The Milky Way is on a collision course with the closest major galaxy, Andromeda. What’s going to happen to our solar system when the two objects finally collide? Find out in this episode of The Astroholic Explains!

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Welcome to the Astroholic explains. I’m your host Chris.

And I’m Alfredo the Astroholic.

This is a podcast where I throw some cosmic conundrums your way,

and I try my best to answer them. Let’s get started.

My question today, what will happen when Andromeda merges with our galaxy? If hypothetically, life survives, what would we see in the sky?

That’s a very interesting question. As always, jumping..

into the deep end!

Yes. Okay. So, for the people that don’t know, our galaxy, the Milky Way is on a collision course with Andromeda, the closest major galaxy to our system. So each large galaxy is surrounded by many little galaxies so we can see two easily in the sky in the southern hemisphere.

Who are they?

The Magellanic clouds

Wow… We saw one!

We saw both of them…. Sidetrack on the story: We were in a petrol station in the middle of the Australian desert. And I was hoping they would see something but lights at the petrol station were very bright. So we walk through the petrol station on the other side, there were no lights for probably thousands of kilometers. And suddenly, the entire sky was shimmering and much on the clouds where there! Beautiful! And I started crying, because well, I get emotional of the night sky. So you can see them and I recommend everyone if they have the possibility to be in the southern hemisphere and a place with low light pollution to go and check them out. Back to the point. So there are lots of small galaxies around both us and Andromeda, but us and Andromeda are going to come together, I think now it’s estimated to be in around 5 billion years in the future.

So no time soon, then!

No time soon. It’s nothing that we should worry about. But even if we assume that somehow we could extend our lives that long if you want to live that long, it’s very unlikely that there’s going to be any major changes to the sun or place the probability they’re going to be major changes to the solar system is quite low.

So nothing will happen to the Sun by that point.

Not yet, then. The sun at that point might be ready to become a red giant.

Okay, so it’s about to swell up.

It’s about to swell up. At the moment, the sun is middle-aged


But I think “middle-aged” as the term means, rather than middle-aged, as we intended us now somebody that is in their 50s/60s

It’s not about to go and buy a boat.

No, the sun is not going to go and buy an expensive car or something. What’s going to happen? What we think is that two galaxies are going to orbit around each other and they’re going to be spinning around a bit. And their interaction would probably strip material from each other and create this sort of bridge of stars and gas between the two objects. These are called tidal bridges. Because of the tidal forces that is just..

Tidal forces on each other? Or tidal forces the Earth would experience?

No, no, It’s between the galaxies


It’s called tidal force because it’s due to gravity and guess we knew what tides were before we knew what gravity was.

Yeah, I guess

And usually, in these tidal bridges, we have new stars forming because the gravitational push and pull compresses the gas. So you have brand-new stars. And a galaxy merger usually is a lot, a lot of new stars being born and stars dying. There is a lot of activity and it’s fascinating to see.

So this is pretty much what we’re going to see four or five billion years in the future. We’re going to start seeing bridges of material, the Milky Way will start looking like more than the tight strip of light in the night sky. It will have offshoots and obviously Andromeda will become way clearer in visible light. So that’s very exciting!

And as the two galaxies swirl around one another, we are going to have more complex structures appearing in the sky. Again, it’s just.. just think of the Milky Way light strip in the sky but think that it will have vortices, swirls, and there’s going to be a lot messier.

Really big sort of swirls? Will you be able to see them with a naked eye like you can sometimes see the Milky Way in the sky?

Yeah, yeah. If you were in a place with low-light pollution like Australia… Yeah go to Australia or Brecon Beacons. So there are a few places in the UK with dark skies. So if you go to any of those places in 5 billion years, and assuming that they’re still above water, or whatever happened to the earth in 5 billion years, you should be able to see a different sky.

That’s pretty awesome.

But that’s not the end… [Chris muttering] Ask the question…

I’m presuming the scientists and astronomers have analyzed all the data they can to see which direction Andromeda will be hitting the Milky Way from which side. Because we’re on a certain part of one of the arms of the Milky Way. Yep. Is it going to be hitting us sort of head-on on our closest side or will it hit a different part or even hit the top of the bottom of the Milky Way?

Astronomers have analyzed that but there are a lot of uncertainties…

It’s a long, long way off.

It’s a long, long way off, quite distant. And although we made huge steps in working on the precision, every simulation will have major uncertainties. 

I guess you can’t really determine it.

