Let's Talk Physics

A stochasticly updated blog about interesting topics in Physics & Astronomy

How it’s All Going to End: Andromeda (Part 3)

We live in the a galaxy called the Milky Way. I’m not even going to try and convey the sense of size here. The best I can do is quote Douglas Adams, and say “Space is big. You just won’t believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.” When we start talking about Galaxies, the distances, when presented in kilometers, are just too big to make sense to us. For example, the radius of the Milky Way is 771,400,000,000,000,000 km. So to stop the eyes from popping out of your head when you see that many zeroes, I’m going to start talking in units of parsecs. 1 parsec is 30,860,000,000,000 km. So now, in our new units, the radius of the Milk Way is 25,000 pc. That’s a much easier number to handle, but we’ve kind of lost sense of scale. Oh well, let’s just roll with it, it’ll save screen space since I won’t have to write so many 0’s now.

So, below is an image of the Milky Way (an artists conception of it anyway, we haven’t managed to get anything above the Milky Way to take a picture yet). We, not even the tiniest of specs in this image, are about 8,000 pc from the center. This puts us…well…nowhere special really. but that’s a topic for another day.

An artists impression of the Milky way (click for source)

An artists impression of the Milky way (click for source)

What I’m concerned with today is the galaxy in the picture below (that’s not an artists conception, it’s an image from the Wide Infared Field Explorer run by NASA) – our nearest spiral galaxy neighbour. Andromeda has a radius of roughly 21,500 pc, has slightly less mass, but, unfortunately, is heading right towards us.

WISE Infrared View of Andromeda Galaxy and Companions

WISE Infrared View of Andromeda Galaxy and Companions

Ok, maybe I was a bit dramatic with that last part. When I say it’s coming right for us, it is going pretty fast. The galaxy is travelling at roughly 119,000 km per second. However, due to how far away it is (roughly 770,000 pc) it won’t reach us for another 6 and a half billion years. Which, if you remember how the Sun is going to eat us, is roughly 2 billion years after the Earth will have been destroyed. So, for arguments sake, let’s say that the human race will have spread out amoungst the stars by this time, which mightn’t be a bad approximation (however, the idea of leaving the Milky Way is a bit far fetched). So we’re hanging out around the Milky Way. What’s going to happen to us? Well, according to the simulator Universe Sandbox, what should happen is shown in the video (sadly not my own, due to my PC not being strong enough to handle such a powerful simulation) below.

Looks great, especially if it were used in a Hollywood blockbuster, but just like a blockbuster, it’s a complete dramatization. What actually happens when galaxies collide is a lot more boring. Most of the time the galaxies aren’t moving fast enough through each other to cause much of a problem – they literally just slip through each other without a lot of problems occurring. In fact, astronomers are currently observing 3 small galaxies passing through a massive galaxy called Abel 2199 (see image below). Now, I’m not saying that what you saw in the video won’t happen over a very VERY long time. Actually, simulations we’ve run show that this might happen a couple of hundred of million years or so after the collision. But there won’t be a big pretty explosion for people to watch (aliens living in other galaxies might observe our galactic merger like we’re currently doing with Abell 2199 though).

Abel 2199

Abel 2199 and surrounding galaxies

So what will this mean for us? Well, probably nothing. More than likely, Andromeda will pass right through without affecting us. But, there is a small chance that it could do some nasty things to us.

  1. The central supermassive blackhole of Andromeda might collide with our own supermassive blackhole, which would completely reconfigure our Galaxy, and probably affect the planets we are living on by then.
  2. A star from the Andromeda might collide with the star that we are orbiting at the time. The resulting energy from the collision would destroy us and the local system.
  3. Our planet of choice might get caught in the gravitational field of a passing star, and be pulled out of orbit of the original star we were orbiting. This could have 2 consequences:
    a) We get caught behind this new star, too far to be properly heated by it’s energy, and the planet freezes, or we get caught too close, completely toasting our planet
    b) We don’t get caught by this new star, nor do we remain with our old star. We get cast out into inter-stellar space, and become a wandering planet, forever frozen, with no sun to heat us and no nearby planet to flee to.

And that’s about it. The great collision with Andromeda might not affect us, or it might doom us all. And of course, there’s no way of stopping this collision. It’ll be a “get out of the way” or perish situation.



Sorry about the delay on this post, I ended up surviving the UCC Zombie Apocalypse yesterday, which took up most of the afternoon. I’ll be posting again on Monday, talking about wandering blackholes. Have a good weekend!


About irishphysicist

I'm a PhD student with the Departments of Physics in University College Cork, Ireland and University of Notre Dame, Indiana. I want to try and bring astrophysics to the public, and also would like world domination. But that's a story for another day.

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This entry was posted on February 15, 2013 by in Astrophysics and tagged , , , , , , , , , , , .
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