/2 billion years since it formed, the sun has become about 40% more luminous. And it's still going! The sun's power comes from fusing hydrogen into helium, and as time goes on, that hydrogen (at least in the center of the sun) becomes more and more scarce. Counter-intuitively, this means that the sun will burn hotter and hotter as time goes on. By the time the collision begins, the Earth will be so hot that the seas will evaporate, which pretty much spells disaster for anybody who happens to remain.

Hopefully, our robot progeny will find the beauty of the galactic collision incredibly moving, because the flesh-and-bone version of humanity will have long since boiled away.


Dave Goldberg is a Physics Professor at Drexel University, and the author of "A User's Guide to the Universe," and the upcoming "Universe in the Rearview Mirror" (coming July 11!). You should send him your questions about the universe, or even better become a fan of his awesome facebook page.