Also, when we think of collisions… although they are objects, they are bound by gravity, etc. They’re also diffuse objects. So you’re thinking more of like, a collision between two clouds you know, rather than the collision between two cars..

That’s a lot cuter.

Yeah, but the clouds are very heavy and they will swirl around each other

and destroy a lot of things.

The destruction it’s, again, it’s a bit complicated. There was a paper, I think, four years ago, five years ago. So maybe the probability needs to be revisited or updated. And they calculated that the chance that the solar system would be directly affected by the merger was only 12%. Which is quite high on one hand, but also tells you that most of…

it’s not a massive percent…

And there was only a tiny, tiny fraction of percent that the sun would be sort of swung out of the Milky Way-Andromeda merger galaxy, which nobody has come up with a cute name for it!

For the new combination galaxy? Andromeda Way…… It doesn’t sound right. Milky Dromeda. No, no, that sounds horrible.

Yes. Say I think we’re past the point in which we do cute names for celebrity couples, and nobody is on the right Zeitgeist, to be thinking about a good name, maybe in five billion years.

So it’s unlikely the main changes are going to happen to the solar system with such a low percentage, around 12%, as scientists calculated. So we’re not going to be looking at things like stars from Andromeda smashing into stars in our galaxy, or are we going to be looking at stars colliding with exoplanets, anything like that?

No. Like space is big. We know that stars passing each other really can create effects. They can throw comets inward. They can sort of free planets and throw planets into space. It’s an interesting question, but it’s more for Hollywood rather than sorry, reality. Space is big, like, really, really big. Things affect each other. So you don’t have to have two stars colliding with each other to wreak havoc in a star system, just a star passing by can bring in comets or can sort of pull away a planet. So things can happen.

Doesn’t have to be super dramatic. And certainly, stars from Andromeda and the star from the Milky Way will end up orbiting in the same galaxy. We should consider that the Milky Way and on have roughly the same mass. So it’s not that one is colliding into the other, they’re colliding with each other, they are merging. So they will be throwing the galaxy all over the place. The Solar System likely will be moving on different orbit around the common core of this galactic fusion.

Of course, there’s going to be a brand new galactic center.

Yes, we’ll get there in a minute.

Sorry, sorry…

Always jumping ahead..


So there’s going to be new stars and not just new stars from the other galaxy but as I said the gas, the interstellar gas will be compressed and will form new stars. And suddenly, we’re going to have more.. more stuff going on in the galaxy for maybe a billion, a couple of billion years of very high activity.

So that actually was going to be my next question was how long would this entire procedure take from the immediate approach of Andromeda? Obviously, it’s not going to be very, very quick, because I think if I’m not mistaken, the Milky Way takes about 240 million years to rotate. So if these galaxies are turning around each other, then it’s going to take millions and millions and millions of years for this process, surely?

Yes. So they think they will collide in about 5 billion years. As I mentioned, I think probably at first is going to be a flyby. So they will come together and then separate again. Before coming together again and properly merging the process, we tend to think merger to post-merger can be a billion to 2 billion years. It’s a little bit arbitrary to decide when a galaxy is relaxed when it’s settled.

Some people say when the two cores cannot be distinguished anymore, then the galaxy is not a merger anymore, but are different interpretation because depends how in-depth you want to go. Because at the core of most galaxies, and certainly at the core of the Milky Way and Andromeda, there is a supermassive black hole. And so is the merger complete when the supermassive black holes merge? Potentially; but also know that that phenomena usually have consequences because the two supermassive black holes might be active.

Or maybe sometimes only one is active and when they merge, then the final black hole becomes active, and affects the galaxy. I guess one of the issues with science that we have a way to describe the world, the universe that is limited to the definition that we give them. It’s a bit arbitrary. This whole process probably lasts a couple of billion years. And that’s when we finally end up seeing in the sky, at the end of this process, instead of having the strip of light that is the milky way there’s going to be central bright or brighter spots, the Milky Way & Andromeda, they’re not going to stay spiral galaxies. The merger will be so dramatic that their entire geometry will change, and they will become an elliptical galaxy.

That’s incredible. So it’s a brand new sky at night. 

Definitely a brand new sky at night!

That’s fantastic! Thank you very, very much for answering my question today.

Always my pleasure.

Do you have any burning questions for the Astroholic? If so send them into me out  @illucifer on Twitter, and I will spring it on him in an upcoming episode. See you next time!

Image Credit: HST/NASA/ESA